Publications

High-momentum two-particle correlations are a useful tool for studying jet-quenching effects in the quark-gluon plasma. Angular correlations between neutral-pion triggers and charged hadrons with transverse momenta in the range 412 GeV/ and 0.57 GeV/, respectively, have been measured by the PHENIX experiment in 2014 for Au+Au collisions at √=200 GeV. Suppression is observed in the yield of high-momentum jet fragments opposite the trigger particle, which indicates jet suppression stemming from in-medium partonic energy loss, while enhancement is observed for low-momentum particles. The ratio and differences between the yield in Au+Au collisions and + collisions, and Δ, as a function of the trigger-hadron azimuthal separation, Δ, are measured for the first time at the BNL Relativistic Heavy Ion Collider. These results better quantify how the yield of low- associated hadrons is enhanced at wide angle, which is crucial for studying energy loss as well as medium-response effects.

The invariant yield of electrons from open-heavy-flavor decays for 1

Reported here are transverse single-spin asymmetries (AN) in the production of charged hadrons as a function of transverse momentum (pT) and Feynman-x (xF) in polarized p↑+p, p↑+Al, and p↑+Au collisions at √sNN=200GeV. The measurements have been performed at forward and backward rapidity (1.4

The recently approved Electron-Ion Collider (EIC) will provide a unique new opportunity for searches of charged lepton flavor violation (CLFV) and other new physics scenarios. In contrast to the e↔μ CLFV transition for which very stringent limits exist, there is still a relatively large discovery space for the e→τ CLFV transition, potentially to be explored by the EIC. With the latest detector design of ECCE (EIC Comprehensive Chromodynamics Experiment) and projected integral luminosity of the EIC, we find the τ-leptons created in the DIS process ep→τX are expected to be identified with high efficiency. A first ECCE simulation study, restricted to the 3-prong τ-decay mode and with limited statistics for the Standard Model backgrounds, estimates that the EIC will be able to improve the current exclusion limit on e→τ CLFV by an order of magnitude. The very high vertex resolution of the ECCE detector configuration plays a critical role in τ identification.

A search for flavour-changing neutral current (FCNC) tqH interactions involving a top quark, another up-type quark (q = u, c), and a Standard Model (SM) Higgs boson decaying into a τ-lepton pair (H → τ ⁺ τ ⁻) is presented. The search is based on a dataset of pp collisions at √s = 13 TeV that corresponds to an integrated luminosity of 139 fb ⁻¹ recorded with the ATLAS detector at the Large Hadron Collider. Two processes are considered: single top quark FCNC production in association with a Higgs boson (pp → tH), and top quark pair production in which one of top quarks decays into Wb and the other decays into qH through the FCNC interactions. The search selects events with two hadronically decaying τ-lepton candidates (τ _had) or at least one τ _had with an additional lepton (e, μ), as well as multiple jets. Event kinematics is used to separate signal from the background through a multivariate discriminant. A slight excess of data is observed with a significance of 2.3σ above the expected SM background, and 95% CL upper limits on the t → qH branching ratios are derived. The observed (expected) 95% CL upper limits set on the t → cH and t → uH branching ratios are 9.4×10⁻⁴(4.8_−1.4^+2.2×10⁻⁴) and 6.9×10⁻⁴(3.5_−1.0^+1.5×10⁻⁴) , respectively. The corresponding combined observed (expected) upper limits on the dimension-6 operator Wilson coefficients in the effective tqH couplings are C_cφ uφ 

Time Projection Chambers (TPC)s are excellent tracking detectors for high multiplicity events and can intrinsically be high-rate, but are limited by the ions created in their avalanche stage. GEMs and Micromegas can reduce IBF through their geometry and E͐-field ratios, but these can lead to gain fluctuations and still leave IBF as the dominant source of space charge. An active BPG can block all IBF ions, but their slow drift speed creates too much dead time. A passive BPG will overcome this limitation by using an external B͐-field to allow the electrons to pass through while still blocking all ions. Since the grid changes the electron's trajectory, a loss of resolution will occur. The trajectory is shifted symmetrically along the wires so the wire alignment with respect to the detection pads is a specific question not studied before. We present completed IBF analysis from data collected at Weizmann Institute of Science (WIS), along with an intro to our test on wire resolution.

The electroweak production of Z(νν¯) γ in association with two jets is studied in a regime with a photon of high transverse momentum above 150 GeV using protonproton collisions at a centre-of-mass energy of 13 TeV at the Large Hadron Collider. The analysis uses a data sample with an integrated luminosity of 139 fb⁻¹ collected by the ATLAS detector during the 20152018 LHC data-taking period. This process is an important probe of the electroweak symmetry breaking mechanism in the Standard Model and is sensitive to quartic gauge boson couplings via vector-boson scattering. The fiducial Z(νν¯) γjj cross section for electroweak production is measured to be 0.77_−0.30^+0.34 fb and is consistent with the Standard Model prediction. Evidence of electroweak Z(νν¯) γjj production is found with an observed significance of 3.2σ for the background-only hypothesis, compared with an expected significance of 3.7σ. The combination of this result with the previously published ATLAS observation of electroweak Z(νν¯) γjj production yields an observed (expected) signal significance of 6.3σ (6.6σ). Limits on anomalous quartic gauge boson couplings are obtained in the framework of effective field theory with dimension-8 operators. 

Polarized proton-proton collisions provide leading-order access to gluons, presenting an opportunity to constrain gluon spin-momentum correlations within transversely polarized protons and enhance our understanding of the three-dimensional structure of the proton. Midrapidity open-heavy-flavor production at s=200 GeV is dominated by gluon-gluon fusion, providing heightened sensitivity to gluon dynamics relative to other production channels. Transverse single-spin asymmetries of positrons and electrons from heavy-flavor hadron decays are measured at midrapidity using the PHENIX detector at the Relativistic Heavy Ion Collider. These charge-separated measurements are sensitive to gluon correlators that can in principle be related to gluon orbital angular momentum via model calculations. Explicit constraints on gluon correlators are extracted for two separate models, one of which had not been constrained previously.

The measurement of direct photons from Au+Au collisions at sNN=39 and 62.4 GeV in the transverse-momentum range 0.4

The Electron Ion Collider (EIC) is the next generation of precision QCD facility to be built at Brookhaven National Laboratory in conjunction with Thomas Jefferson National Laboratory. There are a significant number of software and computing challenges that need to be overcome at the EIC. During the EIC detector proposal development period, the ECCE consortium began identifying and addressing these challenges in the process of producing a complete detector proposal based upon detailed detector and physics simulations. In this document, the software and computing efforts to produce this proposal are discussed; furthermore, the computing and software model and resources required for the future of ECCE are described.

From the first principles, particles with the same quark content and similar masses should have similar kinematic distributions. Transverse mass scaling may be employed to estimate possible differences in the momentum distribution of such particles. Based on this scaling, the excited bottomonium states measured at the LHC are found to be significantly different from Υ(1S) to the extent that the integrated yield of Υ(2S) is 1.6 times less and Υ(3S) 2.4 times less than would be explained by the mass difference. This contribution explains how the estimate is worked out and relates it to other measurements performed at the LHC.

The ATLAS experiment at the Large Hadron Collider has a broad physics programme ranging from precision measurements to direct searches for new particles and new interactions, requiring ever larger and ever more accurate datasets of simulated Monte Carlo events. Detector simulation with Geant4 is accurate but requires significant CPU resources. Over the past decade, ATLAS has developed and utilized tools that replace the most CPU-intensive component of the simulationthe calorimeter shower simulationwith faster simulation methods. Here, AtlFast3, the next generation of high-accuracy fast simulation in ATLAS, is introduced. AtlFast3 combines parameterized approaches with machine-learning techniques and is deployed to meet current and future computing challenges, and simulation needs of the ATLAS experiment. With highly accurate performance and significantly improved modelling of substructure within jets, AtlFast3 can simulate large numbers of events for a wide range of physics processes.

Searches are performed for nonresonant and resonant di-Higgs boson production in the bb¯γγ final state. The dataset used corresponds to an integrated luminosity of 139 fb^-1 of proton-proton collisions at a center-of-mass energy of 13 TeV recorded by the ATLAS detector at the CERN Large Hadron Collider. No excess above the expected background is found and upper limits on the di-Higgs boson production cross sections are set. A 95% confidence-level upper limit of 4.2 times the cross section predicted by the Standard Model is set on pp→HH nonresonant production, where the expected limit is 5.7 times the Standard Model predicted value. The expected constraints are obtained for a background hypothesis excluding pp→HH production. The observed (expected) constraints on the Higgs boson trilinear coupling modifier κλ are determined to be [-1.5,6.7] ([-2.4,7.7]) at 95% confidence level, where the expected constraints on κλ are obtained excluding pp→HH production from the background hypothesis. For resonant production of a new hypothetical scalar particle X (X→HH→bb¯γγ), limits on the cross section for pp→X→HH are presented in the narrow-width approximation as a function of m_X in the range 251 GeV≤m_X≤1000 GeV. The observed (expected) limits on the cross section for pp→X→HH range from 640 fb to 44 fb (391 fb to 46 fb) over the considered mass range.

