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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 beunderstood by taking into account the internal and external pair creationsfollowing the direct proton radiative capture by $^{3}$H. However, theseprocesses cannot account for the observed peak around 115$^\circ$ in theangular correlation spectra. This anomalous excess of $e^+e^-$ pairs can bedescribed by the creation and subsequent decay of a light particle during thedirect 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 themass and branching ratio ($B_x=5.1(13)\times10^{-6}$), this is likely the sameX17 particle, which we recently suggested [Phys. Rev. Lett. 116, 052501 (2016)]for describing the anomaly observed in the decay of $^8$Be.

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