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Register a new userFlavor Changing Effects in Family Nonuniversal Z Models
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Flavor-changing and CP-violating interactions of Z to fermions are generally present in models with extra U(1) gauge symmetry that are string-inspired or related to broken gauged family symmetry. We study the consequences of such couplings in fermion electric dipole moments, muon g-2, and K and B meson mixings. From experimental limits or measured values, we constrain the off-diagonal Z couplings to fermions. Some of these constraints are comparable or stronger than the existing constraints obtained from other observables.
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We explore a Z boson with family-nonuniversal couplings to charged leptons. The general effect of Z-Z mixing, of both kinetic and mass types, is included in the analysis. Adopting a model-independent approach, we perform a comprehensive study of constraints on the leptonic Z couplings from currently available experimental data on a number of flavor-conserving and flavor-changing transitions. Detailed comparisons are made to extract the most stringent bounds on the leptonic couplings. Such information is fed into predictions of various processes that may be experimentally probed in the near future.
Models with a non-universal Z exhibit in general flavor changing neutral currents (FCNC) at tree-level. When the Z couplings favor the third generation, flavor changing transitions of the form Ztc and Ztu could be large enough to be observable at the LHC. In this paper we explore this possibility using the associated production of a single top-quark with the Z and find that integrated luminosities of a few hundred fb$^{-1}$ are necessary to probe the interesting region of parameter space.
Rare $B$ meson decays offer an opportunity to probe a light hidden $Z$ boson. In this work we explore a new channel $B_q to gamma Z$ ($q = d, s$) followed by a cascade decay of $Z$ into an invisible (neutrino or dark matter) or charged lepton pair $ell^+ ell^-$ ($ell=e ,mu)$. The study is based on a simplified effective model where the down quark sector has tiny flavor-changing neutral current couplings with $Z$. For the first time, we calculate ${rm BR}(B_q to gamma Z)$ at the leading power of $1/m_b$ and $1/E_gamma$. Confronting with the strong constraints from semi-invisible decays of $B$ meson, we find that the branching ratio for $B_d to {rm invisible} + gamma$ can be larger than its Standard Model prediction, leaving a large room for new physics, in particular for light dark matter. Additionally, the branching ratio for $B_d to e^+ e^- gamma$ can also be sizable when the corresponding flavor violating $Z$ coupling to quarks is of the axial-vector type. On the other hand, the predicted branching ratios of $B_d to mu^+ mu^- gamma$ and $B_s to ell^+ ell^- gamma$ are severely constrained by the experimental measurements.
We report on a detailed analysis of a SO(10) SUSY GUT model of Dermisek and Raby (DR) with a D3 family symmetry. The model is completely specified in terms of only 24 parameters and is able to successfully describe both quark and lepton masses and mixings, except for |Vub| that turns out to be too low. However, a global fit shows that flavor changing (FC) processes like Bs --> mu+ mu-, Bs-mixing, B+ --> tau+ nu, B --> Xs gamma and B --> Xs l+ l- pose a serious problem to the DR model. The simultaneous description of these FC processes forces squarks to have masses well above 1 TeV, not appealing on grounds of naturalness and probably beyond the reach of the LHC.
A plethora of ultraviolet completions of the Standard Model have extra U(1) gauge symmetries. In general, the associated massive $Z^prime$ gauge boson can mediate flavor-changing neutral current processes at tree level. We consider a situation where the $Z^prime$ boson couples solely via flavor-changing interactions to quarks and leptons. In this scenario the model parameter space is, in general, quite well constrained by existing flavor bounds. However, we argue that cancellation effects shelter islands in parameter space from strong flavor constraints and that these can be probed by multipurpose collider experiments like ATLAS or CMS as well as LHCb in upcoming runs at the LHC. In still allowed regions of parameter space these scenarios may help to explain the current tension between theory and experiment of $(g-2)_mu$ as well as a small anomaly in $tau$ decays.
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