Non-universal gauge bosons Z′ and lepton flavor-violating tau decays

Abstract

There are many models beyond the standard model predicting the existence of non-universal gauge bosons Z ′ , which can give rise to very rich phenomena.We calculate the contributions of the non-universal gauge bosons Z ′ , predicted by topcolor-assisted technicolor (TC2) models and flavor-universal TC2 models, to the lepton flavor-violation tau decays τ → l i γ and τ → l i l j l k . We find that the branching ratio B r (τ −→ l i l j l k ) is larger than that of the process τ −→ l i γ in all of the parameter space. Over a sizable region of the parameter space, we have B r (τ −→ l i l j l k ) ∼ 10 −8 , which may be detected in the future experiments. * E-mail:cxyue@public.xxptt.ha.cn 1 Although the standard model (SM) has been successful in describing the physics of electroweak interactions, it is quite possible that the SM is only an effective theory valid below some high energy scale. Extra gauge bosons Z ′ are the best motivated extensions of the SM. If discovered they would represent irrefutable proof of new physics, most likely that the SM gauge groups must be extended [1]. If these extensions are associated with flavor symmetry breaking, the gauge interactions will not be flavor-universal [2], which predict the existence of non-universal gauge bosons Z ′ . Furthermore, the possible anomalies in the Z pole bb asymmetries may suggest a non-universal Z ′ [3].After the mass diagonalization from the flavor eigenbasis into the mass eigenbasis, the non-universal gauge interactions result in the tree level flavor-changing neutral currents (FCNC's) couplings. Thus, the non-universal gauge bosons Z ′ may have significant contributions to some FCNC processes. For example, the non-universal gauge bosons Z ′ predicted by strong top dynamical models, such as topcolor-assisted technicolor (TC2) models[4] and flavor-universal TC2 models [5], can give significant contributions to some FCNC processes [6,7,8]. If these effects are indeed detected at LHC, LC or other experiments, it will be helpful to identify the gauge bosons Z ′ , and hence unravel underlying processes, such as l i → l j γ and l i → l j l k l l . These LFV processes are practically suppressed to zero in the SM, due to the unitarity of the leptonic analog of CKM mixing matrix and the near masslessness of the three neutrinos. Thus, the observation of any rate for one of these processes would be a signal of new physics. The improvement of their experimental measurements forces one to make more elaborate theoretical calculation in the framework of some specific models beyond the SM and see whether the LFV effects can be tested in the future experiments. For instance, the LFV tau decays, such as τ → µγ, τ → eγ, τ → 3µ and τ → 3e, have been studied in a model independent way in Ref. [12], in the SM with extended right-handed and left-handed neutrino sectors [13], in supersymmetric 2 models [14], in the two Higgs doublets model type III [15] and in Zee model [16].In this letter, we focus our attention on the LFV decays of tau lepton. Now the bounds on the LF...

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