Publications

Scalar ultralight dark matter (ULDM) interacting with neutrinos can induce, under certain conditions, time dependent modifications to neutrino oscillation probabilities. The limit in which the ULDM perturbation can be treated as constant throughout the neutrino propagation time has been addressed by several previous works. We complement these by systematically analyzing the opposite limit - accounting for the temporal variations of the ULDM field by solving time dependent Schrödinger equations. In particular, we study a novel two-generations-like CP violating (CPV) signature unique to rapidly oscillating ULDM. We derive the leading order, time dependent corrections to the oscillation probabilities, for both CP conserving and CPV couplings, and explain how they can be measured in current and future experiments.

If Ultra-light dark matter (ULDM) exists and couples to neutrinos, the neutrino oscillation probability might be significantly altered by a parametric resonance. This resonance can occur if the typical frequency of neutrino flavor-oscillations ∆m2/(2E), where ∆m2 is the mass-squared difference of the neutrinos and E is the neutrino energy, matches the oscillation frequency of the ULDM field, determined by its mass, mϕ. The resonance could lead to observable effects even if the ULDM coupling is very small, and even if its typical oscillation period, given by τϕ = 2π/mϕ, is much shorter than the experimental temporal resolution. Defining a small parameter ϵϕ to be the ratio between the contribution of the ULDM field to the neutrino mass and the vacuum value of the neutrino mass, the impact of the resonance is particularly significant if ϵϕmϕL ≳ 4, where L is the distance between the neutrino source and the detector. An outlier in the data collected by the KamLAND experiment which, until now, has been assumed to constitute a statistical fluctuation, or associated with the binning, can actually be explained by such narrow parametric resonance, without affecting the measurements of other current neutrino oscillation experiments. This scenario will be tested by the JUNO experiment.

The LHCb experiment measured the time-dependent CP asymmetries CKK and SKK in Bs → K+K− decay. Combining with the corresponding CP asymmetries Cππ and Sππ in B → π+π− decay, we find that the size of U-spin breaking in this system is of order 20%. Moreover, the data suggest that these effects are dominated by factorizable contributions. We further study the constraints on new physics contributions to b → uu¯¯¯q (q = s, d). New physics that is minimally flavor violating (MFV) cannot be distinguished from the Standard Model (SM) in these decays. However, new physics that is not MFV can mimic large U-spin breaking. Requiring that the U-spin breaking parameters remain below the size implied by the data leads to a lower bound of 5 − 10 TeV on the scale of generic new physics. If the new physics is subject to the selection rules that follow from the Froggatt-Nielsen (FN) mechanism or from General Minimal Flavor Violation (GMFV), the bound is relaxed to 2 TeV.

The upper bounds from the ATLAS and CMS experiments on the decay rate of the Higgs boson to two muons provide the strongest constraint on an imaginary part of the muon Yukawa coupling. This bound is more than an order of magnitude stronger than bounds from CP-violating observables, specifically the electric dipole moment of the electron. It excludes a scenario-which had been viable prior to these measurements-that a complex muon Yukawa coupling is the dominant source of the baryon asymmetry. Even with this bound, the muon source can still contribute O(16%) of the asymmetry.

The Higgs program is relevant to many of the open fundamental questions in particle physics and cosmology. Thus, when discussing future collider experiments, one way of comparing them is by assessing their potential contributions to progress on these questions. We discuss in detail the capabilities of various proposed experiments in searching for singlet scalars, which are relevant to several of the open questions, and in measuring Higgs decays into fermion pairs, which are relevant to the flavor puzzles. With regard to other interesting questions, we list the most relevant observables within the Higgs program.

The Review summarizes much of particle physics and cosmology. Using data from previous editions, plus 2,873 new measurements from 758 papers, we list, evaluate, and average measured properties of gauge bosons and the recently discovered Higgs boson, leptons, quarks, mesons, and baryons. We summarize searches for hypothetical particles such as supersymmetric particles, heavy bosons, axions, dark photons, etc. Particle properties and search limits are listed in Summary Tables. We give numerous tables, figures, formulae, and reviews of topics such as Higgs Boson Physics, Supersymmetry, Grand Unified Theories, Neutrino Mixing, Dark Energy, Dark Matter, Cosmology, Particle Detectors, Colliders, Probability and Statistics. Among the 118 reviews are many that are new or heavily revised, including a new review on Neutrinos in Cosmology.Starting with this edition, the Review is divided into two volumes. Volume 1 includes the Summary Tables and all review articles. Volume 2 consists of the Particle Listings. Review articles that were previously part of the Listings are now included in volume 1.The complete Review (both volumes) is published online on the website of the Particle Data Group (http://pdg.1b1.gov) and in a journal. Volume 1 is available in print as the PDG Book. A Particle Physics Booklet with the Summary Tables and essential tables, figures, and equations from selected review articles is also available.

