arXiv subjects

Yuval Grossman

Publications and source records attributed to Yuval Grossman.

At least 145 records · Page 8Linked to original sources

arXiv1999

Split Fermions in Extra Dimensions and Exponentially Small Cross-Sections at Future Colliders

We point out a dramatic new experimental signature for a class of theories with extra dimensions, where quarks and leptons are localized at slightly separated parallel ``walls'' whereas gauge and Higgs fields live in the bulk of the extra dimensions. The separation forbids direct local couplings between quarks and leptons, allowing for an elegant solution to the proton decay problem. We show that scattering cross sections for collisions of fermions which are separated in the extra dimensions vanish exponentially at energies high enough to probe the separation distance. This is because the separation puts a lower bound on the attainable impact parameter in the collision. We present cross sections for two body high energy scattering and estimate the power with which future colliders can probe this scenario, finding sensitivity to inverse fermion separations of order 10-70 TeV.

hep-ph↗

arXiv1999

Can lepton flavor violating interactions explain the atmospheric neutrino problem?

We investigate whether flavor changing neutrino interactions (FCNIs) can be sufficiently large to provide a viable solution to the atmospheric neutrino problem. Effective operators induced by heavy boson exchange that allow for flavor changing neutrino scattering off quarks or electrons are related by an $SU(2)_L$ rotation to operators that induce anomalous tau decays. Since $SU(2)_L$ violation is small for New Physics at or above the weak scale, one can use the upper bounds on lepton flavor violating tau decays or on lepton universality violation to put severe, model-independent bounds on the relevant non-standard neutrino interactions. Also $Z$-induced flavor changing neutral currents, due to heavy singlet neutrinos, are too small to be relevant for the atmospheric neutrino anomaly. We conclude that the FCNI solution to the atmospheric neutrino problem is ruled out.

hep-ph↗

arXiv1999

Neutrino masses and sneutrino mixing in R-parity violating supersymmetry

R-parity-violating supersymmetry with a conserved baryon number B provides a framework for particle physics with lepton number (L) violating interactions. Two important probes of the L-violating physics are neutrino masses and sneutrino-antisneutrino mass-splittings. We evaluate these quantities in the context of the most general CP-conserving, R-parity-violating B-conserving extension of the minimal supersymmetric standard model. In generic three-generation models, three sneutrino-antisneutrino mass splittings are generated at tree-level. In contrast, only one neutrino mass is generated at tree-level; the other two neutrinos acquire masses at one-loop. In many models, the dominant contribution to the radiative neutrino masses is induced by the non-zero sneutrino-antisneutrino mass splitting.

hep-ph↗

arXiv1998

(S)neutrino properties in R-parity violating supersymmetry: I. CP-conserving Phenomena

R-parity-violating supersymmetry (with a conserved baryon number B) provides a framework for particle physics with lepton number (L) violating interactions. We examine in detail the structure of the most general R-parity-violating (B-conserving) model of low-energy supersymmetry. We analyze the mixing of Higgs bosons with sleptons and the mixing of charginos and neutralinos with charged leptons and neutrinos, respectively. Implications for neutrino and sneutrino masses and mixing and CP-conserving sneutrino phenomena are considered. L-violating low-energy supersymmetry can be probed at future colliders by studying the phenomenology of sneutrinos. Sneutrino-antisneutrino mass splittings and lifetime differences can provide new opportunities to probe lepton number violation at colliders.

hep-ph↗

arXiv1998

Can lepton flavor violating interactions explain the LSND results?

If the atmospheric and the solar neutrino problem are both explained by neutrino oscillations, and if there are only three light neutrinos, then all mass-squared differences between the neutrinos are known. In such a case, existing terrestrial neutrino oscillation experiments cannot be significantly affected by neutrino oscillations, but, in principle there could be an anomaly in the neutrino flux due to new neutrino interactions. We discuss how a non-standard muon decay $μ^+ \to e^+ \barν_e ν_\ell$ would modify the neutrino production processes of these experiments. Since $SU(2)_L$ violation is small for New Physics above the weak scale one can use related flavor-violating charged lepton processes to constrain these decays in a model independent way. We show that the upper bounds on $μ\to 3e$, muonium-antimuonium conversion and $τ\to μe e$ rule out any observable effect for the present experiments due to $μ^+ \to e^+ \barν_e ν_\ell$ for $\ell=e,μ,τ$, respectively. Applying similar arguments to flavor-changing semi-leptonic reactions we exclude the possibility that the "oscillation signals" observed at LSND are due to flavor-changing interactions that conserve total lepton number.

