Observation of the X17 anomaly in the decay of the Giant Dipole Resonance of ${}^8$Be

Abstract

Angular correlation spectra of $e^{+}{e}^{-}$ pairs produced in the ${}^7$Li($p$,${\hat{I}}^3$)${}^8$Be nuclear reaction were studied at a proton beam energy of $E_p$~=~4.0~MeV, which corresponds to the excitation energy of the Giant Dipole Resonance (GDR) in ${}^8$Be. The spectra measured show a peak like anomaly at 120${}^{\circ }$ and a broader anomaly also above 140${}^{\circ }$. Both anomalies could consistently be described by assuming that the same hypothetical X17 particle was created both in the ground-state transition and in the transition going to the broad ($\hat{I}“$=1.5~MeV), first excited state in ${}^8$Be. The invariant mass of the particle, which was derived to be $m_X{c}^2=16.95\pm 0.48$(stat.)~MeV, agrees well with our previously published values.

5 pages, 7 figures. arXiv admin note: text overlap with arXiv:2209.10795