Light Weakly Coupled Axial Forces: Models, Constraints, and Projections

TL;DR

This paper explores constraints and models of MeV-GeV scale hidden U(1) forces with axial couplings to Standard Model fermions, addressing experimental bounds and potential explanations for low-energy anomalies.

Contribution

It introduces a UV-complete model with adjustable axial and vector couplings, analyzing constraints and experimental prospects for such dark photons.

Findings

Certain parameter regions are consistent with existing constraints.

Projected experiments can probe significant parts of the parameter space.

Axial couplings may explain low-energy anomalies like pi^0 and 8-Be* decay.

Abstract

We investigate the landscape of constraints on MeV-GeV scale, hidden U(1) forces with nonzero axial-vector couplings to Standard Model fermions. While the purely vector-coupled dark photon, which may arise from kinetic mixing, is a well-motivated scenario, several MeV-scale anomalies motivate a theory with axial couplings which can be UV-completed consistent with Standard Model gauge invariance. Moreover, existing constraints on dark photons depend on products of various combinations of axial and vector couplings, making it difficult to isolate the effects of axial couplings for particular flavors of SM fermions. We present a representative renormalizable, UV-complete model of a dark photon with adjustable axial and vector couplings, discuss its general features, and show how some UV constraints may be relaxed in a model with nonrenormalizable Yukawa couplings at the expense of fine-tuning. We survey the existing parameter space and the projected reach of planned experiments, briefly commenting on the relevance of the allowed parameter space to low-energy anomalies in pi^0 and 8-Be* decay.