A Flavor Specific Chiral $U(1)_X$ Framework for Explaining the ATOMKI Anomaly

TL;DR

This paper proposes a chiral $U(1)_X$ gauge extension of the Standard Model with a 17 MeV $Z'$ boson, explaining ATOMKI anomalies while satisfying experimental constraints.

Contribution

It constructs a flavor-specific $U(1)_X$ model with a two Higgs doublet setup that generates axial couplings for the light $Z'$ and is consistent with various experimental bounds.

Findings

The model can explain ATOMKI anomalies within a viable parameter space.

It remains consistent with atomic parity violation and meson decay constraints.

The framework provides a theoretically consistent solution to the observed anomalies.

Abstract

Recent anomalies in nuclear transitions observed by the ATOMKI collaboration suggest the existence of a new boson with a mass of $\sim 17$ MeV. A theoretically consistent interpretation requires a framework that not only matches the kinematics but also reproduces the observed decay rates while satisfying stringent experimental constraints. Among various possibilities, an axial-vector or mixed vector-axial-vector mediator $Z'$ emerges as the most viable candidate. However, getting such couplings for a light $Z'$ gauge boson is highly non trivial task. In this work, we construct a gauged chiral, flavor specific $U(1)_X$ extensions of the Standard Model where the associated $Z'$ boson acts as the $17$ MeV particle. By employing a two Higgs doublet framework, we generate the necessary non-vanishing axial-vector couplings while ensuring gauge anomaly cancellation and consistent fermion mass generation. Focusing on the $^8\mathrm{Be}$ and $^4\mathrm{He}$ signals, we show that in this model the viable parameter space to resolve the ATOMKI anomalies is also consistent with a diverse set of experimental constraints, including atomic parity violation, beam dump experiments, meson decays, and neutrino nucleus and neutrino electron scatterings. Our results demonstrate that this framework offers a theoretically sound and phenomenologically robust solution to the ATOMKI anomaly.