The 17 MeV Anomaly in Beryllium Decays and $U(1)$ Portal to Dark Matter

TL;DR

This paper investigates a potential new light gauge boson, $Z'$, as an explanation for the 17 MeV anomaly in beryllium decays, exploring its role as a portal to dark matter and analyzing experimental constraints.

Contribution

It introduces a detailed analysis of $U(1)$ gauge models, distinguishing hidden and non-hidden scenarios, and evaluates their compatibility with experimental data and dark matter searches.

Findings

Hidden $U(1)$ models are excluded by experiments.

Non-hidden $U(1)$ models remain viable for MeV-scale dark matter.

Future detectors can test the $U(1)$ portal scenario.

Abstract

The experiment of Krasznahorkay \textit{et al} observed the transition of a $\rm{^{8}Be}$ excited state to its ground state and accompanied by an emission of $e^{+}e^{-}$ pair with 17 MeV invariant mass. This 6.8$\sigma$ anomaly can be fitted by a new light gauge boson. We consider the new particle as a $U(1)$ gauge boson, $Z'$, which plays as a portal linking dark sector and visible sector. In particular, we study the new $U(1)$ gauge symmetry as a hidden or non-hidden group separately. The generic hidden $U(1)$ model, referred to as dark $Z$ model, is excluded by imposing various experimental constraints. On the other hand, a non-hidden $Z'$ is allowed due to additional interactions between $Z'$ and Standard Model fermions. We also study the implication of the dark matter direct search on such a scenario. We found the search for the DM-nucleon scattering excludes the range of DM mass above 500 MeV. However, the DM-electron scattering for MeV-scale DM is still allowed by current constraints for non-hidden $U(1)$ models. It is possible to test the underlying $U(1)$ portal model by the future Si and Ge detectors with $5e^{-}$ threshold charges.