New evidence supporting the existence of the hypothetic X17 particle

TL;DR

This study provides new experimental evidence for the existence of the hypothetical X17 particle by observing electron-positron pairs from helium-4, supporting its potential role in explaining nuclear anomalies.

Contribution

The paper reports the detection of a peak consistent with the X17 particle in helium-4, adding evidence to its existence and linking it to previous observations in beryllium-8.

Findings

Observation of a peak at 115° in e+e- angular correlations

Mass of the X17 particle estimated at 16.84 MeV

Supports the hypothesis of X17 as a new light boson

Abstract

We observed electron-positron pairs from the electro-magnetically forbidden M0 transition depopulating the 21.01 MeV 0$^-$ state in $^4$He. A peak was observed in their $e^+e^-$ angular correlations at 115$^\circ$ with 7.2$\sigma$ significance, and could be described by assuming the creation and subsequent decay of a light particle with mass of $m_\mathrm{X}c^2$=16.84$\pm0.16 (stat) \pm 0.20 (syst)$ MeV and $\Gamma_\mathrm{X}$= $3.9\times 10^{-5}$ eV. According to the mass, it is likely the same X17 particle, which we recently suggested [Phys. Rev. Lett. 116, 052501 (2016)] for describing the anomaly observed in $^8$Be.