arXiv subjects
Publications and source records attributed to Yuval Grossman.
The lepton sector masses and mixing angles can be explained in models based on $A_4$ symmetry. $A_4$ is a non-Abelian discrete group. Therefore, one issue in constructing models based on it is explaining the origin of $A_4$. A plausible mechanism is that $A_4$ is an unbroken subgroup of a continuous group that is broken spontaneously. We construct a model of leptons where the $A_4$ symmetry is obtained by spontaneous symmetry breaking of SO(3).
If Micro Black Holes (MBHs) can be produced at the LHC, they will decay very fast. We study hypothetical MBHs that do not decay; in particular, QCD effects on accretion by MBHs that are produced at rest. We explain why accretion of a nucleon by such MBHs is associated with pion emission. This pion emission results in a kick to the MBHs, such that their velocities are large enough to escape the Earth. Our study provides an extra assurance that MBHs which might be produced at the LHC are not dangerous.
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.
If new CP violating physics contributes to neutral meson mixing, but its contribution to CP violation in decay amplitudes is negligible, then there is a model independent relation between four (generally independent) observables related to the mixing: The mass splitting (x), the width splitting (y), the CP violation in mixing (1-|q/p|), and the CP violation in the interference of decays with and without mixing (ϕ). For the four neutral meson systems, this relation can be written in a simple approximate form: y tanϕ~ x(|q/p|-1). This relation is already tested (successfully) in the neutral K system. It provides predictions for the B_s and D systems. The success or failure of these relations will probe the physics that is responsible for the CP violation.
New physics at high energy scale often contributes to K-\bar K and D-\bar D mixings in an approximately SU(2)_L invariant way. In such a case, the combination of measurements in these two systems is particularly powerful. The resulting constraints can be expressed in terms of misalignments and flavor splittings.
We present rules for determining the number of physical parameters in models with exact flavor symmetries. In such models the total number of parameters (physical and unphysical) needed to described a matrix is less than in a model without the symmetries. Several toy examples are studied in order to demonstrate the rules. The use of global symmetries in studying the minimally supersymmetric standard model (MSSM) is examined.
Models with composite singlet neutrinos can give small Majorana or Dirac masses to the active neutrinos. The mechanism is based on the fact that conserved chiral symmetries give massless neutrinos at the renormalizable level. Thus, they acquire very small masses due to non-renormalizable terms. We investigate such models in two aspects. First, we find UV completions for them and then we investigate the possibility of giving leptogenesis. We find that these models offer new possibilities for leptogenesis. Models with Majorana masses can exhibit standard leptogenesis. Models with Dirac masses can provide a realization of Dirac type leptogenesis with mass scale that can be as low as 10 TeV.
Measurements of lifetimes can be done in two ways. For very short lived particles, the width can be measured. For long lived ones, the lifetime can be directly measured, for example, using a displaced vertex. Practically, the lifetime cannot be extracted for particles with intermediate lifetimes. We show that for such cases information about the lifetime can be extracted for heavy colored particles that can be produced with known polarization. For example, a $t$-like particle with intermediate lifetime hadronizes into a superposition of the lowest two hadronic states, $T^*$ and $T$ (the equivalent of $B^*$ and $B$). Depolarization effects are governed by time scales that are much longer than the hadronization time scale, $\lqcd^{-1}$. After a time of order $1/Δm$, with $Δm \equiv m(T^*)-m(T)$, half of the initial polarization is lost. The polarization is totally lost after a time of order $1/Γ_γ$, with $Γ_γ= Γ(T^*\to Tγ)$. Thus, by comparing the initial and final polarization, we get information on the particle's lifetime.
If the LHC experiments discover new particles that couple to the Standard Model fermions, then measurements by ATLAS and CMS can contribute to our understanding of the flavor puzzles. We demonstrate this statement by investigating a scenario where extra SU(2)-singlet down-type quarks are within the LHC reach. By measuring masses, production cross sections and relative decay rates, minimal flavor violation (MFV) can in principle be excluded. Conversely, these measurements can probe the way in which MFV applies to the new degrees of freedom. Many of our conclusions are valid in a much more general context than this specific extension of the Standard Model.
We argue that fast interactions of the lightest singlet neutrino $N_1$ would project part of a preexisting lepton asymmetry $L_p$ onto a direction that is protected from $N_1$ washout effects, thus preventing it from being erased. In particular, we consider an asymmetry generated in $N_2$ decays, assuming that $N_1$ interactions are fast enough to bring $N_1$ into full thermal equilibrium. If $N_1$ decays occur at $T\gsim 10^9$ GeV, that is, before the muon Yukawa interactions enter into thermal equilibrium, then generically part of $L_p$ survives. In this case some of the constraints implied by the standard $N_1$ leptogenesis scenario hold only if $L_p \approx 0$. For $T\lsim 10^9$ GeV, $L_p$ is generally erased, unless special alignment/orthogonality conditions in flavor space are realized.
