arXiv subjects

Yosef Nir

Publications and source records attributed to Yosef Nir.

At least 127 records · Page 7Linked to original sources

arXiv1997

Recent Developments in Theory of CP Violation

Four topics in theory of CP violation are reviewed. (a) CP violation in $B$ decays: We describe a new clean way of constraining the angle $γ$ of the unitarity triangle and how new CP violation in decay amplitudes can signal new physics. (b) CP violation in $K$ decays: We explain the special features of the decay $K_L\raπ^0ν\barν$ both as a measurement of Standard Model CP violating parameters and as a probe of new physics. (c) CP violation in $D$ decays: We describe the consequences of CP violation from new physics in $D-\bar D$ mixing. (d) CP violation in Supersymmetry: We explain how a combination of measurements of CP violating processes will give insight into the flavor and CP structure of supersymmetry.

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arXiv1997

Implications of the Fleischer-Mannel Bound

Fleischer and Mannel (FM) have shown that it may become possible to constrain the angle $γ$ of the unitarity triangle from measurements of various $B\toπK$ decays. This constraint is independent of hadronic uncertainties to the few percent level. We show that, within the Standard Model, the FM bound gives 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 the 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 $B-\bar B$ 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.

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arXiv1997

The Role of the Vacuum Insertion Approximation in Calculating CP Asymmetries in B Decays

CP asymmetries in $B$ decays into final CP eigenstates are in many cases theoretically clean. In particular, they do not depend on the values of hadronic parameters. The sign of the asymmetries, however, does depend on the sign of the $B_B$ parameter. Furthermore, the information from $ε_K$ that all angles of the unitarity triangles lie in the range $\{0,π\}$ depends on the sign of the $B_K$ parameter. Consequently, in the (unlikely) case that the vacuum insertion approximation is such a poor approximation that either $B_B$ or $B_K$ is negative, the sign of CP asymmetries in neutral $B$ decays will be opposite to the standard predictions. Various subtleties concerning the role of $K-\bar K$ mixing in the case of final states with a single $K_S$ or $K_L$, such as the $B\toψK_S$ decay, are clarified.

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arXiv1997

CP Violation Beyond the Standard Model

We review CP violation in various extensions of the electroweak sector of the Standard Model. A particular emphasis is put on supersymmetric models. We describe the two CP problems of supersymmetry, concerning $d_N$ and $ε_K$. We critically review the various mechanisms that have been suggested to solve these problems: exact universality, approximate CP symmetry, alignment, approximate universality and heavy squarks. We explain how future measurements of CP violation will test these mechanisms. We describe extensions of the quark sector and their implications on CP asymmetries in neutral B decays, on the $K_L \to πν\barν$ decay and on $ΔΓ(B_s)$. We discuss CP violation in charged scalar exchange in models with natural flavor conservation and explain how transverse lepton polarization in meson decays can probe such models. CP violation in neutral scalar exchange arises in models of horizontal symmetries and may be manifest in heavy quark (b and t) physics. We describe the implications of Left-Right Symmetric models on $d_N$, $ε_K$, $ε'/ε$ and CP asymmetries in B decays. Finally, we briefly discuss the potential of future measurements of CP violation to discover New Physics.

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arXiv1997

A Model Independent Construction of the Unitarity Triangle

In a large class of models, the only significant new physics effect on the CP asymmetries in $B \to ψK_S$ and $B \to ππ$ decays is a new contribution to the $B-\bar B$ 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.

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arXiv1997

$K_L \to π^0 ν\bar ν$ Beyond the Standard Model

We analyze the decay $K_L \to π^0 ν\bar ν$ in a model independent way. If lepton flavor is conserved the final state is (to a good approximation) purely CP even. In that case this decay mode goes mainly through CP violating interference between mixing and decay. Consequently, a theoretically clean relation between the measured rate and electroweak parameters holds in any given model. Specifically, $Γ(K_L \to π^0 ν\bar ν)/Γ(K^+ \to π^+ ν\bar ν)= \sin^2θ$ (up to known isospin corrections), where $θ$ is the relative CP violating phase between the $K-\bar K$ mixing amplitude and the $s\to dν\barν$ decay amplitude. The experimental bound on $BR(K^+ \to π^+ ν\bar ν)$ provides a model independent upper bound: $BR(K_L \to π^0 ν\bar ν) < 1.1 \times 10^{-8}$. In models with lepton flavor violation, the final state is not necessarily a CP eigenstate. Then CP conserving contributions can dominate the decay rate.