Experimental measurements of the ratios R(D(∗))≡Γ(B→D(*)τv)Γ(B→D(*)ℓv)(ℓ=e,μ) show a 3.9σ deviation from the Standard Model prediction. In the absence of light right-handed neutrinos, a new physics contribution to b → cτν decays necessarily modifies also bb¯ → τ⁺τ⁻ and/or cc¯ → τ⁺τ⁻ transitions. These contributions lead to violation of lepton flavor universality in, respectively, Y and ψ leptonic decays. We analyze the constraints resulting from measurements of the leptonic vector-meson decays on solutions to the R(D^(*)) puzzle. Available data from BaBar and Belle can already disfavor some of the new physics explanations of this anomaly. Further discrimination can be made by measuring Y(1S, 2S, 3S) → ττ in the upcoming Belle II experiment.

Abstract: The ATLAS and CMS experiments at the LHC have reported an excess of diphoton events with invariant mass around 750 GeV, with local significance of about 3.6 σ and 2.6 σ, respectively. We entertain the possibility that this excess is due to new physics, in which case the data suggest a new particle with 13 TeV LHC production cross section times diphoton branching ratio of about 5 fb. Interestingly, ATLAS reports a mild preference for a sizeable width for the signal of about 45 GeV; this result appears consistent with CMS, and is further supported by improving the compatibility of the 8 TeV and 13 TeV analyses. We focus on the possibility that the new state is a scalar. First, we show that, in addition to the new state that is needed directly to produce the diphoton bump, yet more new particles beyond the Standard Model are needed to induce diphoton decay rate of the right size. Second, we note that if the excess is attributed to the Breit-Wigner peak of a single new state, then the signal strength and width taken together suggest a total LHC production cross section of order 10⁵ fb. Restricting to perturbative models without ad-hoc introduction of many new states or exotic charges, we reach the following conclusions: (i) Gluon-fusion cannot explain the required large production cross section. (ii) Tree level production from initial state quarks cannot explain the required branching ratio to two photons. (iii) Tree level production is constrained by flavor data as well as LHC Run-I and Tevatron dijet analyses. Insisting on a large width we are led to suggest that more than one scalar states, nearly degenerate in mass, could conspire to produce an observed wide bump.

The Review summarizes much of particle physics and cosmology. Using data from previous editions, plus 3,283 new measurements from 899 Japers, we list, evaluate, and average measured properties of gauge bosons and the recently discovered Higgs boson, leptons, quarks, mesons, and baryons. We summarize searches for hypothetical particles such as heavy neutrinos, supersymmetric and technicolor particles, axions, dark photons, etc. All the particle properties and search limits are listed in Summary Tables. We also give numerous tables, figures, formulae, and reviews of topics such as Supersymmetry, Extra Dimensions, Particle Detectors, Probability, and Statistics. Among the 112 reviews are many that are new or heavily revised including those on: Dark Energy, Higgs Boson Physics, Electroweak Model, Neutrino Cross Section Measurements, Monte Carlo Neutrino Generators, Top Quark, Dark Matter, Dynamical Electroweak Symmetry Breaking, Accelerator Physics of Colliders, High-Energy Collider Parameters, Big Bang Nucleosynthesis, Astrophysical Constants and Cosmological Parameters.

Within the four-generation standard model, the Higgs couplings to gluons and to photons deviate in a significant way from the predictions of the three-generation standard model. As a consequence, large departures in several Higgs production and decay channels are expected. Recent Higgs search results, presented by ATLAS, CMS, and CDF, hint on the existence of a Higgs boson with a mass around 125 GeV. Using these results and assuming such a Higgs boson, we derive exclusion limits on the four-generation standard model. For m _H=125 GeV, the model is excluded above 99.95% confidence level. For 124.5 GeV≤m_H≤127.5 GeV, an exclusion limit above 99% confidence level is found.