Several observables sensitive to the fragmentation of b quarks into b hadrons are measured using 36 fb^-1 of √s=13 TeV proton-proton collision data collected with the ATLAS detector at the LHC. Jets containing b hadrons are obtained from a sample of dileptonic t¯t events, and the associated set of charged-particle tracks is separated into those from the primary pp interaction vertex and those from the displaced b-decay secondary vertex. This division is used to construct observables that characterize the longitudinal and transverse momentum distributions of the b hadron within the jet. The measurements have been corrected for detector effects and provide a test of heavy-quark-fragmentation modeling at the LHC in a system where the top-quark decay products are color connected to the proton beam remnants. The unfolded distributions are compared with the predictions of several modern Monte Carlo parton-shower generators and generator tunes, and a wide range of agreement with the data is observed, with p values varying from 5×10^-4 to 0.98. These measurements complement similar measurements from e⁺e^- collider experiments in which the b quarks originate from a color singlet Z/γ∗.

A search for events with two displaced vertices from long-lived particle (LLP) pairs using data collected by the ATLAS detector at the LHC is presented. This analysis uses 139 fb^-1 of proton-proton collision data at √s=13 TeV recorded in 2015-2018. The search employs techniques for reconstructing vertices of LLPs decaying to jets in the muon spectrometer displaced between 3 and 14 m with respect to the primary interaction vertex. The observed numbers of events are consistent with the expected background and limits for several benchmark signals are determined. For the Higgs boson with a mass of 125 GeV, the paper reports the first exclusion limits for branching fractions into neutral long-lived particles below 0.1%, while branching fractions above 10% are excluded at 95% confidence level for LLP proper lifetimes ranging from 4 cm to 72.4 m. In addition, the paper present the first results for the decay of LLPs into tt¯ in the ATLAS muon spectrometer.

Small nuclear collisions are mainly sensitive to cold-nuclear-matter effects; however, the collective behavior observed in these collisions shows a hint of hot-nuclear-matter effects. The identified-particle spectra, especially the φ mesons which contain strange and antistrange quarks and have a relatively small hadronic-interaction cross section, are a good tool to study these effects. The PHENIX experiment has measured φ mesons in a specific set of small collision systems p+Al, p+Au, and He3+Au, as well as d+Au [Adare, Phys. Rev. C 83, 024909 (2011)0556-281310.1103/PhysRevC.83.024909], at sNN=200 GeV. The transverse-momentum spectra and nuclear-modification factors are presented and compared to theoretical-model predictions. The comparisons with different calculations suggest that quark-gluon plasma may be formed in these small collision systems at sNN=200 GeV. However, the volume and the lifetime of the produced medium may be insufficient for observing strangeness-enhancement and jet-quenching effects. The comparison with calculations suggests that the main production mechanisms of φ mesons at midrapidity may be different in p+Al versus p/d/He3+Au collisions at sNN=200 GeV. While thermal quark recombination seems to dominate in p/d/He3+Au collisions, fragmentation seems to be the main production mechanism in p+Al collisions.

Suppression of the J/ψ nuclear-modification factor has been seen as a trademark signature of final-state effects in large collision systems for decades. In small systems, the nuclear modification was attributed to cold-nuclear-matter effects until the observation of strong differential suppression of the ψ(2S) state in p+A and d+A collisions suggested the presence of final-state effects. Results of J/ψ and ψ(2S) measurements in the dimuon decay channel are presented here for p+p, p+Al, and p+Au collision systems at sNN=200GeV. The results are predominantly shown in the form of the nuclear-modification factor, RpA, the ratio of the ψ(2S) invariant yield per nucleon-nucleon collision in collisions of proton on target nucleus to that in p+p collisions. Measurements of the J/ψ and ψ(2S) nuclear-modification factor are compared with shadowing and transport-model predictions, as well as to complementary measurements at Large Hadron Collider energies.

The associated production of a Higgs boson and a top-quark pair is measured in events characterised by the presence of one or two electrons or muons. The Higgs boson decay into a b-quark pair is used. The analysed data, corresponding to an integrated luminosity of 139 fb⁻¹, were collected in proton-proton collisions at the Large Hadron Collider between 2015 and 2018 at a centre-of-mass energy of √s = 13 TeV. The measured signal strength, defined as the ratio of the measured signal yield to that predicted by the Standard Model, is 0.35_−0.34^+0.36. This result is compatible with the Standard Model prediction and corresponds to an observed (expected) significance of 1.0 (2.7) standard deviations. The signal strength is also measured differentially in bins of the Higgs boson transverse momentum in the simplified template cross-section framework, including a bin for specially selected boosted Higgs bosons with transverse momentum above 300 GeV.

In 2015, the PHENIX Collaboration has measured single-spin asymmetries for charged pions in transversely polarized p+p collisions at the center-of-mass energy of s=200 GeV. The pions were detected at central rapidities of |η|

We present a measurement of the transverse single-spin asymmetry for π0 and η mesons in p↑+p collisions in the pseudorapidity range |η|

A measurement of event-shape variables in proton-proton collisions at large momentum transfer is presented using data collected at s = 13 TeV with the ATLAS detector at the Large Hadron Collider. Six event-shape variables calculated using hadronic jets are studied in inclusive multijet events using data corresponding to an integrated luminosity of 139 fb⁻¹. Measurements are performed in bins of jet multiplicity and in different ranges of the scalar sum of the transverse momenta of the two leading jets, reaching scales beyond 2 TeV. These measurements are compared with predictions from Monte Carlo event generators containing leading-order or next-to-leading order matrix elements matched to parton showers simulated to leading-logarithm accuracy. At low jet multiplicities, shape discrepancies between the measurements and the Monte Carlo predictions are observed. At high jet multiplicities, the shapes are better described but discrepancies in the normalisation are observed. [Figure not available: see fulltext.]

The integrated fiducial cross-section and unfolded differential jet mass spectrum of high transverse momentum Z -> b (b) over bar decays are measured in Z gamma events in proton-proton collisions at root s = 13 TeV. The data analysed were collected between 2015 and 2016 with the ATLAS detector at the Large Hadron Collider and correspond to an integrated luminosity of 36.1 fb(-1). Photons are required to have a transverse momentum p(T) > 175 GeV. The Z -> b (b) over bar decay is reconstructed using a jet with p(T) > 200 GeV, found with the anti-k(t) R = 1.0 jet algorithm, and groomed to remove soft and wide-angle radiation and to mitigate contributions from the underlying event and additional proton-proton collisions. Two different but related measurements are performed using two jet grooming definitions for reconstructing the Z -> b (b) over bar decay: trimming and soft drop. These algorithms differ in their experimental and phenomenological implications regarding jet mass reconstruction and theoretical precision. To identify Zbosons, b-tagged R = 0.2 track-jets matched to the groomed large-R calorimeter jet are used as a proxy for the b-quarks. The signal yield is determined from fits of the data-driven background templates to the different jet mass distributions for the two grooming methods. Integrated fiducial cross-sections and unfolded jet mass spectra for each grooming method are compared with leading-order theoretical predictions. The results are found to be in good agreement with Standard Model expectations within the current statistical and systematic uncertainties.

We present direct photon-hadron correlations in 200 GeV/A Au+Au, d+Au, and p+p collisions, for direct photon pT from 5-12 GeV/c, collected by the PHENIX Collaboration in the years from 2006 to 2011. We observe no significant modification of jet fragmentation in d+Au collisions, indicating that cold nuclear matter effects are small or absent. Hadrons carrying a large fraction of the quark's momentum are suppressed in Au+Au compared to p+p and d+Au. As the momentum fraction decreases, the yield of hadrons in Au+Au increases to an excess over the yield in p+p collisions. The excess is at large angles and at low hadron pT and is most pronounced for hadrons associated with lower momentum direct photons. Comparison to theoretical calculations suggests that the hadron excess arises from medium response to energy deposited by jets.

The PHENIX experiment has measured the spin alignment for inclusive J/ψ→e+e- decays in proton-proton collisions at s=510 GeV at midrapidity. The angular distributions have been measured in three different polarization frames, and the three decay angular coefficients have been extracted in a full two-dimensional analysis. Previously, PHENIX saw large longitudinal net polarization at forward rapidity at the same collision energy. This analysis at midrapidity, complementary to the previous PHENIX results, sees no sizable polarization in the measured transverse momentum range of 0.0

Charmonium is a valuable probe in heavy-ion collisions to study the properties of the quark gluon plasma, and is also an interesting probe in small collision systems to study cold nuclear matter effects, which are also present in large collision systems. With the recent observations of collective behavior of produced particles in small system collisions, measurements of the modification of charmonium in small systems have become increasingly relevant. We present the results of J/Psi measurements at forward and backward rapidity in various small collision systems, p + p, p + Al, p + Au, and He-3+Au, at root s(NN) = 200 GeV. The results are presented in the form of the observable RAB, the nuclear modification factor, a measure of the ratio of the J/Psi invariant yield compared to the scaled yield in p + p collisions. We examine the rapidity, transverse momentum, and collision centrality dependence of nuclear effects on J/Psi production with different projectile sizes p and He-3, and different target sizes Al and Au. The modification is found to be strongly dependent on the target size, but to be very similar for p + Au and He-3+Au. However, for 0%-20% central collisions at backward rapidity, the modification factor for He-3+Au is found to be smaller than that for p + Au, with a mean fit to the ratio of 0.89 +/- 0.03(stat)+/- 0.08(syst), possibly indicating final state effects due to the larger projectile size.

