Particle Physics Models for the 17 MeV Anomaly in Beryllium Nuclear Decays

TL;DR

This paper explores particle physics models, especially a 17 MeV gauge boson, to explain the 8Be decay anomaly, proposing new gauge interactions and analyzing their experimental implications.

Contribution

It introduces anomaly-free models with protophobic gauge bosons that explain the 8Be anomaly and connect to muon g-2 and collider searches.

Findings

A 17 MeV gauge boson can explain the 8Be anomaly.

Models predict lepton couplings around 0.001.

Potential signals in muon g-2 and LHC experiments.

Abstract

The 6.8$\sigma$ anomaly in excited 8Be nuclear decays via internal pair creation is fit well by a new particle interpretation. In a previous analysis, we showed that a 17 MeV protophobic gauge boson provides a particle physics explanation of the anomaly consistent with all existing constraints. Here we begin with a review of the physics of internal pair creation in 8Be decays and the characteristics of the observed anomaly. To develop its particle interpretation, we provide an effective operator analysis for excited 8Be decays to particles with a variety of spins and parities and show that these considerations exclude simple models with scalar particles. We discuss the required couplings for a gauge boson to give the observed signal, highlighting the significant dependence on the precise mass of the boson and isospin mixing and breaking effects. We present anomaly-free extensions of the Standard Model that contain protophobic gauge bosons with the desired couplings to explain the 8Be anomaly. In the first model, the new force carrier is a U(1)B gauge boson that kinetically mixes with the photon; in the second model, it is a U(1)(B-L) gauge boson with a similar kinetic mixing. In both cases, the models predict relatively large charged lepton couplings ~ 0.001 that can resolve the discrepancy in the muon anomalous magnetic moment and are amenable to many experimental probes. The models also contain vectorlike leptons at the weak scale that may be accessible to near future LHC searches.