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A resonant structure was observed in the opening angle of the electron-positron pairs in the $^{7}$Li(p,e$^{+}$e$^{-}$) $^{8}$Be 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 $^{4}$He and $^{12}$C nuclei with the same experimental technique.
The MEG-II apparatus at PSI, designed to search for the $\mu^+ \rightarrow e^+ \gamma$ 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~$\mu$m). 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.
Coordinate ZOOM