The PHENIX experiment has studied nuclear effects in p + Al and p + Au collisions at root S-NN = 200GeV on charged hadron production at forward rapidity (1.4

The PHENIX collaboration presents first measurements of low-momentum (0.4 5 GeV= c), but when results from different collision energies are compared, an additional root s(NN) -dependent multiplicative factor is needed to describe the integrated-direct-photon yield.

Measurements of the differential production of electrons from open-heavy-flavor hadrons with charm-and bottom-quark content in p + p collisions at root s = 200 GeV are presented. The measurements proceed through displaced-vertex analyses of electron tracks from the semileptonic decay of charm and bottom hadrons using the PHENIX silicon-vertex detector. The relative contribution of electrons from bottom decays to inclusive heavy-flavor-electron production is found to be consistent with fixed-order-plus-next-to-leading-log perturbative-QCD calculations within experimental and theoretical uncertainties. These new measurements in p + p collisions provide a precision baseline for comparable forthcoming measurements in A + A collisions.

Experimental studies of the collisions of heavy nuclei at relativistic energies have established the properties of the quark-gluon plasma (QGP), a state of hot, dense nuclear matter in which quarks and gluons are not bound into hadrons(1-4). In this state, matter behaves as a nearly inviscid fluid(5) that efficiently translates initial spatial anisotropies into correlated momentum anisotropies among the particles produced, creating a common velocity field pattern known as collective flow. In recent years, comparable momentum anisotropies have been measured in small-system proton-proton (p+p) and proton-nucleus (p+A) collisions, despite expectations that the volume and lifetime of the medium produced would be too small to form a QGP. Here we report on the observation of elliptic and triangular flow patterns of charged particles produced in proton-gold (p+Au), deuteron-gold (d+Au) and helium-gold (He-3+Au) collisions at a nucleon-nucleon centre-of-mass energy root s(NN) = 200 GeV. The unique combination of three distinct initial geometries and two flow patterns provides unprecedented model discrimination. Hydrodynamical models, which include the formation of a short-lived QGP droplet, provide the best simultaneous description of these measurements.

We present measurements of elliptic and triangular azimuthal anisotropy of charged particles detected at forward rapidity 1

Asymmetric nuclear collisions of p + Al, p + Au, d + Au, and He-3 + Au at root S-NN = 200 GeV provide an excellent laboratory for understanding particle production, as well as exploring interactions among these particles after their initial creation in the collision. We present measurements of charged hadron production dN(ch)/d eta in all such collision systems over a broad pseudorapidity range and as a function of collision multiplicity. A simple wounded quark model is remarkably successful at describing the full data set. We also measure the elliptic flow 12 over a similarly broad pseudorapidity range. These measurements provide key constraints on models of particle emission and their translation into flow.

The PHENIX experiment at the Relativistic Heavy Ion Collider has measured the differential cross section of phi(1020)-meson production at forward rapidity in p + p collisions at root s = 510 GeV via the dimuon decay channel. The partial cross section in the rapidity and P-T ranges 1.2

We have measured the cross section and single-spin asymmetries from forward W-+/- -> mu(+/-)nu production in longitudinally polarized p + p collisions at root s = 510 GeV using the PHENIX detector at the Relativistic Heavy Ion Collider. The cross sections are consistent with previous measurements at this collision energy, while the most forward and backward longitudinal single spin asymmetries provide new insights into the sea quark helicities in the proton. The charge of the W bosons provides a natural flavor separation of the participating partons.

We report the transverse single-spin asymmetries of J/psi production at forward and backward rapidity, 1.2

We present a detailed measurement of charged two-pion correlation functions in 0-30% centrality root= 200 GeV Au+Au collisions by the PHENIX experiment at the Relativistic Heavy Ion Collider. The data are well described by Bose-Einstein correlation functions stemming from Levy-stable source distributions. Using a fine transverse momentum binning, we extract the correlation strength parameter lambda, the Levy index of stability alpha, and the Levy length scale parameter R as a function of average transverse mass of the parr m(T). We find that the positively and the negatively charged pion pairs yield consistent results, and their correlation functions are represented, within uncertainties, by the same Levy-stable source functions. The lambda(m(T)) measurements indicate a decrease of the strength of the correlations at low m(T). The Levy length scale parameter R(m(T)) decreases with increasing m(T), following a hydrodynamically predicted type of scaling behavior. The values of the Levy index of stability a are found to be significantly lower than the Gaussian case of alpha = 2, but also significantly larger than the conjectured value that may characterize the critical point of a second-order quark-hadron phase transition.

We present measurements of the elliptic flow (v2) as a function of transverse momentum (pT), pseudorapidity (η), and centrality in d+Au collisions at sNN=200, 62.4, 39, and 19.6 GeV. The beam-energy scan of d+Au collisions provides a testing ground for the onset of flow signatures in small collision systems. We measure a nonzero v2 signal at all four collision energies, which, at midrapidity and low pT, is consistent with predictions from viscous hydrodynamic models. Comparisons with calculations from parton transport models (based on the ampt Monte Carlo generator) show good agreement with the data at midrapidity to forward (d-going) rapidities and low pT. At backward (Au-going) rapidities and pT>1.5GeV/c, the data diverges from ampt calculations of v2 relative to the initial geometry, indicating the possible dominance of nongeometry related correlations, referred to as nonflow. We also present measurements of the charged-particle multiplicity (dNch/dη) as a function of η in central d+Au collisions at the same energies. We find that in d+Au collisions at sNN=200 GeV the v2 scales with dNch/dη over all η in the PHENIX acceptance. At sNN=62.4, and 39 GeV, v2 scales with dNch/dη at midrapidity and forward rapidity, but falls off at backward rapidity. This departure from the dNch/dη scaling may be a further indication of nonflow effects dominating at backward rapidity.

The cross section of a top-quark pair produced in association with a photon is measured in proton-proton collisions at a centre-of-mass energy of s=8 TeV with 20.2 fb⁻¹ of data collected by the ATLAS detector at the Large Hadron Collider in 2012. The measurement is performed by selecting events that contain a photon with transverse momentum p_T > 15 GeV, an isolated lepton with large transverse momentum, large missing transverse momentum, and at least four jets, where at least one is identified as originating from a b-quark. The production cross section is measured in a fiducial region close to the selection requirements. It is found to be 139 ± 7 (stat.) ± 17 (syst.) fb, in good agreement with the theoretical prediction at next-to-leading order of 151 ± 24 fb. In addition, differential cross sections in the fiducial region are measured as a function of the transverse momentum and pseudorapidity of the photon.[Figure not available: see fulltext.].

The cross section and transverse single-spin asymmetries of μ- and μ+ from open heavy-flavor decays in polarized p+p collisions at s=200 GeV were measured by the PHENIX experiment during 2012 at the Relativistic Heavy Ion Collider. Because heavy-flavor production is dominated by gluon-gluon interactions at s=200 GeV, these measurements offer a unique opportunity to obtain information on the trigluon correlation functions. The measurements are performed at forward and backward rapidity (1.4

We report the first measurement of the fraction of J/ψ mesons coming from B-meson decay (FB→J/ψ) in p+p collisions at s=510 GeV. The measurement is performed using the forward silicon vertex detector and central vertex detector at PHENIX, which provide precise tracking and distance-of-closest-approach determinations, enabling the statistical separation of J/ψ due to B-meson decays from prompt J/ψ. The measured value of FB→J/ψ is 8.1%±2.3%(stat)±1.9%(syst) for J/ψ with transverse momenta 0

The PHENIX Collaboration has measured the ratio of the yields of ψ(2S) to ψ(1S) mesons produced in p+p, p+Al, p+Au, and He3+Au collisions at sNN=200 GeV over the forward and backward rapidity intervals 1.2

We report the double-helicity asymmetry, ALLJ/ψ, in inclusive J/ψ production at forward rapidity as a function of transverse momentum pT and rapidity |y|. The data analyzed were taken during s=510 GeV longitudinally polarized p+p collisions at the Relativistic Heavy Ion Collider in the 2013 run using the PHENIX detector. At this collision energy, J/ψ particles are predominantly produced through gluon-gluon scatterings, thus ALLJ/ψ is sensitive to the gluon polarization inside the proton. We measured ALLJ/ψ by detecting the decay daughter muon pairs μ+μ- within the PHENIX muon spectrometers in the rapidity range 1.2

Measurements of anisotropic flow Fourier coefficients (vn) for inclusive charged particles and identified hadrons π±, K±, p, and p produced at midrapidity in Cu+Au collisions at sNN=200 GeV are presented. The data were collected in 2012 by the PHENIX experiment at the Relativistic Heavy-Ion Collider (RHIC). The particle azimuthal distributions with respect to different-order symmetry planes Ψn, for n=1, 2, and 3 are studied as a function of transverse momentum pT over a broad range of collision centralities. Mass ordering, as expected from hydrodynamic flow, is observed for all three harmonics. The charged-particle results are compared with hydrodynamical and transport model calculations. We also compare these Cu+Au results with those in Cu+Cu and Au+Au collisions at the same sNN and find that the v2 and v3, as a function of transverse momentum, follow a common scaling with 1/(nNpart1/3).

