On the Atomki nuclear anomaly after the MEG-II result

TL;DR

This paper critically examines the Atomki nuclear anomalies in light of recent MEG-II results, exploring whether a new boson, particularly a scalar, could explain the observed effects while remaining consistent with other experimental constraints.

Contribution

It investigates the phenomenology of a spin-2 state and re-evaluates the scalar boson hypothesis for the Atomki anomalies, considering recent experimental constraints.

Findings

Spin-2 state is disfavoured by SINDRUM constraints.

A scalar boson can explain Helium and Carbon anomalies.

Scalar explanation remains compatible with other experimental data.

Abstract

Recent experimental results from the Atomki collaboration have reported the observation of anomalous effects in Beryllium, Helium and Carbon nuclear transitions that could hint at physics beyond the Standard Model. However, the MEG-II experiment has recently found no significant anomalous signal in the Beryllium transition ${^8}\text{Be}^\star\to{^8}\text{Be}+e^+e^-$. In view of this result, we critically re-examine the possible theoretical interpretations of the anomalies observed by the Atomki experiment in terms of a new boson $X$ with mass around $17\;$MeV. The present work aims to study the phenomenology of a spin-2 state and revisit the possibility of a pure CP-even scalar, which was initially dismissed due to its inability to explain the Beryllium anomalous signal. Our analysis shows that a spin-2 state is highly disfavoured by the SINDRUM constraint while a scalar boson could explain the Helium and Carbon anomalies while being compatible with other experimental constraints.