Measurements of the Yukawa couplings of the recently discovered boson h to fermion pairs will provide a new arena for studying flavor physics. We analyze the lessons that can be learned by measuring the h decay rates into the charged lepton pairs, τ ⁺τ ^-, μ ⁺ μ ^- and τ ^± μ ^±. We demonstrate how this set of measurements can distinguish in principle between various classes of flavor models such as natural flavor conservation, minimal flavor violation, and Froggatt-Nielsen symmetry.

This biennial Review summarizes much of particle physics. Using data from previous editions, plus 2658 new measurements from 644 papers, we list, evaluate, and average measured properties of gauge bosons, leptons, quarks, mesons, and baryons. We summarize searches for hypothetical particles such as Higgs bosons, heavy neutrinos, and supersymmetric particles. All the particle properties and search limits are listed in Summary Tables. We also give numerous tables, figures, formulae, and reviews of topics such as the Standard Model, particle detectors, probability, and statistics. Among the 112 reviews are many that are new or heavily revised including those on Heavy-Quark and Soft-Collinear Effective Theory, Neutrino Cross Section Measurements, Monte Carlo Event Generators, Lattice QCD, Heavy Quarkonium Spectroscopy, Top Quark, Dark Matter, V-cb & V-ub, Quantum Chromodynamics, High-Energy Collider Parameters, Astrophysical Constants, Cosmological Parameters, and Dark Matter.A booklet is available containing the Summary Tables and abbreviated versions of some of the other sections of this full Review. All tables, listings, and reviews (and errata) are also available on the Particle Data Group website: http://pdg.lbl.gov.

The CDF and LHCb experiments have recently provided two intriguing hints for new physics: a large forward-backward asymmetry in tt̄ production and a direct CP asymmetry in D decays of order of a percent. In both cases, flavor nonuniversal interactions are required in the up sector, raising the possibility that the two effects come from one and the same new physics source. We show that a minimal model, with an extra scalar doublet, previously suggested to explain the top data, gives-without any modifications or additions-a contribution to CP violation in charm decays that is of the right size.

Models of minimal lepton flavor violation where the seesaw scale is higher than the relevant flavor scale predict that all lepton flavor violation is proportional to the charged lepton Yukawa matrix. If extra vectorlike leptons are within the reach of the LHC, it will be possible to test the resulting predictions in ATLAS/CMS.

The D0 collaboration has recently announced evidence for a dimuon CP asymmetry in B_d,s decays of order one percent. If confirmed, this asymmetry requires new physics. We argue that for minimally flavor violating (MFV) new physics, and at low tanβ = ν_u/ν_d, there are only two four-quark operators (Q_2,3) that can provide the required CP violating effect. The scale of such new physics must lie below 260 GeV √tanβ. The effect is universal in the B_s and B _d systems, leading to S_ψk ∼ sin 2β-0.15 and S_ψφ ∼ 0.25. The effects on ∈K and on electric dipole moments are negligible. The most plausible mechanism is tree-level scalar exchange. MFV supersymmetry with low tanβ will be excluded. Finally, we explain how a pattern of deviations from the Standard Model predictions for S_ψφ, S_ψk and ∈K can be used to test MFV and, if MFV holds, to probe its structure in detail.

Future high-precision flavor experiments may discover a pattern of deviations from the standard model predictions for flavor-changing neutral current processes. One of the interesting questions that can be answered then will be whether the flavor structure of the new physics is related to that of the standard model or not. We analyze this aspect of flavor physics within a specific framework: supersymmetric models where the soft breaking terms are dominated by gauge-mediation but get non-negligible contributions from gravitymediation. We compare the possible patterns of non-minimally flavor-violating effects that arise if the gravity-mediated contributions are anarchical vs. the case that they are structured by a Froggatt-Nielsen symmetry. We show that combining information on flavor and CP violation from meson mixing and electric dipole moments is indicative for the flavor structure of gravity-mediation.

An impressive progress in measurements of the D0-D̄0 mixing parameters has been made in recent years. We explore the implications of these measurements to models of new physics, especially in view of recent upper bounds on the amount of CP violation. We update the constraints on nonrenormalizable four-quark operators. We show that the experiments are close to probing minimally flavor violating models with large tan β. The data challenge models with a scale of order TeV where the flavor violation in the down sector is suppressed by alignment and, in particular, certain classes of supersymmetric models and of warped extra dimension models.

