arXiv subjects

Yosef Nir

Publications and source records attributed to Yosef Nir.

At least 109 records · Page 6Linked to original sources

arXiv2000

Implications of a Small CP Asymmetry in B->ψK_S

New measurements of the CP asymmetry in B->ψK_S, a_{ψK_S}, by the BABAR and BELLE collaborations are consistent with the Standard Model prediction for sin2β. These measurements, however, leave open the possibility that a_{ψK_S} is well below the Standard Model prediction. We identify deviations from the `reasonable ranges' of hadronic parameters that can lead to low values of sin2β. New physics, mainly in B^0-\barB^0 mixing and/or K^0-\barK^0 mixing, can explain low values of a_{ψK_S} in two ways: either by allowing for values of sin2βbelow the Standard Model prediction or by modifying the relation between a_{ψK_S} and sin2β.

hep-ph↗

arXiv2000

Pseudo-Dirac Solar Neutrinos

Three of the viable solutions of the solar neutrino problem are consistent with close to maximal leptonic mixing: $\sin^2θ_{12}={1\over2}(1-ε_{12})$ with $|ε_{12}|\ll1$. Flavor models can naturally explain close to maximal mixing if approximate horizontal symmetries force a pseudo-Dirac structure on the neutrino mass matrix. An experimental determination of $|ε_{12}|$ and sign($ε_{12}$) can constrain the structure of the lepton mass matrices and consequently provide stringent tests of such flavor models. If both $|ε_{12}|$ and $Δm^2_{21}$ are known, it may be possible to estimate the mass scale of the pseudo-Dirac neutrinos. Radiative corrections to close to maximal mixing are negligible. Subtleties related to the kinetic terms in Froggatt-Nielsen models are clarified.

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arXiv2000

Lessons from CLEO and FOCUS Measurements of D0-anti-D0 Mixing Parameters

If the true values of the D0-anti-D0 mixing parameters lie within the one sigma ranges of recent measurements, then there is strong evidence for a large width difference, y > 0.01, and large SU(3) breaking effects in strong phases, δ> pi/4. These constraints are model independent, and would become stronger if M_{12}/Gamma_{12} << 1 in the D0-anti-D0 system. The interesting fact that the FOCUS result cannot be explained by a large mass difference is not trivial and depends on the small D0-anti-D0 production asymmetry in FOCUS and the bounds on CP violating effects from CLEO. The large value of δmight help explain why y ~ sin^2(theta_c).

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arXiv2000

Neutrino Parameters, Abelian Flavor Symmetries, and Charged Lepton Flavor Violation

Neutrino masses and mixings have important implications for models of fermion masses, and, most directly, for the charged lepton sector. We consider supersymmetric Abelian flavor models, where neutrino mass parameters are related to those of charged leptons and sleptons. We show that processes such as τto μγ, μto eγand μ-e conversion provide interesting probes. In particular, some existing models are excluded by current bounds, while many others predict rates within reach of proposed near future experiments. We also construct models in which the predicted rates for charged lepton flavor violation are below even the proposed experimental sensitivities, but argue that such models necessarily involve loss of predictive power.

hep-ph↗

arXiv1999

Strong phases and D0-anti-D0 mixing parameters

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

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arXiv1999

CP Violation In and Beyond the Standard Model

The special features of CP violation in the Standard Model are presented. The significance of measuring CP violation in B, K and D decays is explained. The predictions of the Standard Model for CP asymmetries in B decays are analyzed in detail. Then, four frameworks of new physics are reviewed: (i) Supersymmetry provides an excellent demonstration of the power of CP violation as a probe of new physics. (ii) Left-right symmetric models are discussed as an example of an extension of the gauge sector. CP violation suggests that the scale of LRS breaking is low. (iii) The variety of extensions of the scalar sector are presented and their unique CP violating signatures are emphasized. (iv) Vector-like down quarks are presented as an example of an extension of the fermion sector. Their implications for CP asymmetries in B decays are highly interesting.

hep-ph↗

arXiv1999

Constraining New Physics with the CDF Measurement of CP Violation in $B \to ψK_S$

Recently, the CDF collaboration has reported a measurement of the CP asymmetry in the $B\toψK_S$ 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 $M_{12}$ amplitude and the Standard Model contribution to be $2θ_d$, we find a new bound: $\sin2θ_d\gsim-0.6 (-0.87)$ at one sigma (95% CL). Further implications for the CP asymmetry in semileptonic B decays are discussed.

hep-ph↗

arXiv1999

New Physics Effects in Doubly Cabibbo Suppressed D Decays

The most sensitive experimental searches for $D-\bar D$ mixing use $D^0 \to K^+ π^-$ decays. It is often assumed that effects of New Physics and, in particular, CP violation, can appear through the mixing, while the $c \to d u \bar s$ decay amplitude cannot have significant contributions from New Physics and is, therefore, CP conserving to a good approximation. We examine this assumption in two ways. First, we calculate the contributions to the decay in various relevant models of New Physics: Supersymmetry without R-parity, multi-scalar models, left-right symmetric models, and models with extra quarks. We find that phenomenological constraints imply that the New Physics contributions are indeed small compared to the standard model doubly Cabibbo suppressed amplitude. Second, we show that many of our constraints hold model-independently. We find, however, one case where the model-independent bound is rather weak and a CP violating contribution of order 30% is not excluded.

hep-ph↗

arXiv1999

Constraining Extensions of the Quark Sector with the CP Asymmetry in $B\to ψK_{S}$

Models with extended quark sector affect the CP asymmetry in the $B\to ψK_{S}$ decay, $a_{ψK_{S}}$, in two ways: First, the top-mediated box diagram is not necessarily the only important contribution to $B-\bar{B}$ mixing. Second, the $3\times 3$ CKM matrix is no longer unitary. We analyze the constraints that follow from the CDF measurement, $a_{ψK_{S}} =0.79^{+0.41}_{-0.44}$, 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.