We present midrapidity measurements from the PHENIX experiment of large parity-violating single-spin asymmetries of high transverse momentum electrons and positrons from W±/Z decays, produced in longitudinally polarized p+p collisions at center of mass energies of s=500 and 510 GeV. These asymmetries allow direct access to the antiquark polarized parton distribution functions due to the parity-violating nature of the W-boson coupling to quarks and antiquarks. The results presented are based on data collected in 2011, 2012, and 2013 with an integrated luminosity of 240 pb-1, which exceeds previous PHENIX published results by a factor of more than 27. These high Q2 data probe the parton structure of the proton at W mass scale and provide an important addition to our understanding of the antiquark parton helicity distribution functions at an intermediate Bjorken x value of roughly MW/s=0.16.

The ATLAS experiment has performed extensive searches for the electroweak production of charginos, neutralinos, and staus. This article summarizes and extends the search for electroweak supersymmetry with new analyses targeting scenarios not covered by previously published searches. New searches use vector-boson fusion production, initial-state radiation jets, and low-momentum lepton final states, as well as multivariate analysis techniques to improve the sensitivity to scenarios with small mass splittings and low-production cross sections. Results are based on 20 fb^-1 of proton-proton collision data at √s = 8 TeV recorded with the ATLAS experiment at the Large Hadron Collider. No significant excess beyond Standard Model expectations is observed. The new and existing searches are combined and interpreted in terms of 95% confidence-level exclusion limits in simplified models, where a single production process and decay mode is assumed, as well as within phenomenological supersymmetric models.

Measurements of midrapidity charged-particle multiplicity distributions, dNch/dη, and midrapidity transverse-energy distributions, dET/dη, are presented for a variety of collision systems and energies. Included are distributions for Au+Au collisions at sNN=200, 130, 62.4, 39, 27, 19.6, 14.5, and 7.7 GeV, Cu+Cu collisions at sNN=200 and 62.4 GeV, Cu+Au collisions at sNN=200 GeV, U+U collisions at sNN=193 GeV, d+Au collisions at sNN=200 GeV, He3+Au collisions at sNN=200 GeV, and p+p collisions at sNN=200 GeV. Centrality-dependent distributions at midrapidity are presented in terms of the number of nucleon participants, Npart, and the number of constituent quark participants, Nqp. For all A+A collisions down to sNN=7.7 GeV, it is observed that the midrapidity data are better described by scaling with Nqp than scaling with Npart. Also presented are estimates of the Bjorken energy density, BJ, and the ratio of dET/dη to dNch/dη, the latter of which is seen to be constant as a function of centrality for all systems.

We report the measurement of cumulants (Cn,n=1,⋯,4) of the net-charge distributions measured within pseudorapidity (|η|

We present the first measurement of elliptic (v2) and triangular (v3) flow in high-multiplicity He3+Au collisions at sNN=200GeV. Two-particle correlations, where the particles have a large separation in pseudorapidity, are compared in He3+Au and in p+p collisions and indicate that collective effects dominate the second and third Fourier components for the correlations observed in the He3+Au system. The collective behavior is quantified in terms of elliptic v2 and triangular v3 anisotropy coefficients measured with respect to their corresponding event planes. The v2 values are comparable to those previously measured in d+Au collisions at the same nucleon-nucleon center-of-mass energy. Comparisons with various theoretical predictions are made, including to models where the hot spots created by the impact of the three He3 nucleons on the Au nucleus expand hydrodynamically to generate the triangular flow. The agreement of these models with data may indicate the formation of low-viscosity quark-gluon plasma even in these small collision systems.

We present a systematic study of charged-pion and kaon interferometry in Au+Au collisions at sNN=200 GeV. The kaon mean source radii are found to be larger than pion radii in the outward and longitudinal directions for the same transverse mass; this difference increases for more central collisions. The azimuthal-angle dependence of the radii was measured with respect to the second-order event plane and similar oscillations of the source radii were found for pions and kaons. Hydrodynamic models qualitatively describe the similar oscillations of the mean source radii for pions and kaons, but they do not fully describe the transverse-mass dependence of the oscillations.

We present midrapidity charged-pion invariant cross sections, the ratio of the π^- to π⁺ cross sections and the charge-separated double-spin asymmetries in polarized p + p collisions at p√s = 200 GeV. While the cross section measurements are consistent within the errors of next-to-leading-order (NLO) perturbative quantum chromodynamics predictions (pQCD), the same calculations overestimate the ratio of the charged-pion cross sections. This discrepancy arises from the cancellation of the substantial systematic errors associated with the NLO-pQCD predictions in the ratio and highlights the constraints these data will place on flavor-dependent pion fragmentation functions. The charge-separated pion asymmetries presented here sample an x range of ~0.03-0.16 and provide unique information on the sign of the gluon-helicity distribution.

The ATLAS experiment at the LHC has measured the centrality dependence of charged-particle pseudorapidity distribution, charged-particle spectra, and two-particle correlations in p+Pb collisions at a nucleon-nucleon centre-of-mass energy of sNN=5.02 TeV. Charged particles were measured over |η|

Results of a search for the electroweak associated production of charginos and next-to-lightest neutralinos, pairs of charginos or pairs of tau sleptons are presented. These processes are characterised by final states with at least two hadronically decaying tau leptons, missing transverse momentum and low jet activity. The analysis is based on an integrated luminosity of 20.3 fb^-1 of proton-proton collisions at√s = 8 TeV recorded with the ATLAS experiment at the Large Hadron Collider. No significant excess is observed with respect to the predictions from Standard Model processes. Limits are set at 95% confidence level on the masses of the lighter chargino and next-to-lightest neutralino for various hypotheses for the lightest neutralino mass in simplified models. In the scenario of direct production of chargino pairs, with each chargino decaying into the lightest neutralino via an intermediate tau slepton, chargino masses up to 345 GeV are excluded for a massless lightest neutralino. For associated production of mass-degenerate charginos and next-to-lightest neutralinos, both decaying into the lightest neutralino via an intermediate tau slepton, masses up to 410 GeV are excluded for a massless lightest neutralino.

High-energy proton- and deuteron-nucleus collisions provide an excellent tool for studying a wide array of physics effects, including modifications of parton distribution functions in nuclei, gluon saturation, and color neutralization and hadronization in a nuclear environment, among others. All of these effects are expected to have a significant dependence on the size of the nuclear target and the impact parameter of the collision, also known as the collision centrality. In this article, we detail a method for determining centrality classes in p(d)+A collisions via cuts on the multiplicity at backward rapidity (i.e., the nucleus-going direction) and for determining systematic uncertainties in this procedure. For d+Au collisions at sNN=200 GeV we find that the connection to geometry is confirmed by measuring the fraction of events in which a neutron from the deuteron does not interact with the nucleus. As an application, we consider the nuclear modification factors Rp(d)+A, for which there is a bias in the measured centrality-dependent yields owing to auto correlations between the process of interest and the backward-rapidity multiplicity. We determine the bias-correction factors within this framework. This method is further tested using the hijing Monte Carlo generator. We find that for d+Au collisions at sNN=200 GeV, these bias corrections are small and vary by less than 5% (10%) up to pT=10 (20) GeV/c. In contrast, for p+Pb collisions at sNN=5.02 TeV we find that these bias factors are an order of magnitude larger and strongly pT dependent, likely attributable to the larger effect of multiparton interactions.