In recent years, the CKM picture of flavor and CP violation has been confirmed, mainly due to B decay data. Yet, it is likely that there are small corrections to this picture. We expect to find new physics not much above the weak scale. This new physics could modify flavor changing processes compared to their SM expectations. Much larger B decay data sets, which are expected from LHCb and super-B-factories, will be used to search for these deviations with much improved sensitivity. The combination of low and high energy data will be particularly useful to probe the structure of new physics.

We consider supersymmetric models where gauge mediation provides the dominant contributions to the soft supersymmetry breaking terms while gravity mediation provides sub-dominant yet non-negligible contributions. We further assume that the gravity-mediated contributions are subject to selection rules that follow from a Froggatt-Nielsen symmetry. This class of models constitutes an example of viable and natural non-minimally flavor violating models. The constraints from K ⁰- ⁰ mixing imply that the modifications to the Standard Model predictions for B _d- _d and B _s- _s mixing are generically at most at the percent level, but can be of order ten percent for large tan β. The modifications for D ⁰- ⁰ mixing are generically at most of order a few percent, but in a special subclass of models they can be of order one. We point out ΔB = 1 processes relevant for flavor violation in hybrid mediation.

This biennial Review summarizes much of particle physics. Using data from previous editions., plus 2778 new measurements from 645 papers, we list, evaluate, and average measured properties of gauge bosons, leptons, quarks, mesons, and baryons. We also summarize searches for hypothetical particles such as Higgs bosons, heavy neutrinos, and supersymmetric particles. All the particle properties and search limits are listed in Summary Tables. We also give numerous tables, figures, formulae, and reviews of topics such as the Standard Model, particle detectors., probability, and statistics. Among the 108 reviews are many that are new or heavily revised including those on CKM quark-mixing matrix, V-ud & V-us, V-cb & V-ub, top quark, muon anomalous magnetic moment, extra dimensions, particle detectors, cosmic background radiation, dark matter, cosmological parameters, and big bang cosmology.

Cover Story

The mixing between third and second (or first) generation squarks is very small in supersymmetric models with minimal flavor violation such as gauge-, anomaly- or gaugino-mediation. An opportunity to measure this mixing will arise if the lightest stop is close enough in mass to the lightest neutralino, so that the decays into third generation quarks are kinematically forbidden. We analyze under which circumstances it might become possible to measure at the Large Hadron Collider (LHC) the rate of the flavor changing stop decays.

We obtain model independent relations among neutrino masses and leptogenesis parameters. We find exact relations that involve the CP asymmetries _Nα, the washout parameters _α and θ_αβ, and the neutrino masses m_i and M_α, as well as powerful inequalities that involve just _α and m_i. We prove that the Yukawa interactions of at least two of the heavy singlet neutrinos are in the strong washout region (_α 10^-3eV).

The BaBar and Belle experiments have recently presented evidence for D-0 - (D-0) over bar mixing. We explain the following points: (i) The measurements imply width difference y similar to 0.01. In the limit of small CP violation, the CP -odd state is longer-lived; (ii) y similar to 0.01 is consistent with the Standard Model. It suggests that SU(3) breaking from phase space effects is likely to play a major role; (iii) There is no evidence for either large mass splitting or CP violation. Consequently, there is no hint for new ohysics; (iv) The stronger bounds on the mass splitting and on CP violation imply that, if squarks are observed at the LHC, it is unlikely that they will be non-degenerate.

New physics contributions to Bs-B̄s mixing can be parametrized by the size (rs2) and the phase (2θs) of the total mixing amplitude relative to the standard model amplitude. The phase has so far been unconstrained. We first use the D0 measurement of the semileptonic CP asymmetry ASL to obtain the first constraint on the semileptonic CP asymmetry in Bs decays, ASLs=-0.008±0. 011. Then we combine recent measurements by the CDF and D0 Collaborations-the mass difference (ΔMs), the width difference (ΔΓs), and ASLs-to constrain 2θs. The errors on ΔΓs and ASLs should still be reduced to have a sensitive probe of the phase, yet the central values are such that the regions around 2θs∼3π/2 and, in particular, 2θs∼π/2, are disfavored.

