Hicham (Benmansour)

Results on the X17 search with the MEG-II apparatus

Abstract: A resonant structure was observed in the opening angle of the electron-positron pairs in the  7Li(p,e+e-)8Be reaction and interpreted as the production and subsequent decay of a hypothetical particle (X17). Similar excesses, kinematically 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 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 (with an average thickness of  7 um). The gamma and the e+e- pair emerging from the Be* transitions were studied using calorimeters and a spectrometer, featuring a better angular opening resolution and a larger angular acceptance than previous experiments. We present here the results of the X17 search on a four-week data-taking in 2023 with a proton energy varying from 500 keV to 1080 keV, resulting in the excitation of two different resonances with Q-value 17.6 MeV and 18.1 MeV.