Search for the X17 particle in $$^{7}\textrm{Li}(\textrm{p},\textrm{e}^+ \textrm{e}^{-}) ^{8}\textrm{Be}$$ processes with the MEG II detector

The observation of a resonance structure in the opening angle of the electron-positron pairs in the 7Li(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 4He and 12C nuclei with the same experimental technique. The MEG II apparatus at PSI, designed to search for the μ+→e+γ 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 γ and the e+e- pair emerging from the 8Be∗ 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 γ emission) of the two resonances to X17 were set, R17.6<1.8×10-6 and R18.1<1.2×10-5 in the mass range between 16.5MeV/c2 and 17.1MeV/c2.