Observation of the X17 anomaly in the decay of the Giant Dipole Resonance of $^8$Be (2308.06473v1)
Abstract: Angular correlation spectra of $e+e-$ pairs produced in the ${7}$Li($p$,$\gamma$)${8}$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 ($\Gamma$=1.5~MeV), first excited state in $8$Be. The invariant mass of the particle, which was derived to be $m_Xc2 = 16.95 \pm 0.48$(stat.)~MeV, agrees well with our previously published values.