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

Atmospheric ν_μDeficit from Decoherence

The simplest explanation for the observed deficit of atmospheric muon neutrinos is that they have oscillated into tau or sterile neutrinos with an oscillation length of the order of the Earth diameter. In order to confirm this hypothesis, all other possible explanations should be ruled out. We propose that a viable alternative hypothesis is that the muon neutrino deficit is caused by flavor sampling events that result in a loss of coherence. The coherence length of the muon neutrinos is expected to be approximately one Earth diameter. We give predictions and experimental tests of this scenario.

hep-ph↗

arXiv1998

Fast CP Violation

$B$ flavor tagging will be extensively studied at the asymmetric $B$ factories due to its importance in CP asymmetry measurements. The primary tagging modes are the semileptonic decays of the $b$ (lepton tag), or the hadronic $b \to c (\to s)$ decays (kaon tag). We suggest that looking for time dependent CP asymmetries in events where one $B$ is tagged leptonically and the other one is tagged with a kaon could result in an early detection of CP violation. Although in the Standard Model these asymmetries are expected to be small, $\sim 1%$, they could be measured with about the same amount of data as in the ``gold-plated'' decay $B_d \to ψK_S$. In the presence of physics beyond the Standard Model, these asymmetries could be as large as $\sim 5%$, and the first CP violation signal in the $B$ system may show up in these events. We give explicit examples of new physics scenarios where this occurs.

hep-ph↗

arXiv1998

Light active and sterile neutrinos from compositeness

Neutrinos can have naturally small Dirac masses if the Standard Model singlet right-handed neutrinos are light composite fermions. Theories which produce light composite fermions typically generate many of them, three of which can marry the left-handed neutrinos with small Dirac masses. The rest can serve as sterile states which can mix with the Standard Model neutrinos. We present explicit models illustrating this idea.

hep-ph↗

arXiv1997

Bounding the effect of penguin diagrams in $a_{CP}(B^0 \to π^+π^-)$

A clean determination of the angle $α$ of the unitary triangle from $B\to ππ$ decays requires an isospin analysis. If the $B \to π^0π^0$ and $\bar B \to π^0π^0$ decay rates are small it may be hard to carry out this analysis. Here we show that an upper bound on the error on $\sin 2α$ due to penguin diagram effects can be obtained using only the measured rate $\BR(B^\pm \to π^\pm π^0)$ and an upper bound on the combined rate $\BR(B \to π^0 π^0) + \BR(\bar B \to π^0 π^0)$. Since no b flavor tagging is needed to measure this combined rate, the bound that can be achieved may be significantly better than any approach which requires separate flavor-tagged neutral pion information.

hep-ph↗

arXiv1997

Sneutrino Mixing

In supersymmetric models with nonvanishing Majorana neutrino masses, the sneutrino and antisneutrino mix. The conditions under which this mixing is experimentally observable are studied, and mass-splitting of the sneutrino mass eigenstates and sneutrino oscillation phenomena are analyzed.

hep-ph↗

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.

hep-ph↗

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.

hep-ph↗

arXiv1997

CP Asymmetry in B_d --> phi K_S: Standard Model Pollution

The difference in the time dependent CP asymmetries between the modes $B --> psi K_S$ and $B --> phi K_S$ is a clean signal for physics beyond the Standard Model. This interpretation could fail if there is a large enhancement of the matrix element of the $b --> u ubar s$ operator between the $B_d$ initial state and the $phi K_S$ final state. We argue against this possibility and propose some experimental tests that could shed light on the situation.

hep-ph↗

arXiv1997

CP Violation due to new Delta B =1 Amplitudes

We make a systematic analysis of the effects of new physics in the B decay amplitudes on the CP asymmetries in neutral B decays. Although these are expected to be smaller than new physics effects on the mixing amplitude, they are easier to probe in some cases. The effects of new contributions to the mixing amplitude are felt universally across all decay modes, whereas the effects of new decay amplitudes could vary from mode to mode. In particular the prediction that the CP asymmetries in the B decay modes with $b\to c\bar c s$, $b \to c\bar c d$, $ b\to c \bar u d$ and $ b\to s \bar s s$ should all measure the same quantity (sin(2β) in the Standard Model) could be violated.

hep-ph↗

arXiv1997

Removing Discrete Ambiguities in CP Asymmetry Measurements

We discuss methods to resolve the ambiguities in CP violating phase angles $ϕ$ that are left when a measurement of $\sin 2 ϕ$ is made. We show what knowledge of hadronic quantities will be needed to fully resolve all such ambiguities.

hep-ph↗

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.

hep-ph↗

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.

hep-ph↗