A new anomaly observed in $^4$He supports the existence of the hypothetical X17 particle

TL;DR

This study reports an anomaly in $^4$He nuclear reactions that suggests the possible existence of the X17 particle, supporting previous findings of this hypothetical particle in other nuclear decay anomalies.

Contribution

The paper provides experimental evidence for a new anomaly in $^4$He that supports the existence of the X17 particle, linking it to prior observations in $^8$Be decay.

Findings

Observed an excess of $e^+e^-$ pairs at a specific angle.

Estimated the mass of the hypothetical particle as approximately 17 MeV.

Supported the hypothesis that X17 is a real particle related to nuclear anomalies.

Abstract

Energy-sum and angular correlation spectra of $e^+e^-$ pairs produced in the $^{3}$H(p,$\gamma$)$^{4}$He nuclear reaction have been studied at $E_p$=510, 610 and 900 keV proton energies. The main features of the spectra can be understood by taking into account the internal and external pair creations following the direct proton radiative capture by $^{3}$H. However, these processes cannot account for the observed peak around 115$^\circ$ in the angular correlation spectra. This anomalous excess of $e^+e^-$ pairs can be described by the creation and subsequent decay of a light particle during the direct capture process. The derived mass of the particle is $m_\mathrm{X}c^2$=16.94$\pm0.12 (stat) \pm 0.21 (syst)$~MeV. According to the mass and branching ratio ($B_x=5.1(13)\times10^{-6}$), this is likely the same X17 particle, which we recently suggested [Phys. Rev. Lett. 116, 052501 (2016)] for describing the anomaly observed in the decay of $^8$Be.