Results are presented from data recorded in 2009 by the PHENIX experiment at the Relativistic Heavy Ion Collider for the double-longitudinal spin asymmetry, ALL, for π0 and η production in s=200GeV polarized p+p collisions. Comparison of the π0 results with different theory expectations based on fits of other published data showed a preference for small positive values of gluon polarization, ΔG, in the proton in the probed Bjorken x range. The effect of adding the new 2009 π0 data to a recent global analysis of polarized scattering data is also shown, resulting in a best fit value ΔGDSSV[0.05,0.2]=0.06-0.15+0.11 in the range 0.05

The PHENIX experiment has measured open heavy-flavor production via semileptonic decay over the transverse momentum range 1

We present results for three charmonia states (ψ, χ_c, and J/ψ) in d+Au collisions at |y|

The transverse momentum (p_T) spectra and ratios of identified charged hadrons (π^±, K^±, p, p̄) produced in √sN_N=200 GeV Au+Au and d+Au collisions are reported in five different centrality classes for each collision species. The measurements of pions and protons are reported up to p_T=6 GeV/c (5 GeV/c), and the measurements of kaons are reported up to p_T=4 GeV/c (3.5 GeV/c) in Au+Au (d+Au) collisions. In the intermediate p_T region, between 2 and 5 GeV/c, a significant enhancement of baryon-to-meson ratios compared to those measured in p+p collisions is observed. This enhancement is present in both Au+Au and d+Au collisions and increases as the collisions become more central. We compare a class of peripheral Au+Au collisions with a class of central d+Au collisions which have a comparable number of participating nucleons and binary nucleon-nucleon collisions. The p_T-dependent particle ratios for these classes display a remarkable similarity, which is then discussed.

Direct photons have been measured in √sN_N=200 GeV d+Au collisions at midrapidity. A wide p_T range is covered by measurements of nearly real virtual photons (1

We present a panel discussion about our current state of knowledge about the Quark-Gluon Plasma. The nine panelists were asked to address the question: What do we want to know about the quark-gluon plasma, and how can we find the answers? The contributions illuminate our present understanding and highlight various aspects of the ongoing debate about the future directions of relativistic heavy-ion collision research.

A search for long-lived particles is performed using a data sample of 4.7 fb^-1 from proton-proton collisions at a centre-of-mass energy. √s = 7 TeV collected by the ATLAS detector at the LHC. No excess is observed above the estimated background and lower limits, at 95% confidence level, are set on the mass of the long-lived particles in different scenarios, based on their possible interactions in the inner detector, the calorimeters and the muon spectrometer. Long-lived staus in gauge-mediated SUSY-breaking models are excluded up to a mass of 300 GeV for tan β = 5-20. Directly produced long-lived sleptons are excluded up to a mass of 278 GeV. R-hadrons, composites of gluino (stop, sbottom) and light quarks, are excluded up to a mass of 985 GeV (683 GeV, 612 GeV) when using a generic interaction model. Additionally two sets of limits on R-hadrons are obtained that are less sensitive to the interaction model for R-hadrons. One set of limits is obtained using only the inner detector and calorimeter observables, and a second set of limits is obtained based on the inner detector alone. (c) 2013 CERN. Published by Elsevier B.V. All rights reserved.

We present measured J/ψ production rates in d+Au collisions at √s_NN=200 GeV over broad ranges of transverse momentum (p_T=0-14 GeV/c) and rapidity (-2.2

The PHENIX experiment has measured electrons and positrons at midrapidity from the decays of hadrons containing charm and bottom quarks produced in d+Au and p+p collisions at √s_NN=200GeV in the transverse-momentum range 0.85≤pTe≤8.5GeV/c. In central d+Au collisions, the nuclear modification factor R_dA at 1.5

We report the measurement of direct photons at midrapidity in Au+Au collisions at √s _NN=200GeV. The direct photon signal was extracted for the transverse momentum range of 4GeV/c

The differential cross section for the production of direct photons in p+p collisions at √s=200GeV at midrapidity was measured in the PHENIX detector at the Relativistic Heavy Ion Collider. Inclusive direct photons were measured in the transverse momentum range from 5.5-25GeV/c, extending the range beyond previous measurements. Event structure was studied with an isolation criterion. Next-to-leading-order perturbative-quantum-chromodynamics calculations give a good description of the spectrum. When the cross section is expressed versus x _T, the PHENIX data are seen to be in agreement with measurements from other experiments at different center-of-mass energies.

Background: Heavy-flavor production in p + p collisions is a good test of perturbative-quantum-chromodynamics (pQCD) calculations. Modification of heavy-flavor production in heavy-ion collisions relative to binary-collision scaling from p + p results, quantified with the nuclear-modification factor (R_AA), provides information on both cold- and hot-nuclear-matter effects. Midrapidity heavy-flavor R_AA measurements at the Relativistic Heavy Ion Collider have challenged parton-energy-loss models and resulted in upper limits on the viscosity-entropy ratio that are near the quantum lower bound. Such measurements have not been made in the forward-rapidity region. Purpose: Determine transverse-momentum (p_T) spectra and the corresponding R_AA for muons from heavy-flavor meson decay in p + p and Cu + Cu collisions at √s_NN=200 GeV and y=1.65. Method: Results are obtained using the semileptonic decay of heavy-flavor mesons into negative muons. The PHENIX muon-arm spectrometers measure the p_T spectra of inclusive muon candidates. Backgrounds, primarily due to light hadrons, are determined with a Monte Carlo calculation using a set of input hadron distributions tuned to match measured-hadron distributions in the same detector and statistically subtracted. Results: The charm-production cross section in p + p collisions at √s=200 GeV, integrated over p_T and in the rapidity range 1.4

The PHENIX experiment was upgraded with the Hadron Blind Detector (HBD) for the measurement of low-mass electron pairs. The HBD reduces the combinatorial background by providing additional electron identification in a field-free region close to the vertex and by exploiting the small opening angle of the pi(0) Dalitz and conversion pairs. The HBD was successfully operated during Run 9 and Run 10 at the RHIC in the measurements of e(+)e(-) pairs in p+p and Au+Au collisions, respectively. This paper reports on the present status of the dielectron analysis in Au+Au collisions at root s(NN) = 200 GeV.

Heavy quarkonia are observed to be suppressed in relativistic heavy-ion collisions relative to their production in p+p collisions scaled by the number of binary collisions. In order to determine if this suppression is related to color screening of these states in the produced medium, one needs to account for other nuclear modifications including those in cold nuclear matter. In this paper, we present new measurements from the PHENIX 2007 data set of J/ψ yields at forward rapidity (1.2

A search for long-lived charged particles reaching the muon spectrometer is performed using a data sample of 37 pb^-1 from pp collisions at √s = 7 TeV collected by the ATLAS detector at the LHC in 2010. No excess is observed above the estimated background. Stable τ over bar sleptons are excluded at 95% CL up to a mass of 136 GeV, in GMSB models with N-5 = 3 ,m_messenger = 250 TeV, sign(μ) = 1 and tan β = 5. Electroweak production of sleptons is excluded up to a mass of 110 GeV. Gluino R-hadrons in a generic interaction model are excluded up to masses of 530 GeV to 544 GeV depending on the fraction of R-hadrons produced as (g) over bar -balls. 

A Hadron Blind Detector (HBD) has been developed, constructed and successfully operated within the PHENIX detector at RHIC. The HBD is a Cherenkov detector operated with pure ^CF4. It has a 50 cm long radiator directly coupled in a windowless configuration to a readout element consisting of a triple GEM stack, with a CsI photocathode evaporated on the top surface of the top GEM and pad readout at the bottom of the stack. This paper gives a comprehensive account of the construction, operation and in-beam performance of the detector.

We report on the event structure and double helicity asymmetry (A _LL) of jet production in longitudinally polarized p+p collisions at √s=200GeV. Photons and charged particles were measured by the PHENIX experiment at midrapidity |η|2GeV/c) photon in the event. Event structure, such as multiplicity, p_T density and thrust in the PHENIX acceptance, were measured and compared with the results from the pythia event generator and the geant detector simulation. The shape of jets and the underlying event were well reproduced at this collision energy. For the measurement of jet A_LL, photons and charged particles were clustered with a seed-cone algorithm to obtain the cluster p_T sum (pTreco). The effect of detector response and the underlying events on pTreco was evaluated with the simulation. The production rate of reconstructed jets is satisfactorily reproduced with the next-to-leading-order and perturbative quantum chromodynamics jet production cross section. For 4

Transverse momentum distributions and yields for π^±, K^±, p, and p in p+p collisions at √s = 200 and 62.4 GeV at midrapidity are measured by the PHENIX experiment at the Relativistic Heavy Ion Collider (RHIC). These data provide important baseline spectra for comparisons with identified particle spectra in heavy ion collisions at RHIC. We present the inverse slope parameter T_inv, mean transverse momentum p_T, and yield per unit rapidity dN/dy at each energy, and compare them to other measurements at different √s in p+p and p+p collisions. We also present the scaling properties such as m_T scaling and x _T scaling on the p_T spectra between different energies. To discuss the mechanism of the particle production in p+p collisions, the measured spectra are compared to next-to-leading-order or next-to-leading- logarithmic perturbative quantum chromodynamics calculations.

The Hadron Blind Detector (HBD) was developed, installed and successfully operated in the PHENIX detector during RHIC Runs in 2009 (p+p) and 2010 (Au+Au). The HBD is a windowless Cherenkov detector, operated with CF4 gas in proximity focus configuration. It uses triple GEM elements for signal amplification with CsI photocathode evaporated on top of the first GEM. The purpose of the HBD is to reduce the combinatorial background from the dielectron invariant mass spectrum by recognizing and rejecting e(+) or e(-) originating from pi(0) Dalitz decays and conversion. This article reports on the in-beam performance of the HBD.

