Search for the X17 particle in 7Li(p, e+e−)8Be processes with the MEG II detector

The observation of a resonance structure in the opening angle of the electron-positron pairs in the Li-7(p,e(+)e(-)) 8Be reaction was claimed and interpreted as the production and subsequent decay of a hypothetical particle (X17). Similar excesses, consistent with this particle, were later observed in processes involving He-4 and C-12 nuclei with the same experimental technique. The MEG II apparatus at PSI, designed to search for the mu (+) -> e(+)gamma decay, can be exploited to investigate the existence of this particle and study its nature. Protons from a Cockroft-Walton accelerator, with an energy up to 1.1 MeV, were delivered on a dedicated Li-based target. The gamma and the e(+)e(-)pair emerging from the Be-8(& lowast;) transitions were studied with calorimeters and a spectrometer, featuring a broader angular acceptance than previous experiments. We present in this paper the analysis of a four-week data-taking in 2023 with a beam energy of 1080 keV, resulting in the excitation of two different resonances with Q-value 17.6 and 18.1 MeV. No significant signal was found, and limits at 90% C.L. on the branching ratios (relative to the gamma emission) of the two resonances to X17 were set, R-17.6 < 1.8 x 10(-6 )and R-18.1 < 1.2 x 10(-5) in the mass range between 16.5 MeV/c(2) and 17.1 MeV/c(2).