Search for the X(17) particle in the 7Li(p, e+e−)8Be reaction with the MEG II detector

A recent measurement of the Internal Pair Creation in the 7Li(p,e+e-)8Be reaction performed at the Atomki laboratory in Hungary showed an unexpected excess in their angular distribution. The significance of the observed bump is ~7 standard deviations, and following measurements of the same reaction but with an upgraded experimental apparatus showed analogous results. Moreover, a measurement of the 3H(p,e+e-)4He reaction showed another significant excess at a different angle. Both these observations can be explained by the creation of a new physics boson with a 17 MeV mass called X(17). The MEG II experiment was designed to search for Charged Lepton Flavor Violation in the mu o e gamma decay, but its detectors are able to measure the 7Li(p,e+e-)8Be reaction as well. The experiment is in fact equipped with a photon detector and a magnetic spectrometer, and has access to a Cockroft-Walton accelerator for the photon detector calibrations. The accelerator can be used to generate the necessary proton beam, and the magnetic spectrometer can be used to measure the decay products of the reaction. An independent measurement with an experiment that guarantees a better invariant mass resolution and a larger angular acceptance can confirm that the anomaly observed at Atomki is not an artifact of the detector geometry, but a real anomaly not explained by any known nuclear physics effect. This measurement is possible only through a redesign of the target region of the CW accelerator used for MEG II calibrations, which was carefully selected after thermomechanical and physics simulations. The signal and background simulations, modeled on the state of the art knowledge of the IPC theory, showed that the MEG II experiment can perform the 7Li(p,e+e-)8Be measurement with a resolution on the invariant mass of 504 keV and reach a sensitivity of 5 in few days of data acquisition. A first prototype of this experimental setup, different from the final design, was built in such a way to not interfere with the MEG II data taking. It was used to take some preliminary data during the 2021 MEG II engineering run to test the CDCH reconstruction algorithms and to find the optimal trigger strategy for the final measurement. The construction of the definitive target region is underway and will be ready for the beginning of 2022, when the X(17) measurement at MEG II will begin.