Probing new $U(1)$ gauge symmetries via exotic $Z\to Z^{\prime }{\hat{I}}^3$ decays

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 $Z-Z^{\prime }-{\hat{I}}^3$ vertex function, reviewing the connection between the full model and the effective Wess-Zumino operator. We calculate the exotic $Z\to Z^{\prime }{\hat{I}}^3$ 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 $Z\to Z_{BL}^{\prime }{\hat{I}}^3$ 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 $Z\to Z_B^{\prime }{\hat{I}}^3$ decay width. We show that the current LEP limits on the exotic $Z\to Z_B^{\prime }{\hat{I}}^3$ 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 $Z\to Z_B^{\prime }{\hat{I}}^3$ exotic decay, and emphasize how the $Z\to Z^{\prime }{\hat{I}}^3$ decay is emblematic of new anomalous $U(1)$ gauge symmetries.

17 pages, 7 Figures