The X17 search with the MEG II apparatus

Based in Debrecen, Hungary, the Atomki group observed in 2015 an anomalous ex- cess in the angular distribution of electron-positron pairs from the 7Li(p, e+e−)8Be reaction. This excess, which could not be explained by the monotonous decline of the Internal Pair Conversion background, was interpreted as evidence for a new light neutral boson with a mass of approximately 17 MeV/c2, nicknamed X17. Additionally, several excesses observed in the decays of excited states of 4He and 12C were found to be kinematically consistent with X17, further thickening the plot. The MEG-II apparatus, originally designed to search for the charged lepton flavour-violating decay μ+ → e+γ, can be adapted to investigate X17’s existence and properties, offering improved resolution and broader angular acceptance with respect to Atomki. Utilizing a Cockcroft-Walton generator that accelerates protons up to 1.1 MeV, MEG-II employs a specially designed lithium target along with a magnetic spectrometer consisting of a cylindrical drift chamber and two fast scintillator arrays immersed in a gradient magnetic field. This setup allows MEG- II to study the 7Li(p,e+e−)8Be reaction and analyze its Angular Opening and energy sum (Esum = E_e+ + E_e− ) spectra. Here, we present the analysis of data collected over four weeks in 2023, with proton energies ranging from 500 keV to 1080 keV, resulting in the excitation of two different resonances with Q-values of 17.6 MeV and 18.1 MeV. No significant excess was found, and limits at 90% C.L. were set on the X17 branching ratios relative to γ emission from the two resonances: RX17/γ,17.6 < 1.8 × 10−6 and RX17/γ,18.1 < 1.2 × 10−5. The X17 hypothesis, based on Atomki results, is incompatible at 94% confidence with our best fit.