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

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