The observation of a resonance structure in the opening angle of the electron-positron pairs in the Li-7(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 He-4 and C-12 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. The gamma and the e(+)e(-)pair emerging from the Be-8(& lowast;) 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 gamma emission) of the two resonances to X17 were set, R-17.6 < 1.8 x 10(-6 )and R-18.1 < 1.2 x 10(-5) in the mass range between 16.5 MeV/c(2) and 17.1 MeV/c(2).
Search for the X17 particle in 7Li(p, e+e−)8Be processes with the MEG II detector / Null, Null; Afanaciev, K.; Baldini, A. M.; Ban, S.; Benmansour, H.; Boca, G.; Cattaneo, P. W.; Cavoto, G.; Cei, F.; Chiappini, M.; Corvaglia, A.; Maso, G. Dal; De Bari, A.; De Gerone, M.; Barusso, L. Ferrari; Francesconi, M.; Galli, L.; Gallucci, G.; Gatti, F.; Gerritzen, L.; Grancagnolo, F.; Grandoni, E. G.; Grassi, M.; Grigoriev, D. N.; Hildebrandt, M.; Ignatov, F.; Ikeda, F.; Iwamoto, T.; Karpov, S.; Kettle, P. -R.; Khomutov, N.; Kolesnikov, A.; Kravchuk, N.; Krylov, V.; Kuchinskiy, N.; Leonetti, F.; Li, W.; Malyshev, V.; Matsushita, A.; Meucci, M.; Mihara, S.; Molzon, W.; Mori, T.; Nicolò, D. N.; Nishiguchi, H.; Ochi, A.; Ootani, W.; Oya, A.; Palo, D.; Panareo, M.; Papa, A.; Pettinacci, V.; Popov, A.; Renga, F.; Ritt, S.; Rossella, M.; Rozhdestvensky, A.; Scarpellini, S.; Schwendimann, P.; Signorelli, G.; Takahashi, M.; Uchiyama, Y.; Venturini, A.; Vitali, B.; Voena, C.; Yamamoto, K.; Yokota, R.; Yonemoto, T.. - In: EUROPEAN PHYSICAL JOURNAL. C, PARTICLES AND FIELDS. - ISSN 1434-6052. - 85:7(2025), pp. 1-11. [10.1140/epjc/s10052-025-14345-0]
Search for the X17 particle in 7Li(p, e+e−)8Be processes with the MEG II detector
Cavoto, G.
Writing – Original Draft Preparation
;Meucci, M.Software
;Scarpellini, S.Membro del Collaboration Group
;Vitali, B.Membro del Collaboration Group
;Voena, C.Membro del Collaboration Group
;
2025
Abstract
The observation of a resonance structure in the opening angle of the electron-positron pairs in the Li-7(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 He-4 and C-12 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. The gamma and the e(+)e(-)pair emerging from the Be-8(& lowast;) 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 gamma emission) of the two resonances to X17 were set, R-17.6 < 1.8 x 10(-6 )and R-18.1 < 1.2 x 10(-5) in the mass range between 16.5 MeV/c(2) and 17.1 MeV/c(2).| File | Dimensione | Formato | |
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