We study a simple extension of the standard model where scalar singlets that mix with the Higgs doublet are added. This modification to the standard model could have a significant impact on Higgs searches at the LHC. The Higgs doublet is not a mass eigenstate and therefore the expected nice peak of the standard model Higgs disappears. We analyze this scenario finding the required properties of the singlets in order to make the Higgs "invisible" at the LHC. In some part of the parameter space even one singlet could make the discovery of the SM Higgs problematic. In other parts, the Higgs can be discovered even in the presence of many singlets.
We obtain model independent relations among neutrino masses and leptogenesis parameters. We find exact relations that involve the CP asymmetries $ε_{N_α}$, the washout parameters $\tilde m_α$ and $θ_{αβ}$, and the neutrino masses $m_i$ and $M_α$, as well as powerful inequalities that involve just $\tilde m_α$ and $m_i$. We prove that the Yukawa interactions of at least two of the heavy singlet neutrinos are in the strong washout region ($\tilde m_α\gg10^{-3} eV$).
The weak phase γcan be determined using untagged B^0\to DK_S or B_s\to Dϕ, Dη^{(')} decays. In the past, the small lifetime difference y\equiv ΔΓ/(2Γ) has been neglected in B^0, while the CP violating parameter ε\equiv 1-|q/p|^2 has been neglected in both B^0-\bar B^0 and B_s-\bar B_s mixing. We estimate the effect of neglecting y and ε. We find that in D decays to flavor states this introduces a systematic error, which is enhanced by a large ratio of Cabibbo-allowed to doubly Cabibbo-suppressed D decay amplitudes.
We analyze various theoretical aspects of CP violation in singly Cabibbo suppressed (SCS) D-meson decays, such as $D \to K K,ππ$. In particular, we explore the possibility that CP asymmetries will be measured close to the present level of experimental sensitivity of $O(10^{-2})$. Such measurements would signal new physics. We make the following points: (i) The mechanism at work in neutral D decays could be indirect or direct CP violation (or both). (ii) One can experimentally distinguish between these possibilities. (iii) If the dominant CP violation is indirect, then there are clear predictions for other modes. (iv) Tree-level direct CP violation in various known models is constrained to be much smaller than $10^{-2}$. (v) SCS decays, unlike Cabibbo favored or doubly Cabibbo suppressed decays, are sensitive to new contributions from QCD penguin operators and especially from chromomagnetic dipole operators. This point is illustrated with supersymmetric gluino-squark loops, which can yield direct CP violating effects of $O(10^{-2})$.
New physics contributions to B_s-\bar{B}_s mixing can be parametrized by the size (r_s^2) and the phase (2θ_s) 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 A_SL to obtain the first constraint on the semileptonic CP asymmetry in B_s decays, A_SL^s=-0.008 \pm 0.011. Then we combine recent measurements by the CDF and D0 collaborations - the mass difference (ΔM_s), the width difference (ΔΓ_s) and A_SL^s - to constrain 2θ_s. The errors on ΔΓ_s and A_SL^s should still be reduced to have a sensitive probe of the phase, yet the central values are such that the regions around 2θ_s ~ 3π/2 and, in particular, 2θ_s ~ π/2, are disfavored.
We investigate whether postulating the existence of Lorentz-violating, CPT-conserving interactions allows three-neutrino solutions to the LSND anomaly that are also consistent with all other neutrino data. We show that Lorentz-violating interactions that couple only to one of the active neutrinos have the right properties to explain all the data. The details of the data make this solution unattractive. We find, for example, that a highly non-trivial energy dependence of the Lorentz-violating interactions is required.
Methods are proposed for measuring the ratio $R^{+/0} = Γ(Υ(4S) \to B^+ B^-)/$ $Γ(Υ(4S) \to \bo \ob)$ without assuming isospin invariance in exclusive hadronic decays such as $B \to J/ψK$. The validity of isospin invariance in various B decays is discussed. Isospin violations are expected to be especially small in inclusive semileptonic decays, so that $Γ(B^+ \to X_c^0 \ell^+ ν_\ell) = Γ(B^0 \to X_c^- \ell^+ ν_\ell)$ is expected to hold at the sub-percent level. Suggestions are made for utilizing this relation. The accuracy of a ``double-tag'' method for obtaining $R^{+/0}$, such as that used for measuring the $D^+ D^-/D^0 \od$ ratio at the $ψ'' \equiv ψ(3770)$, is also estimated.
While flavor symmetries are useful for studying hadronic B decays, symmetry relations for amplitudes and decay rates are usually violated by first order symmetry breaking corrections. We point out two cases in which first order symmetry breaking is suppressed by a small ratio of amplitudes: (1) An isospin sum rule for four $B\to Kπ$ decays, where isospin breaking is shown to be negligible. (2) An SU(3) sum rule for pairs of $B\to Kπ$ and $B\to Kη_8$, generalized to pairs of $B\to Kπ, B\to Kη$ and $B\to Kη'$.