This proceeding gives the most complete review of the measurements of the ø-meson production in relativistic heavy ion and proton-proton collisions performed by the PHENIX experiment at the Relativistic Heavy Ion Collider at Brookhaven National Laboratory. The measurements of the f-meson are consistent in the analysis of various decay modes, using different techniques. The results show expected similarities when analyzed in Au + Au and Cu + Cu collision systems for the corresponding centrality classes. In other systems PHENIX observes not only the difference between the suppression of f-meson and the proton, reflecting generally different behavior between mesons and baryons, but also a significant difference in suppression of different mesons. These results are hard to explain due to the mass or quark content of the f-meson. PACS 21.65.Jk-Mesons in nuclear matter. PACS 25.75.Dw-Particle and resonance production.

Large parity-violating longitudinal single-spin asymmetries ALe ⁺=-0.86-0.14+0.30 and ALe^-=0.88-0.71+0.12 are observed for inclusive high transverse momentum electrons and positrons in polarized p+p collisions at a center-of-mass energy of √s=500GeV with the PHENIX detector at RHIC. These e^± come mainly from the decay of W^± and Z0 bosons, and their asymmetries directly demonstrate parity violation in the couplings of the W^± to the light quarks. The observed electron and positron yields were used to estimate W ^± boson production cross sections for the e^± channels of σ(pp→W⁺X)×BR(W⁺→e ⁺ν_e)=144.1±21.2(stat)-10.3+3.4(syst)±21. 6(norm)pb, and σ(pp→W^-X)×BR(W ^-→e^-ν_̄e)=31.7±12.1(stat)-8. 2+10.1(syst)±4.8(norm)pb.

We have measured the azimuthal anisotropy of π0 production for 1

New measurements by the PHENIX experiment at the Relativistic Heavy Ion Collider for η production at midrapidity as a function of transverse momentum (pT) and collision centrality in √sNN=200 GeV Au+Au and p+p collisions are presented. They indicate nuclear modification factors (RAA) which are similar in both magnitude and trend to those found in earlier π0 measurements. Linear fits to RAA as a function of pT in 5-20 GeV/c show that the slope is consistent with zero within two standard deviations at all centralities, although a slow rise cannot be excluded. Having different statistical and systematic uncertainties, the π0 and η measurements are complementary at high pT; thus, along with the extended pT range of these data they can provide additional constraints for theoretical modeling and the extraction of transport properties.

We perform a systematic comparison of the statistical model parametrization of hadron abundances observed in high-energy pp, AA and e⁺e^- collisions. The basic aim of the study is to test if the quality of the description depends on the nature of the collision process. In particular, we want to see if nuclear collisions, with multiple initial interactions, lead to "more thermal" average multiplicities than elementary pp collisions or e⁺e^- annihilation. Such a comparison is meaningful only if it is based on data for the same or similar hadronic species and if the analyzed data has quantitatively similar errors. When these requirements are maintained, the quality of the statistical model description is found to be the same for the different initial collision configurations.

PHENIX has measured the e⁺e^- pair continuum in √sNN=200□ GeV Au+Au and p+p collisions over a wide range of mass and transverse momenta. The e⁺e^- yield is compared to the expectations from hadronic sources, based on PHENIX measurements. In the intermediate-mass region, between the masses of the φ and the J/ψ meson, the yield is consistent with expectations from correlated cc□ production, although other mechanisms are not ruled out. In the low-mass region, below the φ, the p+p inclusive mass spectrum is well described by known contributions from light meson decays. In contrast, the Au+Au minimum bias inclusive mass spectrum in this region shows an enhancement by a factor of 4.7±0. 4stat±1.5syst±0.9model. At low mass (mee

The PHENIX experiment at RHIC has measured the phi-meson production at mid-rapidity in p+p, d+Au, Cu+Cu and Au+Au collisions at root s(NN) = 200 GeV via the K+K- decay mode. The transverse momentum spectra of the phi-meson and the nuclear modification factor as a function of centrality are reviewed here.

Bose-Einstein correlations of charged kaons are used to probe Au+Au collisions at sNN=200GeV and are compared to charged pion probes, which have a larger hadronic scattering cross section. Three-dimensional Gaussian source radii are extracted, along with a one-dimensional kaon emission source function. The centrality dependences of the three Gaussian radii are well described by a single linear function of Npart1/3 with a zero intercept. Imaging analysis shows a deviation from a Gaussian tail at r10fm, although the bulk emission at lower radius is well described by a Gaussian. The presence of a non-Gaussian tail in the kaon source reaffirms that the particle emission region in a heavy-ion collision is extended, and that similar measurements with pions are not solely due to the decay of long-lived resonances.

We present inclusive charged hadron elliptic flow (v2) measured over the pseudorapidity range |η|6.5) pseudorapidities and (2) the two-particle cumulant method extracted using correlations between particles detected at midrapidity. The two event-plane results are consistent within systematic uncertainties over the measured pT and in centrality 0-40%. There is at most a 20% difference in the v2 between the two event-plane methods in peripheral (40-60%) collisions. The comparisons between the two-particle cumulant results and the standard event-plane measurements are discussed.

We report the observation at the Relativistic Heavy Ion Collider of suppression of back-to-back correlations in the direct photon+jet channel in Au+Au relative to p+p collisions. Two-particle correlations of direct photon triggers with associated hadrons are obtained by statistical subtraction of the decay photon-hadron (γ-h) background. The initial momentum of the away-side parton is tightly constrained, because the parton-photon pair exactly balance in momentum at leading order in perturbative quantum chromodynamics, making such correlations a powerful probe of the in-medium parton energy loss. The away-side nuclear suppression factor, IAA, in central Au+Au collisions, is 0.32±0.12stat±0.09syst for hadrons of 3

The PHENIX experiment at RHIC has measured a variety of light neutral mesons (pi (0), K (S) (0) , eta, omega, eta ('), phi) via multi-particle decay channels over a wide range of transverse momentum. A review of the recent results on the production rates of light mesons in p + p and their nuclear modification factors in d + Au, Cu + Cu and Au + Au collisions at different energies is presented.

A comprehensive survey of event-by-event fluctuations of charged hadron multiplicity in relativistic heavy ions is presented. The survey covers Au+Au collisions at sNN=62.4 and 200 GeV, and Cu+Cu collisions at sNN=22.5,62.4, and 200 GeV. Fluctuations are measured as a function of collision centrality, transverse momentum range, and charge sign. After correcting for nondynamical fluctuations due to fluctuations in the collision geometry within a centrality bin, the remaining dynamical fluctuations expressed as the variance normalized by the mean tend to decrease with increasing centrality. The dynamical fluctuations are consistent with or below the expectation from a superposition of participant nucleon-nucleon collisions based upon p+p data, indicating that this dataset does not exhibit evidence of critical behavior in terms of the compressibility of the system. A comparison of the data with a model where hadrons are independently emitted from a number of hadron clusters suggests that the mean number of hadrons per cluster is small in heavy ion collisions.

Measurements in Au+Au collisions at sNN=200GeV of jet correlations for a trigger hadron at intermediate transverse momentum (pT,trig) with associated mesons or baryons at lower pT,assoc indicate strong modification of the away-side jet. The ratio of jet-associated baryons to mesons increases with centrality and pT,assoc. For the most central collisions, the ratio is similar to that for inclusive measurements. This trend is incompatible with in-vacuum fragmentation but could be due to jetlike contributions from correlated soft partons, which recombine upon hadronization.

The PHENIX experiment has measured the suppression of semi-inclusive single high-transverse-momentum π0's in Au+Au collisions at sNN=200 GeV. The present understanding of this suppression is in terms of energy loss of the parent (fragmenting) parton in a dense color-charge medium. We have performed a quantitative comparison between various parton energy-loss models and our experimental data. The statistical point-to-point uncorrelated as well as correlated systematic uncertainties are taken into account in the comparison. We detail this methodology and the resulting constraint on the model parameters, such as the initial color-charge density dNg/dy, the medium transport coefficient q, or the initial energy-loss parameter ε0. We find that high-transverse-momentum π0 suppression in Au+Au collisions has sufficient precision to constrain these model-dependent parameters at the ±20-25% (one standard deviation) level. These constraints include only the experimental uncertainties, and further studies are needed to compute the corresponding theoretical uncertainties.

Azimuthal angle (Δ) correlations are presented for charged hadrons from dijets for 0.4

Longitudinal density correlations of produced matter in Au+Au collisions at sNN=200 GeV have been measured from the inclusive charged particle distributions as a function of pseudorapidity window sizes. The extracted αξ parameter, related to the susceptibility of the density fluctuations in the long-wavelength limit, exhibits a nonmonotonic behavior as a function of the number of participant nucleons, Npart. A local maximum is seen at Npart~90, with corresponding energy density based on the Bjorken picture of εBjτ~2.4 GeV/(fm2c) with a transverse area size of 60 fm2. This behavior may suggest a critical phase boundary based on the Ginzburg-Landau framework.