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arXiv1996

Variations on Minimal Gauge Mediated Supersymmetry Breaking

We study various modifications to the minimal models of gauge mediated supersymmetry breaking. We argue that, under reasonable assumptions, the structure of the messenger sector is rather restricted. We investigate the effects of possible mixing between messenger and ordinary squark and slepton fields and, in particular, violation of universality. We show that acceptable values for the $μ$ and $B$ parameters can naturally arise from discrete, possibly horizontal, symmetries. We claim that in models where the supersymmetry breaking parameters $A$ and $B$ vanish at tree level, $\tanβ$ could be large without fine tuning. We explain how the supersymmetric CP problem is solved in such models.

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arXiv1996

On Supersymmetric CP Violation

We discuss the discovery potential for New Physics of various measurements of CP violation. If nature is supersymmetric, then the flavor problem is even more mysterious than in the standard model. We show how we can learn about the mechanism that solves the supersymmetric flavor problem from measurements of mixing and CP violation in $K$, $D$ and $B$ decays. (Invited talk given at the Workshop on K Physics, Orsay, France, May 30 - June 4, 1996.)

hep-ph↗

arXiv1996

Neutrino Masses and Mixing in Supersymmetric Models without $R$ Parity

We study neutrino masses and mixing in Supersymmetric Models without $R$ parity and with generic soft Supersymmetry breaking terms. Neutrinos acquire mass from various sources: tree level neutrino--neutralino mixing, loop effects and non--renormalizable operators. Abelian horizontal symmetries (invoked to explain the smallness and hierarchy in quark parameters) replace $R$ parity in suppressing neutrino masses. We find lower bounds on the mixing angles: $\sinθ_{ij} \gsim m(\ell_i^-)/m(\ell_j^-)$ ($i<j$) and unusual order of magnitude predictions for neutrino mass ratios: $m(ν_e)/m(ν_μ)\sim\sin^2θ_{12}$; $m(ν_i)/m(ν_τ)\sim 10^{-7} \sin^2θ_{i3}$ ($i=1,2$). Bounds from laboratory experiments exclude $m_{ν_τ} \gsim 3\ MeV$ and the cosmological constraint that excludes $m_{ν_τ} \gsim 100\ eV$ is not evaded. Neither the solar nor the atmospheric neutrino problems are likely to be solved by $ν_μ-ν_e$ oscillations. These conclusions can be evaded if holomorphy plays an important role in the lepton Yukawa couplings.

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arXiv1996

Solving the Supersymmetric CP Problem with Abelian Horizontal Symmetries

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 $Ø(1)$ and responsible, through Standard Model box diagrams, to $ε_K$. The supersymmetric CP violating phases are suppressed, $ϕ_A\simλ^6$ and $ϕ_B\simλ^{8}$ ($λ\sim0.2$), leading to an electric dipole moment of the neutron that is about 2--3 orders of magnitude below the experimental bound.

hep-ph↗

arXiv1995

New Tools for Low Energy Dynamical Supersymmetry Breaking

We report the construction of large new classes of models which break supersymmetry dynamically. We then turn to model building. Two of the principal obstacles to constructing simple models of dynamical supersymmetry breaking are the appearance of Fayet-Iliopoulos D terms and difficulties in generating a $μ$ term for the Higgs fields. Among the new models are examples in which symmetries prevent the appearance of Fayet-Iliopoulos terms. A gauge singlet field, that may play a role in explaining the hierarchy in quark and lepton parameters, can generate a suitable $μ$ term. The result is a comparatively simple model, with a low energy structure similar to that of the MSSM, but with far fewer arbitrary parameters. We begin the study of the phenomenology of these models.

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arXiv1995

Exploring New Physics with CP Violation in Neutral D and B Decays

If New Physics contributes significantly to neutral meson mixing, then it is quite likely that it does so in a CP violating manner. In $D^0-\bar D^0$ mixing measured through $D^0\rightarrow K^+π^-$, CP violation induces a term $\propto te^{-Γt}$ with important implications for experiments. For $B_s-\bar B_s$ mixing, a non-vanishing CP asymmetry (above a few percent) $a_{CP}(B_s \rightarrow D_s^+ D_s^-)$ is a clear signal of New Physics. Interestingly, this would test precisely the same Standard Model ingredients as the question of whether $α+β+γ=π$.

