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 π0 and 8Be∗ decay.
Highlights
New sub-GeV abelian gauge bosons are simple, well-motivated extensions of the Standard Model (SM)
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
Dark force carriers at the MeV scale are a fascinating possibility for physics beyond the Standard Model
Summary
New sub-GeV abelian gauge bosons are simple, well-motivated extensions of the Standard Model (SM). If U(1)D gauges a subset of SM quantum numbers, the new gauge boson couples directly to a current of SM fields, which can radiatively induce a nonzero kinetic mixing as well; popular examples include the anomaly-free combinations B − L [3,4,5], Li − Lj [6, 7], B − 3Li [8], and B − L + xY [9], where x ∈ R These abelian extensions are ubiquitous in the model-building literature and regularly invoked to explain anomalies in dark matter detection [10,11,12] and resolve discrepancies in precision physics measurements [13, 14], to name only a few applications. The appendices contain a taxonomy of the vector and axial coupling dependence of the low-energy constraints we consider, as well as details on the calculation of the A contribution to pseudoscalar decays to lepton pairs
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