The PHENIX experiment at the BNL Relativistic Heavy Ion Collider (RHIC) has measured J/ψ production for rapidities -2.2

We present azimuthal angle correlations of intermediate transverse momentum (1-4GeV/c) hadrons from dijets in Cu+Cu and Au+Au collisions at sNN=62.4 and 200 GeV. The away-side dijet induced azimuthal correlation is broadened, non-Gaussian, and peaked away from Δ =π in central and semicentral collisions in all the systems. The broadening and peak location are found to depend upon the number of participants in the collision, but not on the collision energy or beam nuclei. These results are consistent with sound or shock wave models, but pose challenges to Cherenkov gluon radiation models.

The PHENIX experiment at the BNL Relativistic Heavy Ion Collider (RHIC) has measured electrons with 0.3

The production of the low-mass dielectrons is considered to be a powerful tool to study the properties of the hot and dense matter created in the ultra-relativistic heavy-ion collisions. We present the preliminary results on the first measurements of the low-mass dielectron continuum in Au + Au collisions and the phi-meson production measured in Au + Au and d + Au collisions at root s(NN) = 200 GeV performed by the PHENIX experiment.

Inclusive transverse momentum spectra of η mesons in the range pTf2-12 GeV/c have been measured at midrapidity (|η|

Azimuthal correlations of jet-induced high-pT charged hadron pairs are studied at midrapidity in Au+Au collisions at sNN=200GeV. The distribution of jet-associated partner hadrons (1.0

The properties of jets produced in p+p collisions at s=200GeV are measured using the method of two-particle correlations. The trigger particle is a leading particle from a large transverse momentum jet while the associated particle comes from either the same jet or the away-side jet. Analysis of the angular width of the near-side peak in the correlation function determines the jet-fragmentation transverse momentum jT. The extracted value, jT2 =585±6(stat)±15(sys)MeV/c, is constant with respect to the trigger particle transverse momentum, and comparable to the previous lower s measurements. The width of the away-side peak is shown to be a convolution of jT with the fragmentation variable, z, and the partonic transverse momentum, kT. The z is determined through a combined analysis of the measured π0 inclusive and associated spectra using jet-fragmentation functions measured in e+e- collisions. The final extracted values of kT are then determined to also be independent of the trigger particle transverse momentum, over the range measured, with value of kT2 =2.68±0.07(stat)±0.34(sys)GeV/c.

The azimuthal distribution of identified π0 and inclusive photons has been measured in sNN=200GeV Au+Au collisions with the PHENIX experiment at the Relativistic Heavy-Ion Collider (RHIC). The second-harmonic parameter (v2) was measured to describe the observed anisotropy of the azimuthal distribution. The measured inclusive photon v2 is consistent with the value expected for the photons from hadron decay and is also consistent with the lack of direct photon signal over the measured pT range 1-6GeV/c. An attempt is made to extract v2 of direct photons.

J/ψ production in d+Au and p+p collisions at sNN=200GeV has been measured by the PHENIX experiment at rapidities -2.2

We present an improved measurement of the double helicity asymmetry for π0 production in polarized proton-proton scattering at s=200GeV employing the PHENIX detector at the Relativistic Heavy Ion Collider (RHIC). The improvements to our previous measurement come from two main factors: Inclusion of a new data set from the 2004 RHIC run with higher beam polarizations than the earlier run and a recalibration of the beam polarization measurements for the earlier run, which resulted in reduced uncertainties and increased beam polarizations. The results are compared to a Next to Leading Order (NLO) perturbative Quantum Chromodynamics (pQCD) calculation with a range of polarized gluon distributions.

Dihadron correlations at high transverse momentum pT in d+Au collisions at sNN=200 GeV at midrapidity are measured by the PHENIX experiment at the Relativistic Heavy Ion Collider. From these correlations, we extract several structural characteristics of jets: the root-mean-squared transverse momentum of fragmenting hadrons with respect to the jet jT2, the mean sine-squared of the azimuthal angle between the jet axes sin2 jj, and the number of particles produced within the dijet that are associated with a high-pT particle (dN/dxE distributions). We observe that the fragmentation characteristics of jets in d+Au collisions are very similar to those in p+p collisions and that there is little dependence on the centrality of the d+Au collision. This is consistent with the nuclear medium having little influence on the fragmentation process. Furthermore, there is no statistically significant increase in the value of sin2 jj from p+p to d+Au collisions. This constrains the effect of multiple scattering that partons undergo in the cold nuclear medium before and after a hard collision.

The PHENIX experiment at RHIC has observed a large enhancement of baryon and anti-baryon production at p(T) approximate to 2-5 GeV/c, compared to expectations from jet fragmentation. While a number of theoretical interpretations of the data are available, there is not yet a definitive answer to the "baryon puzzle". We investigate the centrality dependence of phi-meson production at mid-rapidity in Au+Au collisions with root s(NN) = 200 GeV. Comparison with the proton and anti-proton spectra reveal similar shapes, as expected for soft production described by hydrodynamics. However, the absolute yields show a different centrality dependence. The nuclear modi. cation factors for phi are similar to those of pions, rather than (anti) protons that have similar mass. At intermediate p(T), baryon/meson effects seem to be more important than the mass effects, in support of recombination models.

The PHENIX experiment at RHIC measured single electron spectra in p+p, d+Au and Au+Au collisions at root s(NN) = 200 GeV, and in Au+Au collisions at root s(NN) = 62.4 GeV. In these spectra, electrons from semi-leptonic decays of charmed particles are the dominant contribution after subtraction of all 'photonic' sources ( photon conversions, Dalitz decays, decays of light vector mesons). The p+p open charm production cross-section is found to be in good agreement with pQCD NLO calculations. The shape of the distributions obtained for p+p interactions is compared with those observed for nucleus-nucleus collisions. From p+p to d+Au and Au+Au interactions, open charm production is found to scale with the number of binary collisions N-coll. Au+Au data at root s(NN) = 62.4 GeV is compatible with the ISR p+p results scaled by Ncoll. The elliptic flow parameter v(2) of heavy flavor electrons has also been measured, and is found to be non-zero in the intermediate p(T) range.

The transverse momentum dependence of the azimuthal anisotropy parameter v2, the second harmonic of the azimuthal distribution, for electrons at midrapidity (|η|

The PHENIX experiment measured J/psi production in pp, d+Au and Au+Au reactions at root s(NN) = 200 GeV over a wide range of rapidity and transverse momentum. The nuclear modi. cation factor obtained by comparing the d+Au and pp cross sections as a function of rapidity, is consistent with shadowing of the gluon distribution functions. J/psi production in Au+Au collisions was compared to the production in pp collisions and it was found to be inconsistent with models that predict strong enhancement relative to binary collision scaling.

A Hadron Blind Detector (HBD) is being developed for the PHENIX experiment at RHIC. It consists of a Cherenkov radiator operated with pure CF4 directly coupled in a windowless configuration to a triple-GEM detector element with a CsI photocathode and pad readout. The HBD operates in the bandwidth 6-11.5 eV (110-200 nm). We studied the detector response to minimum ionizing particles and to electrons. We present measurements of the CsI quantum efficiency, which are in very good agreement with previously published results over the bandwidth 6-8.3 eV and extend them up to 10.3 eV. Discharge probability and aging studies of the GEMs and the CsI photocathode in pure CF4 are presented.

The first measurement of direct photons in Au+Au collisions at root s(NN)=200 GeV is presented. The direct photon signal is extracted as a function of the Au+Au collision centrality and compared to next-to-leading order perturbative quantum chromodynamics calculations. The direct photon yield is shown to scale with the number of nucleon-nucleon collisions for all centralities.

Two particle correlations between identified meson and baryon trigger particles with 2.5

The production of deuterons and antideuterons in the transverse momentum range 1.1

The PHENIX experiment has measured midrapidity transverse momentum spectra (0.4

The use of PHENIX experiment at midrapidity in Au + Au collision at √s_NN = 200 GeV for calculating Bose-Einstein correlation of identical charged ion pairs was discussed. It was found that the Bertsch-Partt radius parameters were determined as a function of the transverse momentum of the pair and as function of the collision. It was also observed that the value of R_out/R_side, as function of k_T decrease from ∼1.1 to ∼ 0.8 over the range of k_T. The results show that the measurements of the transverse momentum dependence of the HBT radii.

The centrality dependence of transverse momentum distributions and yields for π^±,K^±, p, and p in Au +Au collisions at √s_NN=200 GeV at midrapidity are measured by the PHENIX experiment at the Relativistic Heavy Ion Collider. We observe a clear particle mass dependence of the shapes of transverse momentum spectra in central collisions below ∼2 GeV/c in p_T. Both mean transverse momenta and particle yields per participant pair increase from peripheral to midcentral and saturate at the most central collisions for all particle species. We also measure particle ratios of π^-/π⁺, K ^-/K⁺, ̄p/p, K/π, p/π, and ̄p/π as a function of p_T and collision centrality. The ratios of equal mass particle yields are independent of p_T and centrality within the experimental uncertainties. In central collisions at intermediate transverse momenta ∼1.5-4.5 GeV/c, proton and antiproton yields constitute a significant fraction of the charged hadron production and show a scaling behavior different from that of pions.