This contribution is aimed at graduate students in the field of (theoretical and experimental) high-energy physics. The main topics covered are (i) the flavour sector of the Standard Model and the Kobayashi-Maskawa mechanism of CP violation; (ii) formalism and theoretical interpretation of CP violation in meson decays; (iii) K decays; (iv) D decays; (v) B decays (b → cc̄s, b → ss̄s, b → uūd and b → cūs, uc̄s); and (vi) CP violation as a probe of new physics and, in particular, of supersymmetry.

This is a report of the low energy and flavour physics working group at WHEPP-8, held at the Indian Institute of Technology, Mumbai, India, during 5-16 January 2004.

We argue that neutrino flavor parameters may exhibit features that are very different from those of quarks and charged leptons. Specifically, within the Froggatt-Nielsen (FN) framework, charged fermion parameters depend on the ratio between two scales, while for neutrinos a third scale - that of lepton number breaking - is involved. Consequently, the selection rules for neutrinos may be different. In particular, if the scale of lepton number breaking is similar to the scale of horizontal symmetry breaking, neutrinos may become flavor-blind even if they carry different horizontal charges. This provides an attractive mechanism for neutrino flavor anarchy.

Recent experimental results on B → ρρ decays indicate that the CP asymmetry S_ρ+ρ- will give an interesting determination of α=arg[-(V_td,V*_tb)/(V_udV* _ub)]. In the limit when the ρ width is neglected, the B → ππ isospin analysis can also be applied to B → ρρ, once an angular analysis is used to separate transversity modes. The present bound on the shift of S_ρ+ρ- from the true sin 2α is already stronger than it is for S_π+π-. We point out a subtle violation of the isospin relations when the two ρ mesons are observed with different invariant masses, and how to constrain this effect experimentally.

Measurements of various features of the fluxes of atmospheric and solar neutrinos have provided evidence for neutrino oscillations and therefore for neutrino masses and mixing. The authors review the phenomenology of neutrino oscillations in vacuum and in matter. They present the existing evidence from solar and atmospheric neutrinos as well as the results from laboratory searches, including the final status of the Liquid Scintillator Neutrino Detector (LSND) experiment. The theoretical inputs that are used to interpret the experimental results are described in terms of neutrino oscillations. The allowed ranges for the mass and mixing parameters are derived in two frameworks: First, each set of observations is analyzed separately in a two-neutrino framework; Second, the data from solar and atmospheric neutrinos are analyzed in a three-active-neutrino framework. The theoretical implications of these results are then discussed, including the existence of new physics, the estimate of the scale of this new physics, and the lessons for grand unified theories, for models of extra dimensions and singlet fermions in the bulk, and for flavor models.

We show that soft supersymmetry breaking terms involving the heavy sneutrinos can lead to sneutrino-antisneutrino mixing and to new sources of [Formula presented] violation, which are present even if a single generation is considered. These terms are naturally present in supersymmetric versions of leptogenesis scenarios, and they induce indirect [Formula presented] violation in the decays of the heavy sneutrinos, eventually generating a baryon asymmetry. This new contribution can be comparable to or even dominate over the asymmetry produced in traditional leptogenesis scenarios.

Recent experimental searches for A_SL, the CP asymmetry in semileptonic B decay, have reached an accuracy of the order of one percent. Consequently, they give meaningful constraints on new physics. We find that cancellations between the standard model (SM) and new physics contributions to B⁰-B̄⁰ mixing cannot be as strong as was allowed prior to these measurements. The predictions for this asymmetry within the SM and within models of minimal flavor violation (MFV) are below the reach of present and near future measurements. Including order m_c²/m_b² and Λ_QCD/m_b corrections we obtain the SM prediction -1.3x10^-3

We study grand unified theories (GUT) based on an (Formula presented) gauge group in which the GUT scale, (Formula presented) is the vacuum expectation value of an exact or approximate modulus, and in which fast proton decay is avoided through a combination of a large triplet mass and small triplet couplings. These features are achieved by discrete symmetries. In many of our models, (Formula presented) is generated naturally by the balance of higher dimension terms that lift the GUT modulus potential, and soft supersymmetry breaking masses. The theories often lead to interesting patterns of quark and lepton masses. We also discuss some distinctions between grand unified theories and string unification.

