$e^{+}e^{-}$ pairs from a nuclear transition signaling an elusive light neutral boson

TL;DR

This study reports the observation of electron-positron pairs from a nuclear transition in oxygen-16, suggesting the possible emission of a light neutral boson with a mass around 8.5 MeV/c², which could be relevant for dark matter research.

Contribution

First evidence of a nuclear transition possibly mediated by a light neutral boson, indicating a new particle with a mass near 8.5 MeV/c².

Findings

Observation of e+e- pairs in 16O decay with an anomalous angular correlation

Detection of a sharp peak consistent with a 8.5 MeV/c² boson

Implication for light dark matter searches

Abstract

Electron-positron pairs have been observed in the 10.95-MeV $0^-\to0^+$ decay in $^{16}$O. The branching ratio of the e$^+$e$^-$ pairs compared to the 3.84-MeV $0^-\to2^+$ $\gamma$ decay of the level is deduced to be $20(5)\times10^{-5}$. This magnetic monopole (M0) transition cannot proceed by $\gamma$-ray decay and is, to first order, forbidden for internal pair creation. However, the transition may also proceed by the emission of a light neutral $0^{-}$ or $1^{+}$ boson. Indeed, we do observe a sharp peak in the $e^{+}e^{-}$ angular correlation with all the characteristics belonging to the intermediate emission of such a boson with an invariant mass of 8.5(5) MeV/c$^2$. It may play a role in the current quest for light dark matter in the universe.