Abstract
New U(1) gauge theories involving Standard Model (SM) fermions typically require additional electroweak fermions for anomaly cancellation. We study the non-decoupling properties of these new fermions, called anomalons, in the Z − Z′ − γ vertex function, reviewing the connection between the full model and the effective Wess-Zumino operator. We calculate the exotic Z → Z′γ decay width in U(1)B−L and U(1)B models, where B and L denote the SM baryon and lepton number symmetries. For U(1)B−L gauge symmetry, each generation of SM fermions is anomaly free and the exotic Z → {Z}_{BL}^{prime}gamma decay width is entirely induced by intragenerational mass splittings. In contrast, for U(1)B gauge symmetry, the existence of two distinct sources of chiral symmetry breaking enables a heavy, anomaly-free set of fermions to have an irreducible contribution to the Z → {Z}_B^{prime}gamma decay width. We show that the current LEP limits on the exotic Z → {Z}_B^{prime}gamma decay are weaker than previously estimated, and low-mass {Z}_B^{prime } dijet resonance searches are currently more constraining. We present a summary of the current collider bounds on U(1)B and a projection for a TeraZ factory on the Z → {Z}_B^{prime}gamma exotic decay, and emphasize how the Z → Z′γ decay is emblematic of new anomalous U(1) gauge symmetries.
Highlights
U(1) gauge extensions to the Standard Model (SM) include refs. [18,19,20,21]
We have presented the calculation and phenomenology of the exotic Z decay to a Z boson and a photon
While reminiscent of textbook calculations of anomaly coefficients, focuses on the full Z − Z − γ vertex function, where the various Ward-Takahashi identities are obtained by taking the appropriate divergences of the external currents
Summary
We calculate the partial width for an exotic decay of the SM Z boson decaying to a Z boson and a photon, where the loop is mediated by fermions. Where Q and m are the electric charge and the mass of the fermion in the loop, gv, ga are the vector and axial coupling factors to the denoted massive Z and Z gauge bosons, p1 and p2 are the external outgoing momenta and k is the loop momentum, as depicted in figure 1. Since we calculate in the flavor conserving limit, a given mass eigenstate fermion can only have one non-zero axial-vector coupling At this point, we could naively adopt the method by Rosenberg [38] to set w and z for each fermion such that the vector WIs are vanishing and the anomaly contributes only the axial-vector divergence. Since the anomaly cancelling fermions can become massive independently of the SM Higgs vev, their contribution to the Z −Z −γ vertex can be non-decoupling regardless of the scale set by the Z mass. The SM-like nature of the 125 GeV Higgs precludes this scenario, but it is a curious fact that the decoupling of anomaly free sets is not guaranteed in theories with two sources of chiral symmetry breaking
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