The possible existence of maximal or near-maximal lepton mixing constitutes an intriguing challenge for fundamental theories of flavor. We study the phenomenological consequences of maximal and near-maximal mixing of the electron neutrino with other (cursive chi=tau and/or muon) neutrinos. We describe the deviations from maximal mixing in terms of a parameter ∈≡1-2 sin² θ_{ecursive chi} and quantify the present experimental status for |∈|

New measurements of the CP asymmetry in B --> psiK(S), a(psi Ks), by the BABAR and BELLE collaborations are consistent with the Standard Model prediction for sin 2 beta. These measurements, however, leave open the possibility that a(psi Ks) is well below the Standard Model prediction. We identify deviations from the "reasonable ranges" of hadronic parameters that can lead to low values of sin 2 beta. New physics, mainly in B-0 - (B-0) over bar mixing and/or K-0 - (K-0) over bar mixing, can explain low values of a(psi Ks) in two ways: either by allowing for values of sin 2 beta below the Standard Model prediction or by modifying the relation between a(psi Ks) and sin 2 beta.

We argue that there could be significant SU(3) violating resonance contributions to D → Kπ decays which would affect the extraction of the D⁰-D⁰ mixing parameters from experiment. Such contributions can induce a strong phase in the interference between the doubly Cabibbo suppressed and the mixing induced Cabibbo favored contributions to the D⁰ → Kπ^- decays. Consequently, the interpretation of a large, CP conserving interference term can involve a large mass difference AM rather than a large width difference AF.

We discuss various implications of recent experimental results concerning CP violation and mixing in K → ππ, B → ψK_S, D → Kπ and D → KK decays.

We discuss the supersymmetric contribution to epsilon'/epsilon in various supersymmetric flavor models. We find that in alignment models the supersymmetric contribution could be significant while in heavy squark models it is expected to be small. The situation is particularly interesting in models that solve the flavor problems by either of the above mechanisms and the remaining CP problems by means of approximate CP, that is, all CP violating phases are small. In such models, the standard model contributions cannot account for epsilon'/epsilon and a failure of the supersymmetric contributions to do so would exclude the model. In models of alignment and approximate CP, the supersymmetric contributions can account for epsilon'/epsilon only if both the supersymmetric model parameters and the hadronic parameters assume rather extreme values. Such models are then strongly disfavored by the epsilon'/epsilon measurements. Models of heavy squarks and approximate CP are excluded.

Models with extended quark sector affect the CP asymmetry in the B --> psi K-S decay, a psi K-s, in two ways: First, the top-mediated box diagram is not necessarily the only important contribution to B - (B) over bar mixing. Second, the 3 x 3 CKM matrix is no longer unitary. We analyze the constraints that follow from the CDF measurement, a psi K-S = 0.79(-0.44)(+0.41), on the mixing parameters of extended quark sectors. Most noticeably, we find significant constraints on the phase of the relevant flavor changing Z coupling in models with extra down quarks in vector-like representations. Further implications for the CP asymmetry in semileptonic B decays are discussed.

Recently, the CDF collaboration has reported a measurement of the CP asymmetry in the B→ψK_S decay: a_ψK_S=0.79^+0.41_-0.44. We analyze the constraints that follow from this measurement on the size and the phase of contributions from new physics to $B-\barB$ mixing. Defining the relative phase between the full M₁₂ amplitude and the Standard Model contribution to be 2θ_d, we find a new bound: $\sin2\theta_d\gsim-0.6 (-0.87)$ at one sigma (95% CL). Further implications for the CP asymmetry in semileptonic B decays are discussed.

The experimental data on atmospheric and solar neutrinos are used to test the framework of non-anomalous Abelian horizontal gauge symmetries with only three light active neutrinos. We assume that the hierarchy in mass-squared splittings is not accidental and that the small breaking parameters are not considerably larger than 0.2. We find that the small angle MSW solution of the solar neutrino problem can only be accommodated if the v_μ - v_τ mass hierarchy depends on the charges of at least three sterile neutrinos. The large angle MSW solution can be accommodated in simpler models if v_e and v_μ form a pseudo-Dirac neutrino, but it is difficult to induce large enough deviation from maximal mixing. The vacuum oscillation solution can be accommodated rather simply. We conclude that it is possible to accommodate the neutrino parameters in the framework of Abelian horizontal symmetries, but it seems that these parameters by themselves will not provide convincing evidence for this framework.