The production of Jψ, measured in proton-proton collisions at √s=200 GeV is discussed. Distributions of the transverse momentum and rapidity, along with the measurements of the mean transverse momentum and total production cross section are presented. It is found that the total J/ψ cross section is 4.0±0.6(stat)±0.6(syst)±0.4(abs) μb. Results show that the mean transverse momentum is 1.80±0.23(stat)±0.16(syst) GeV/c.

Transverse momentum spectra and yields of hadrons are measured by the PHENIX collaboration in Au +Au collisions at √S_NN=130 GeV at the Relativistic Heavy Ion Collider. The time-of-flight resolution allows identification of pions to transverse momenta of 2 GeV/c and protons and antiprotons to 4 GeV/c. The yield of pions rises approximately linearly with the number of nucleons participating in the collision, while the number of kaons, protons, and antiprotons increases more rapidly. The shape of the momentum distribution changes between peripheral and central collisions. Simultaneous analysis of all the p_T spectra indicates radial collective expansion, consistent with predictions of hydrodynamic models. Hydrodynamic analysis of the spectra shows that the expansion velocity increases with collision centrality and collision energy. This expansion boosts the particle momenta, causing the yield from soft processes to exceed that for hard to large transverse momentum, perhaps as large as 3 GeV/c.

A Hadron Blind Detector (HBD) is being developed for an upgrade of the PHENIX experiment at RHIC. The HBD is a windowless Cherenkov detector, operated with pure CF ₄ in a special proximity focus configuration. The detector consists of a 50cm long radiator, directly coupled to a triple GEM detector which has a Csl photocathode evaporated on the top surface of the upper-most GEM foil, and a pad readout at the bottom of the GEM stack. Detailed studies of the detector performance, including hadron rejection, figure of merit, N ₀, number of photoelectrons and efficiency are presented. These studies include measurements performed with a UV lamp, an ⁵⁵Fe x-ray source and an ²⁴¹Am alpha source. Results will also be given on aging studies of the GEM foils and the CsI photocathode in pure CF ₄.

The invariant differential cross section for inclusive neutral-pion production in p+p collisions at √s=200 GeV was measured at midrapidity (|η|

We report on the yield of protons and antiprotons, as a function of centrality and transverse momentum, in [Formula presented] collisions at [Formula presented] measured at midrapidity by the PHENIX experiment at the BNL Relativistic Heavy Ion Collider. In central collisions at intermediate transverse momenta ([Formula presented]) a significant fraction of all produced particles are protons and antiprotons. They show a centrality-scaling behavior different from that of pions. The [Formula presented] and [Formula presented] ratios are enhanced compared to peripheral [Formula presented], [Formula presented], and [Formula presented] collisions. This enhancement is limited to [Formula presented] as deduced from the ratio of charged hadrons to [Formula presented] measured in the range [Formula presented].

The measurement of transverse momentum spectra of the neutral pions in Au + Au collisions was discussed. It was found that the π⁰ multiplicity of the central reactions was significantly below the yields measured in p + p and Au + Au reactions. It was also found that this deficit increased smoothly with the centrality and was present in the semiperipheral reactions.

Transverse momentum spectra of neutral pions in the range [Formula presented] have been measured at midrapidity by the PHENIX experiment at BNL RHIC in [Formula presented] collisions at [Formula presented]. The [Formula presented] multiplicity in central reactions is significantly below the yields measured at the same [Formula presented] in peripheral [Formula presented] and [Formula presented] reactions scaled by the number of nucleon-nucleon collisions. For the most central bin, the suppression factor is [Formula presented] at [Formula presented] and increases to [Formula presented] at [Formula presented]. At larger [Formula presented], the suppression remains constant within errors. The deficit is already apparent in semiperipheral reactions and increases smoothly with centrality.

PHENIX has measured the centrality dependence of charged hadron p_T spectra from Au + Au collisions at s_NN√ = 130 GeV. The truncated mean p_T decreases with centrality for p_T > 2 GeV/c, indicating an apparent reduction of the contribution from hard scattering to high p_T hadron production. For central collisions the yield at high p_T is shown to be suppressed compared to binary nucleon-nucleon collision scaling of p + p data. This suppression is monotonically increasing with centrality, but most of the change occurs below 30% centrality, i.e., for collisions with less than ∼ 140 participating nucleons. The observed p_T and centrality dependence is consistent with the particle production predicted by models including hard scattering and subsequent energy loss of the scattered partons in the dense matter created in the collisions.

Keywords: CENTRALITY DEPENDENCE; TRANSVERSE ENERGY; MULTIPLICITY; PARTICLE; QCD

The PHENIX detector is designed to perform a broad study of A-A, p-A, and p-p collisions to investigate nuclear matter under extreme conditions. A wide variety of probes, sensitive to all timescales, are used to study systematic variations with species and energy as well as to measure the spin structure of the nucleon. Designing for the needs of the heavy-ion and polarized-proton programs has produced a detector with unparalleled capabilities. PHENIX measures electron and muon pairs, photons, and hadrons with excellent energy and momentum resolution. The detector consists of a large number of subsystems that are discussed in other papers in this volume. The overall design parameters of the detector are presented.

Two-particle azimuthal correlation functions are presented for charged hadrons produced in [Formula presented] collisions at the Relativistic Heavy Ion Collider ([Formula presented]). The measurements permit determination of elliptic flow without event-by-event estimation of the reaction plane. The extracted elliptic flow values ([Formula presented]) show significant sensitivity to both the collision centrality and the transverse momenta of emitted hadrons, suggesting rapid thermalization and relatively strong velocity fields. When scaled by the eccentricity of the collision zone [Formula presented], the scaled elliptic flow shows little or no dependence on centrality for charged hadrons with relatively low [Formula presented]. A breakdown of this [Formula presented] scaling is observed for charged hadrons with [Formula presented].

Distributions of event-by-event fluctuations of the mean transverse momentum and mean transverse energy near mid-rapidity have been measured in Au+Au collisions at √S_NN= 130 GeV at the Relativistic Heavy-Ion Collider. By comparing the distributions to what is expected for statistically independent particle emission, the magnitude of nonstatistical fluctuations in mean transverse momentum is determined to be consistent with zero. Also, no significant nonrandom fluctuations in mean transverse energy are observed. By constructing a fluctuation model with two event classes that preserve the mean and variance of the semi-inclusive P_T or e_T spectra, we exclude a region of fluctuations in √S_NN= 130 GeV Au+Au collisions.

The results from an analysis of net charge fluctuations for particles produced in Au+Au interactions at √^sNN=130 GeV. The fluctuations are studied in the variables R=n₊/n_-, the ratio between positive and negative particles, and Q=n₊-n_-, the net charge. This paper discusses the advantages and disadvantages of these variables.

During the 1999 lead run, CERES has measured hadron and electron-pair production at 40 A GeV/c beam momentum with the spectrometer upgraded by the addition of a radial TPC. Here the analysis of Λ and Λ̄ will be presented.

An intriguing new behavior was reported in elementary hadron production at relativistic heavy ion collider (RHIC). In central Au-Au collisions the antiprotons yield was comparable to the π^- at high transverse momentum. The results showed that the transverse energy density and particle multiplicities were higher than the previously observed multiplicities in the relativistic heavy-ion collisions.

Two-pion correlations in [Formula presented] [Formula presented] collisions at RHIC have been measured over a broad range of pair transverse momentum [Formula presented] by the PHENIX experiment at RHIC. The [Formula presented] dependent transverse radii are similar to results from heavy-ion collisions at [Formula presented], 4.9, and 17.3 GeV, whereas the longitudinal radius increases monotonically with beam energy. The ratio of the outwards to sidewards transverse radii [Formula presented] is consistent with unity and independent of [Formula presented].

We present results for the charged-particle multiplicity distribution at midrapidity in Au - Au collisions at √s_NN = 130 GeV measured with the PHENIX detector at RHIC. For the 5% most central collisions we find dN_ch/dη|η=0 = 622±1(stat)±41(syst). The results, analyzed as a function of centrality, show a steady rise of the particle density per participating nucleon with centrality.

Transverse momentum spectra for charged hadrons and for neutral pions in the range 1GeV/c

The first measurement of energy produced transverse to the beam direction at the Relativistic Heavy-Ion Collider at Brookhaven National Laboratory is presented. The midrapidity transverse energy density per participating nucleon rises steadily with the number of participants, closely paralleling the rise in charged-particle density, such that ⟨E_T⟩/⟨N_ch⟩ remains relatively constant as a function of centrality. The energy density calculated via Bjorkens prescription for the 2% most central Au + Au collisions at √sNN = 130GeV is at least ε_Bj = 4.6GeV/fm³, which is a factor of 1.6 larger than found at √sNN = 17.2GeV(Pb + Pb at CERN).