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

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.