We construct phenomenologically viable supersymmetric models where CP is an approximate symmetry. The full high energy theory has exact CP and horizontal symmetries that are spontaneously broken with a naturally induced hierarchy of scales, Λ_CP ≪ Λ_H. Consequently, the effective low energy theory, i.e. the supersymmetric Standard Model, has CP broken explicitly but by a small parameter. The ε_K parameter is accounted for by supersymmetric contributions. The predictions for other CP-violating observables are very different from the Standard Model. In particular, CP-violating effects in neutral B decays into final CP eigenstates such as B → ψK_s and in K → πvv̄ decays are very small.

Fleischer and Mannel (FM) have shown that it may become possible to constrain the angle γ of the unitarity triangle from measurements of various B → πK decays. This constraint is independent of hadronic uncertainties to the few percent level. We show that, within the Standard Model, the FM bound can give strong constraints on the CKM parameters. In particular, it could predict a well-defined sign for sin2γ and sin2α. In a class of extensions of the Standard Model, where New Physics affects only ΔB = 2 (and, in particular, not ΔB = 1 ) processes, the FM bound can lead to constraints on CP asymmetries in B decays into final CP eigenstates even if B - B mixing is dominated by unknown New Physics. In our analysis, we use a new method to combine in a statistically meaningful way the various measurements that involve CKM parameters.

Within the standard model, and if one assumes that soft rescattering effects are negligible, the (Formula presented) asymmetry (Formula presented) is predicted to be very small and the ratio (Formula presented) provides a bound on the angle (Formula presented) of the unitarity triangle, (Formula presented) We estimate the corrections from soft rescattering effects using an approach based on Regge phenomenology, and find effects of order (Formula presented) with large uncertainties. In particular, we conclude that (Formula presented) and (Formula presented) could not be taken unambiguously to signal new physics. Using SU(3) relations, we suggest experimental tests that could constrain the size of the soft rescattering effects thus reducing the related uncertainty. Finally, we study the effect of various models of new physics on (Formula presented) and on (Formula presented).

In a large class of models, the only significant new physics effect on the CP asymmetries in B → ψK_s and B → ππ decays is a new contribution to the B - B̄ mixing amplitude. This allows a model independent construction of the CKM Unitarity Triangle (up to hadronic uncertainties). Furthermore, the contributions to the mixing from the Standard Model and from the new physics can be disentangled. A serious obstacle to this analysis is an eightfold discrete ambiguity in solving for the angles of the triangle. Several ways to reduce the ambiguity either by making further measurements, or by making further assumptions about the nature of the new physics are described.

Models that combine Abelian horizontal symmetries and spontaneous CP violation can (i) explain the smallness and hierarchy in quark parameters; (ii) satisfactorily suppress supersymmetric contributions to flavor changing neutral current processes; (iii) solve the μ-problem; and (iv) suppress supersymmetric contributions to CP violating observables to an acceptable level. The CKM phase is script O sign(1) and responsible, through Standard Model box diagrams, to ε_K. The supersymmetric CP violating phases are suppressed, φ_A ∼ λ⁶ and φ_B ∼ λ⁸ (λ ∼ 0.2), leading to an electric dipole moment of the neutron that is about 2-3 orders of magnitude below the experimental bound.

In current searches for D⁰D⁰ mixing, the time evolution of "wrong-sign" decays is used to distinguish between a potential mixing signal and the dominant background from doubly-Cabibbo-suppressed decays. A term proportional to d M t in the expression for the time evolution is often neglected in theoretical discussions and experimental analyses of these processes. We emphasize that, in general, this term does not vanish even in the case of CP invariance. Furthermore, CP invariance is likely to be violated if the rate of D⁰D⁰ mixing is close to the experimental bound. The consequence of either of these two facts is that the strongest existing measured bound is not applicable for constraining New Physics.

We investigate low energy implications of string loop corrections to supergravity couplings which break a possible flavor universality of the tree level. If supersymmetry is broken by the dilaton F-term, universal soft scalar masses arise at the leading order but string loop corrections generically induce flavor-non-diagonal soft terms. Constraints from flavor changing neutral currents (FCNC) and CP violation then require a large supersymmetry breading scale and thus heavy gluinos and squarks. If supersymmetry is broken by moduli F-terms, universality at the string tree level can only be guaranteed by extra conditions on the Kahler potential. A large hierarchy between the gluino and squark masses ensures that FCNC and CP-violation constraints are satisfied. If the soft scalar masses vanish at the string tree level, the cosmological problems related to light moduli can be evaded. However, generic string loop corrections violate FCNC bounds and require very heavy squark masses (∼ 100 TeV).

