New anomaly observed in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mmultiscripts><mml:mi>He</mml:mi><mml:mprescripts/><mml:none/><mml:mn>4</mml:mn></mml:mmultiscripts></mml:math> supports the existence of the hypothetical X17 particle
A. Krasznahorkay, M. Csatlós, L. Csige, J. Gulyás, A. Krasznahorkay, B. M. Nyakó, I. Rajta, J. Timár, I. Vajda, N.J. Sas
Abstract
Angular correlation spectra of ${e}^{+}{e}^{\ensuremath{-}}$ pairs produced in the $^{3}\mathrm{H}(p,{e}^{+}{e}^{\ensuremath{-}})^{4}\mathrm{He}$ 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 proton capture by $^{3}\mathrm{H}$. However, these processes cannot account for an observed peak around ${115}^{\ensuremath{\circ}}$ in the angular correlation spectra. This anomalous excess of ${e}^{+}{e}^{\ensuremath{-}}$ 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 ${m}_{\mathrm{X}}{c}^{2}=16.94\ifmmode\pm\else\textpm\fi{}0.12(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}0.21(\mathrm{syst})$ MeV. According to the mass this is likely the same X17 particle, which we recently suggested [Phys. Rev. Lett. 116, 042501 (2016)] for describing the anomaly observed in the decay of $^{8}\mathrm{Be}$.