FCNC decays of standard model fermions into a dark photon

Abstract

We analyze a new class of FCNC processes, the f → f γ decays of a fermion f into a lighter (same-charge) fermion f plus a massless neutral vector boson, a dark photonγ. A massless dark photon does not interact at tree level with observable fields, and the f → f γ decay presents a characteristic signature where the final fermion f is balanced by a massless invisible system. Models recently proposed to explain the exponential spread in the standard-model Yukawa couplings can indeed foresee an extra unbroken dark U (1) gauge group, and the possibility to couple on-shell dark photons to standard-model fermions via one-loop magnetic-dipole kind of FCNC interactions. The latter are suppressed by the characteristic scale related to the mass of heavy messengers, connecting the standard model particles to the dark sector. We compute the corresponding decay rates for the top, bottom, and charm decays (t → cγ, uγ, b → sγ, dγ, and c → uγ), and for the charged-lepton decays (τ → µγ, eγ, and µ → eγ) in terms of model parameters. We find that large branching ratios for both quark and lepton decays are allowed in case the messenger masses are in the discovery range of the LHC. Implications of these new decay channels at present and future collider experiments are briefly discussed.arXiv:1607.05928v2 [hep-ph]

Radiative Yukawa couplings in the simplest left-right symmetric model

“…This mechanism requires the existence of new U (1) F charged dark-scalar fields coupled to dark-fermions and messenger fields only. Remarkably, as suggested in [27], these new fields, when weakly coupled, can naturally explain the correct CKM matrix pattern in this context, provided the U (1) F charges are multiple integers of some unity charge. In Table 3 we report the numerical values of the lowest minima of v R and W R mass, as a function of ξ and in unity of couplings λ ∼ g LR = 1.…”

“…A generalization of the scenario that correctly reproduces the measured CKM mixing can be found in [27]. Theĝ L andĝ R constants in Eq.…”

“…(49) we obtain the following prediction for the generic mass M Q i of the dark fermion associated to the SM quark q i , as a function of the quark mass m i [27]:…”

“…(41) to the flavour mixing effects encapsulated in the CKM matrix. As explained in [27], this can be done by extending the Lagrangian of Eq. (41) to…”

“…(53) in the top-quark case, it is possible to compute the lowest minimum ofm as a function of ξ, which is given by [27] …”