The smallness of the quark sector parameters and the hierarchy between them could be the result of a horizontal symmetry broken by a small parameter. Such an explicitly broken symmetry can arise from an exact symmetry which is spontaneously broken. Constraints on the scales of new physics arise from new flavor-changing interactions and from Landau poles, but do not exclude the possibility of observable signatures at the TeV scale. Such a horizontal symmetry could also lead to many interesting results: (i) quark-squark alignment that would suppress, without squark degeneracy, flavor-changing neutral currents induced by supersymmetric particles, (ii) exact relations between mass ratios and mixing angles, (iii) a solution of the μ-problem and (iv) a natural mechanism for obtaining hierarchy among various symmetry-breaking scales.

Cambining the experimental data on the inclusive decays D→Xeν, B→Xeν, and B→Xτν, we find severe constraints on the Λ̄ and λ1 parameters of the heavy quark effective theory. In particular, we get Λ̄

We calculate the diffenretial decay rate for inclusive B→τνX transitions to order 1 m²_b in the heavy quark expansion, for both polarized and unpolarized tau leptons. We show that using a systematic 1 m_b expansion significantly reduces the theoretical uncertainties in the calculation. We obtain for the total branching ration BR(B→τνX = 2.30 ± 0.25%, and for the tau polarization A_pol=-0.706±0.006. From the experimental measurement of the branching ratio at LEP, we derive the upper bound λ₁≤0.8 GeV² for one of the parameters of the heavy quark effective theory.

A recent study by Randall and Sundrum shows that models of extended technicolor (ETC) have interesting implications on rare B decays. We extend their study to the decay B→μ+μ-X. The decay rate is found to be above the standard model prediction roughly by a factor of 4 in the case of "traditional" ETC models, and by a factor of order 30 for ETC models with a GIM mechanism. The current experimental bound is a factor of 5-15 above the standard model prediction, depending on the top-quark mass.

For generic squark masses, box diagrams with squarks and gluinos give unacceptably large contributions to neutral meson (K, B and D) mixing. The standard solution to this problem is to assume that squarks are degenerate to a very good approximation. We suggest an alternative mechanism to suppress squark contributions to flavor changing neutral currents: the alignment of quark with squark mass matrices. This mechanism arises naturally in the framework of abelian horizontal symmetries.

It is possible that the hierarchy in the masses and mixing of quarks is a result of a horizontal symmetry. The smallness of various parameters is related to their suppression by high powers of a scale of new physics. We analyze in detail the structure of such symmetries in view of new experimental data and present explicit models consistent with all phenomenological constraints. We show that it is possible that the flavor dynamics can be at accessible energies - as low as a TeV.

The values of sin2 and sin2, where and are angles of the unitarity triangle, will be readily measured in a B factory (and maybe also in hadron colliders). We study the standard model constraints in the sin2-sin2 plane. We use the results from recent analyses of fB and b|Vcb|2 which take into account heavy-quark symmetry considerations. We find sin20.15 and most likely sin20.6, and emphasize the strong correlations between sin2 and sin2. Various schemes for quark mass matrices allow much smaller areas in the sin2-sin2 plane. We study the schemes of Fritzsch, of Dimopoulos, Hall, and Raby, and of Giudice, as well as the "symmetric Cabibbo-Kobayashi-Maskawa" idea, and show how CP asymmetries in B decays will crucially test each of these schemes.

Recent experiments suggest that v_τ may be a pseudo-Dirac neutrino with m_vτ≈ 17 keV. If such a neutrino is to be consistent with constraints from nucleosynthesis, then the mass difference between its two Majorana components should be extremely small. If it is small enough because of see-saw suppression then, in the absence of fine-tuning or additional symmetries, one needs to give up the MSW solution for the solar neutrino problem and the singlet majoron solution for the energy density problem. Thus, additional symmetries that can make v_τ an almost exact Dirac neutrino should be sought. Also, the distance required for v_τ to oscillate into its sterile Dirac partner becomes unobservably large.