Eur. Phys. J. C (2025) 85: 763
https://doi.org/10.1140/epjc/s10052-025-14345-0

Regular Article - Experimental Physics

Search for the X17 particle in $_{}^7\text{Li}{(\text{p},{\text{e}}^{+}{\text{e}}^{-})}^8\text{Be}$ processes with the MEG II detector

¹ Joint Institute for Nuclear Research, 141980, Dubna, Russia
² INFN Sezione di Pisa, Dipartimento di Fisica dell’Università, Largo B. Pontecorvo 3, 56127, Pisa, Italy
³ ICEPP, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, 113-0033, Tokyo, Japan
⁴ INFN Sezione di Genova, Dipartimento di Fisica dell’Università, Via Dodecaneso 33, 16146, Genoa, Italy
⁵ INFN Sezione di Pavia, Dipartimento di Fisica dell’Università, Via Bassi 6, 27100, Pavia, Italy
⁶ INFN Sezione di Roma, Dipartimento di Fisica dell’Università “Sapienza”, Piazzale A. Moro, 00185, Rome, Italy
⁷ INFN Sezione di Lecce, Dipartimento di Matematica e Fisica dell’Università del Salento, Via per Arnesano, 73100, Lecce, Italy
⁸ Paul Scherrer Institut PSI, 5232, Villigen, Switzerland
⁹ Institute for Particle Physics and Astrophysics, ETH Zürich, Otto-Stern-Weg 5, 8093, Zurich, Switzerland
¹⁰ INFN Sezione di Napoli, Via Cintia, 80126, Naples, Italy
¹¹ Budker Institute of Nuclear Physics of Siberian Branch of Russian Academy of Sciences, 630090, Novosibirsk, Russia
¹² Novosibirsk State Technical University, 630092, Novosibirsk, Russia
¹³ Oliver Lodge Laboratory, University of Liverpool, L69 7ZE, Liverpool, UK
¹⁴ University of California, 92697, Irvine, CA, USA
¹⁵ KEK, High Energy Accelerator Research Organization, 1-1 Oho, 305-0801, Tsukuba, Ibaraki, Japan
¹⁶ Kobe University, 1-1 Rokkodai-cho, Nada-ku, 657-8501, Kobe, Hyogo, Japan
¹⁷ Physics Department, Center for Experimental Nuclear Physics and Astrophysics, University of Washington, 354290, 98195, Seattle, WA, USA
¹⁸ INFN Sezione di Roma, Dipartimento di Fisica dell’Università “Sapienza”, Piazzale A. Moro, 00185, Rome, Italy

^a gianluca.cavoto@roma1.infn.it
^b gianluca.cavoto@roma1.infn.it

Received: 15 November 2024
Accepted: 22 May 2025
Published online: 11 July 2025

Abstract

The observation of a resonance structure in the opening angle of the electron-positron pairs in the $_{}^7$Li(p,e$_{}^{+}$e$_{}^{-}$) $_{}^8$Be 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 $_{}^4$He and $_{}^{12}$C nuclei with the same experimental technique. The MEG II apparatus at PSI, designed to search for the ${\mu }^{+}\to {\text{e}}^{+}\gamma$ 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 $_{}^8{\text{Be}}^{\ast }$ 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, $R_{17.6}<1.8\times {10}^{-6}$ and $R_{18.1}<1.2\times {10}^{-5}$ in the mass range between $16.5\text{MeV}/{\text{c}}^2$ and $17.1\text{MeV}/{\text{c}}^2$.