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Register a new userPurely flavor-changing Z bosons and where they might hide
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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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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.
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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Simple symmetry arguments applied to the third generation lead to a prediction: there exist new sequential Higgs doublets with masses of order $lesssim 5 $ TeV, with approximately universal Higgs-Yukawa coupling constants, $gsim 1$. This is calibrated by the known Higgs boson mass, the top quark Higgs-Yukawa coupling, and the $b$-quark mass. A new massive weak-isodoublet, $H_b$, coupled to the $b$-quark with $gsim 1$ is predicted, and may be accessible to the LHC at $13$ TeV, and definitively at the energy upgraded LHC of $26$ TeV. The extension to leptons generates a new $H_tau$ and a possible $H_{ u_tau}$ doublet. The accessibility of the latter depends upon whether the mass of the $tau$-neutrino is Dirac or Majorana.
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