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 ' - gamma vertex function, reviewing the connection between the full model and the effective Wess-Zumino operator. We calculate the exotic Z -> Z ' gamma 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 -> ZBL ' 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 -> ZB ' gamma decay width. We show that the current LEP limits on the exotic Z -> ZB ' gamma decay are weaker than previously estimated, and low-mass ZB ' 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 -> ZB ' gamma exotic decay, and emphasize how the Z -> Z ' gamma decay is emblematic of new anomalous U(1) gauge symmetries.