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arXiv1995

QCD corrections to charged Higgs-mediated $b \to c τν$ decay

We calculate the $O(α_s)$ corrections to the charged Higgs mediated inclusive semi-tauonic $B$ decay. By working in the Landau gauge, we demonstrate how to obtain the charged Higgs contributions (both direct and interference terms) from the known QCD corrections to $t \to b H^+$. Combining our results with $O(1/m_b^2)$ corrections to the spectator model and comparing the theoretical prediction with the recent experimental measurements, we find a model-independent $2σ$ upper bound on the ratio of Higgs vacuum expectation values, $\tanβ<0.52(m_H/1 GeV)$.

hep-ph↗

arXiv1995

Supersymmetry without R-Parity and without Lepton Number

We investigate Supersymmetric models where neither R parity nor lepton number is imposed. Neutrino masses can be kept highly suppressed compared to the electroweak scale if the $μ$-terms in the superpotential are aligned with the SUSY-breaking bilinear $B$-terms. This situation arises naturally in the framework of horizontal symmetries. The same symmetries suppress the trilinear R parity violating terms in the superpotential to an acceptable level.

hep-ph↗

arXiv1995

Gauge Unification, Yukawa Hierarchy and the $μ$ Problem

The hierarchy in the Yukawa couplings may be the result of a gauged horizontal $U(1)_H$ symmetry. If the mixed anomalies of the Standard Model gauge group with $U(1)_H$ are cancelled by a Green-Schwarz mechanism, a relation between the gauge couplings, the Yukawa couplings and the $μ$-term arises. Assuming that at a high energy scale $g_3^2=g_2^2={5\over3}g_1^2$ and $(m_e m_μm_τ)/(m_d m_s m_b)\sim λ$ (where $λ$ is of the order of the Cabibbo angle), the $U(1)_H$ symmetry solves the $μ$-problem with $μ\simλm_{3/2}$.

hep-ph↗

arXiv1995

Lepton Mass Matrix Models

The smallness and hierarchy in fermion parameters could be the result of selection rules due to an Abelian horizontal symmetry broken by a small parameter. When applied to the lepton sector, then for a large class of models, a number of interesting order of magnitude relations arise: with $i<j$, $m(ν_i)/m(ν_j)\sim\sin^2θ_{ij}$; $m(\ell^-_i)/m(\ell^-_j)\lsim\sinθ_{ij}$; $m(ν_i)/m(ν_j)\gsim m^2(\ell^-_i)/m^2(\ell^-_j)$; $m(ν_e)\lsim m(ν_μ)\lsim m(ν_τ)$. The relations between neutrino masses and mixings may become exact if the horizontal symmetry together with holomorphy induce certain zero entries in the lepton mass matrices. A full high energy theory is likely to include scalars with flavor changing couplings and heavy leptons in vector representations; however, the masses of these particles are too heavy to be directly observed in experiment. Indirect evidence for the horizontal symmetry may arise from other sectors of the theory: non-degenerate sleptons are allowed as the symmetry aligns lepton and slepton mass matrices; light leptoquarks are allowed as the symmetry can make their couplings diagonal and chiral.

hep-ph↗

arXiv1994

Some Phenomenological Implications of String Loop Effects

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 supersymmtery breaking 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 Kähler 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 ($\approx 100\, TeV$).

hep-ph↗

arXiv1994

Implications of Horizontal Symmetries on Baryon Number Violation in Supersymmetric Models

The smallness of the quark and lepton parameters and the hierarchy between them could be the result of selection rules due to a horizontal symmetry broken by a small parameter. The same selection rules apply to baryon number violating terms. Consequently, the problem of baryon number violation in Supersymmetry may be solved naturally, without invoking any especially-designed extra symmetry. This mechanism is efficient enough even for low-scale flavor physics. Proton decay is likely to be dominated by the modes $K^+\barν_i$ or $K^0μ^+(e^+)$, and may proceed at observable rates.

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