hep-ph↗

arXiv1999

Lepton Parity in Supersymmetric Flavor Models

We investigate supersymmetric models where neither R-parity nor lepton number nor baryon number is imposed. The full high energy theory has an exact horizontal U(1) symmetry that is spontaneously broken. Quarks and Higgs fields carry integer horizontal charges but leptons carry half integer charges. Consequently, the effective low energy theory has two special features: a U(1) symmetry that is explicitly broken by a small parameter, leading to selection rules, and an exact residual Z_2 symmetry, that is lepton parity. As concerns neutrino parameters, the Z_2 symmetry forbids contributions from R_p-violating couplings and the U(1) symmetry induces the required hierarchy. As concerns baryon number violation, the Z_2 symmetry forbids proton decay and the U(1) symmetry provides sufficient suppression of double nucleon decay and of neutron - antineutron oscillations.

hep-ph↗

arXiv1999

CP Violation in B Decays

In the near future, we will have the first significant experimental measurements of CP violation in B decays. These measurements will easily test crucial questions such as whether the Standard Model Kobayashi-Maskawa phase plays a dominant role in CP violation or whether CP is an approximate symmetry in nature. We explain the different types of CP violation in B decays, and the usefulness of measuring them. We use the same formalism to describe the epsilon and epsilon(prime) parameters of the neutral K system and to explain the terms direct and indirect CP violation. We present the Standard Model predictions for the various asymmetries. We argue that certain CP asymmetries in B decays are subject to a very clean theoretical interpretation in terms of fundamental Lagrangian parameters. Within the Standard Model, these asymmetries will provide very accurate measurements of the CKM parameters. In case that deviations from the Standard Model predictions will be found, there is enough information to understand the nature of New Physics that is required to explain them. We demonstrate this statement by analyzing the impact of various Supersymmetric flavor models on CP violation.

hep-ph↗

arXiv1999

Neutrino Masses and Mixing with Non-Anomalous Abelian Flavor Symmetries

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 $ν_μ-ν_τ$ mass hierarchy depends on the charges of at least three sterile neutrinos. The large angle MSW solution can be accommodated in simpler models if $ν_e$ and $ν_μ$ 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.

hep-ph↗

arXiv1998

Flavor Physics and CP Violation

This is a written version of a series of three lectures aimed at graduate students in the field of experimental high energy physics. The emphasis is on physics that is relevant to $B$-factories. The main topics covered are: (i) The flavor sector of the Standard Model; (ii) Determination of the mixing parameters; (iii) CP violation in meson decays (a model independent description); (iv) CP violation in the Standard Model; (v) CP violation as a probe of new physics.

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arXiv1998

Naturally Light Sterile Neutrinos in Gauge Mediated Supersymmetry Breaking

Moduli are generic in string (M) theory. In a large class of gauge-mediated Supersymmetry breaking models, the fermionic components of such fields have very light masses, around the eV scale, and non-negligible mixing with active neutrinos, of order $10^{-4}$. Consequently, these fermions could play the role of sterile neutrinos to which active neutrinos oscillate, thus affecting measurements of solar neutrinos or of atmospheric neutrinos. They could also provide warm dark matter, thus affecting structure formation.

hep-ph↗

arXiv1998

Large Mixing and Large Hierarchy Between Neutrinos with Abelian Flavor Symmetries

The experimental data on atmospheric and solar neutrinos suggest that there is near-maximal mixing between ν_μand ν_τbut that their masses are hierarchically separated. In models of Abelian horizontal symmetries, mixing of O(1) generically implies that the corresponding masses are of the same order of magnitude. We describe two new mechanisms by which a large hierarchy between strongly mixed neutrinos can be achieved in this framework. First, a discrete Abelian symmetry can give the desired result in three ways: mass enhancement, mixing enhancement and mass suppression. Second, holomorphic zeros can give mass suppression.

hep-ph↗

arXiv1998

Probing the Flavor and CP Structure of Supersymmetric Models with K --> pi nu nubar Decays

We study the implications of various supersymmetric models on the rare $K^+ \to π^+ ν\barν$ and $K_L \to π^0 ν\barν$ decays. Although large effects are possible in generic supersymmetric models, most of the known supersymmetric flavor models lead to negligible effects. Thus, it is likely that one can get information about CKM matrix elements from these decays even in the presence of supersymmetry. Moreover, the possibility of large contributions to $K \to πν\barν$ in generic supersymmetric models can be constrained by improved bounds on $D-\bar D$ mixing. We show that it may be possible to distinguish between different supersymmetric flavor models by combining the information from the $K \to πν\barν$ decays with that from $B-\bar B$ and $D-\bar D$ mixing.

hep-ph↗

arXiv1998

Approximate CP in Supersymmetric Models

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, $Λ_{CP}\llΛ_H$. Consequently, the effective low energy theory, that is 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 $B\raψK_S$ and in $K\raπν\barν$ decays are very small.

hep-ph↗

arXiv1997

Final State Interactions and New Physics in B -> pi K Decays

Within the Standard Model, and if one assumes that soft rescattering effects are negligible, the CP asymmetry A^dir_CP (B^\pm -> π^\pm K) is predicted to be very small and the ratio R = BR(B_d -> π^\mp K^\pm)/BR(B^\pm -> π^\pm K) provides a bound on the angle γof the unitarity triangle, sin^2 γ\leq R. We estimate the corrections from soft rescattering effects using an approach based on Regge phenomenology, and find effects of order 10% with large uncertainties. In particular, we conclude that A^dir_CP \sim 0.2 and sin^2 γ\sim 1.2 R 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 A^dir_CP and on R.

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