arXiv subjects
Publications and source records attributed to Yuval Grossman.
The $K_L \to π^0 ν\barν$ decay is analyzed in a model independent way. When lepton flavor is conserved, this decay mode is a manifestation of CP violating interference between mixing and decay. Consequently, a theoretically clean relation between the measured rate and electroweak parameters holds in any given model.
In any model with nonzero Majorana neutrino masses, the sneutrino and antisneutrino of the supersymmetric extended theory mix. We outline the conditions under which sneutrino-antisneutrino mixing is experimentally observable. The mass-splitting of the sneutrino mass eigenstates and sneutrino oscillation phenomena are considered.
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.
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_d 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 ($sin2β$ in the Standard Model) could be violated. Since the above Standard Model prediction is very precise, new decay amplitudes which are a few percent of the Standard Model amplitudes can be probed. Three examples of models where measurable effects are allowed are given: effective supersymmetry, models with enhanced chromomagnetic dipole operators, and supersymmetry without R parity.
The study of $B \to τ^+ τ^- (X)$ decays can provide us with a better understanding of the third generation, and can be a useful probe of physics beyond the standard model. We present a model independent analysis of these decays. We classify new physics that can largely enhance the decay rates and we discuss the constraints implied by other processes. Experimentally, flavor changing neutral current $B$ decays into final state $τ$'s are still unconstrained. Searches for $B$ decays with large missing energy at LEP provide the first limits. We estimate that existing data already imply bounds on the $B_d \to τ^+ τ^-$, $B_s \to τ^+ τ^-$, and $B \to X τ^+ τ^-$ decay rates at the few percent level. Although these bounds are over four orders of magnitude above the standard model predictions, they provide the first constraints on some leptoquarks, and on some R-parity violating couplings.
The inclusive $B \to X_s ν\barν$ decay rate, on which no experimental bound exists to date, can be constrained by searching for large missing energy events in $B$ decays. Carefully examining the experimental and theoretical aspects of such an analysis, we argue that the published ALEPH limit on $\BR(B \to τ\barν)$ implies, conservatively, the bound $\BR(B \to X_s ν\barν)<3.9\times10^{-4}$, which is less than one order of magnitude above the standard model prediction. The LEP collaborations could significantly improve this bound by a dedicated experimental analysis. We study the constraints this new limit imposes on various extensions of the standard model. We derive new bounds on the couplings of third generation fermions in models with leptoquarks, and in supersymmetric models without R-parity. We also constrain models where new gauge bosons are coupled dominantly to the third generation, such as TopColor models and models based on horizontal gauge symmetries. For models which predict an enhanced effective $bsZ$ vertex, the constraint from $B \to X_s ν\barν$ is competitive with the limits from inclusive and exclusive $B \to X_s \ell^+ \ell^-$ decays.
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.
A unique description avoiding confusion is presented for all flavor oscillation experiments in which particles of a definite flavor are emitted from a localized source. The probability for finding a particle with the wrong flavor must vanish at the position of the source for all times. This condition requires flavor-time and flavor-energy factorizations which determine uniquely the flavor mixture observed at a detector in the oscillation region; i.e. where the overlaps between the wave packets for different mass eigenstates are almost complete. Oscillation periods calculated for ``gedanken'' time-measurement experiments are shown to give the correct measured oscillation wave length in space when multiplied by the group velocity. Examples of neutrinos propagation in a weak field and in a gravitational field are given. In these cases the relative phase is modified differently for measurements in space and time. Energy-momentum (frequency-wave number) and space-time descriptions are complementary, equally valid and give the same results. The two identical phase shifts obtained describe the same physics; adding them together to get a factor of two is double counting.
A unique description avoiding confusion is presented for all flavor-oscillation experiments in which particles of a definite flavor are emitted from a localized source. The probability for finding a particle with the wrong flavor must vanish at the position of the source for all times. This condition requires flavor-time and flavor-energy factorizations which determine uniquely the flavor mixture observed at a detector in the oscillation region; i.e. where the overlaps between the wave packets for different mass eigenstates differ negligibly from 100\%. The translation of a ``gedanken'' experiments calculations (where measurement is perform in time) is done using the {\it group} velocity. Energy-momentum (frequency-wave number) and space-time descriptions are complementary, equally valid and give the same results. The two identical phase shifts obtained describe the same physics; adding them together to get a factor of two is double counting.
The Standard Model predicts a large width difference in the $B_s$ system. New physics can contribute significantly to the {\it mass} difference. If this contribution is CP violating, this will always result in a reduction of the {\it width} difference. The analyses of measurements of the width difference using $B_s$ decays into CP eigenstates have to be modified in the presence of new physics. We discuss how the width difference and the new CP violating phase in $B_s$ mixing can be measured.
In analyzing neutrino oscillation experiments it is often assumed that while new physics contributes to neutrino masses, neutrino interactions are given by the Standard Model. We develop a formalism to study new physics effects in neutrino interactions using oscillation experiments. We argue that the notion of branching ratio is not appropriate in this case. We show that a neutrino appearance experiment with sensitivity to oscillation probability $P_{ij}^{exp}$ can detect new physics in neutrino interactions if its strength $G_N$ satisfies $(G_N/G_F)^2 \sim P_{ij}^{exp}$. Using our formalism we show how current experiments on neutrino oscillation give bounds on the new interactions in various new physics scenarios.
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, $\tanβ<0.52(m_H/1 GeV)$.
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.
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.
We calculate, in the framework of multi Higgs doublet models, the $CP$ violating transverse tau polarization, $P_\perp$, in inclusive semileptonic $B$ meson decays. We find that $P_\perp$ diverges at ${\cal O}(1/m_b^2)$ of the heavy quark expansion. We discuss the physical reasons for this divergence and show how to regularize it. We find large $1/m_b^2$ corrections that can suppress the $CP$ asymmetry by as much as 30\%. The maximal allowed asymmetry is $a_{CP}\sim0.34$. We discuss how the allowed range is expected to change in the future.
We calculate in the framework of two Higgs doublet models the differential decay rate for inclusive $\bar B\toτ\,\barν\,X$ transitions to order $1/m_b^2$ in the heavy quark expansion, for both polarized and unpolarized tau leptons. In contrast to the situation in the standard model, we find sizeable $1/m_b^2$ corrections. A systematic heavy quark expansion significantly reduces the theoretical uncertainties in the calculation compared to the existing free quark decay model results. From the experimental measurement of the branching ratio, we derive the bound $\tanβ< 0.51\, m_{H^\pm}\,[{\rm GeV}]$. We point out that the tau polarization is potentially a more sensitive probe of multi scalar models than the branching ratio itself.
We study the most general Multi-Higgs-Doublet Model (MHDM) with Natural Flavor Conservation (NFC). The couplings of a charged scalar $H_i^{\pm}$ to up quarks, down quarks and charged leptons depend on three new complex parameters, $X_i$, $Y_i$ and $Z_i$, respectively. We prove relations among these parameters. We carry out a comprehensive analysis of phenomenological constraints on the couplings of the lightest charged scalar: $X$, $Y$ and $Z$. We find that the general MHDM may differ significantly from its minimal version, the Two-Higgs-Doublet Model (2HDM).
In models of three or more scalar doublets, new CP violating phases appear in charged scalar exchange. These phases affect CP asymmetries in neutral $B$ decays, even if Natural Flavor Conservation holds. The recent upper bound on the decay $b\rightarrow sγ$ constrains the effect to be at most of order a few percent. Modifications of constraints on the CKM parameters open an interesting new region in the $\sin2α-\sin2β$ plane even in the absence of new phases.