Light and Heavy $Z'$ from Flavored Chiral $U(1)_X$ Gauge Symmetries: Purely Axial and Mixed Vector-Axial Couplings

TL;DR

This paper introduces a new class of flavor-specific chiral $U(1)_X$ gauge symmetries that produce $Z'$ bosons with purely axial or mixed couplings, enabling diverse phenomenological signatures and flavor-changing effects.

Contribution

It systematically classifies anomaly-free charge assignments and constructs minimal UV completions for flavor-specific $U(1)_X$ models with novel $Z'$ coupling structures.

Findings

Identified charge assignments leading to flavor-changing neutral currents.

Demonstrated $Z'$ can have purely axial or mixed couplings in various mass regimes.

Presented benchmark models with distinct flavor structures and phenomenological implications.

Abstract

Model independent phenomenological studies, ranging from neutrino to B-physics, often consider effective interactions involving either purely vector (V), purely axial vector (A), or mixed vector and axial vector (V, A) couplings. While pure vector $Z'$ interactions can naturally emerge in gauged $U(1)_X$ extensions of the Standard Model, such as the $B-L$ model, generating other coupling structures from a UV complete theory is highly nontrivial. To realize such couplings, we propose a new class of flavor specific chiral $U(1)_X$ gauge symmetries. Gauge anomaly cancellation is achieved by introducing three right-handed neutrinos charged under the $U(1)_X$ symmetry. We systematically classify anomaly free charge assignments and analyze viable ultraviolet completions with minimal scalar content, requiring no additional fermions beyond the three necessary for anomaly cancellation. We present several benchmark models illustrating the range of possible charge assignments, under which the quark and lepton flavor structures can differ substantially, leading to distinct phenomenological signatures. In particular, such non universal charge configurations naturally give rise to $Z'$ mediated flavor changing neutral currents in both the quark and lepton sectors. We also demonstrate that, within this framework, the $Z'$ boson can naturally acquire purely axial vector or mixed vector-axial couplings to the SM fermions, both in the heavy and light $Z'$ regimes.