Probing new U(1) gauge symmetries via exotic Z → Z′γ decays

Abstract

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 ′ γ $$ {Z}_{BL}^{\prime}\gamma $$ 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 ′ γ $$ {Z}_B^{\prime}\gamma $$ decay width. We show that the current LEP limits on the exotic Z → Z B ′ γ $$ {Z}_B^{\prime}\gamma $$ decay are weaker than previously estimated, and low-mass Z B ′ $$ {Z}_B^{\prime } $$ 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 ′ γ $$ {Z}_B^{\prime}\gamma $$ exotic decay, and emphasize how the Z → Z′γ decay is emblematic of new anomalous U(1) gauge symmetries.

Keywords

• Anomalies in Field and String Theories
• Beyond Standard Model
• Effective Field Theories
• Spontaneous Symmetry Breaking