Shedding light on X17: community report

Eur. Phys. J. C (2023) 83:230 https://doi.org/10.1140/epjc/s10052-023-11271-x Review Shedding light on X17: community report Daniele S. M. Alves1, Daniele Barducci2,3 , Gianluca Cavoto2,3 , Luc Darmé4 , Luigi Delle Rose5,6 , Luca Doria7 , Jonathan L. Feng8 , André Frankenthal9 , Ashot Gasparian10 , Evgueni Goudzovski11 , Carlo Gustavino3 , Shaaban Khalil12 , Venelin Kozhuharov13 , Attila J. Krasznahorkay14 , Tommaso Marchi15 , Manuel Meucci2,3 , Gerald A. Miller16 , Stefano Moretti17 , Marco Nardecchia2,3 , Enrico Nardi18 , Hugo Natal da Luz19 , Giovanni Organtini2,3 , Angela Papa20 , Ann-Kathrin Perrevoort21 , Vlasios Petousis19 , Gabriele Piperno2 , Mauro Raggi2,3,a , Francesco Renga3 , Patrick Schwendimann20 , Rudolf Sýkora19 , Claudio Toni2,3 , Paolo Valente3 , Cecilia Voena3 , Cheuk-Yin Wong22 , Xilin Zhang23 1 Theoretical Division, Los Alamos National Laboratory, Los Alamos, NM 87544, USA 2 Dip. di Fisica Sapienza Univ., p.le A. Moro 2, Rome, Italy 3 INFN sez. Roma, p.le A. Moro 2, Rome, Italy 4 Institut de Physique des 2 Infinis de Lyon (IP2I), UMR5822, CNRS/IN2P3, 69622 Villeurbanne Cedex, France 5 Institut de Fisica d’Altes Energies (IFAE), The Barcelona Institute of Science and Technology, Campus UAB, 08193 Bellaterra, Barcelona, Spain 6 Dipartimento di Fisica, Università della Calabria, 87036 Arcavacata di Rende, Cosenza, Italy 7 Johannes Gutenberg-Universität Mainz, Johann-Joachim-Becher-Weg 45 D, 55128 Mainz, Germany 8 Department of Physics and Astronomy, University of California, Los Angeles, CA 92697, USA 9 Department of Physics, Princeton University, Princeton, USA 10 North Carolina A&T State University, Greensboro, NC 27411, USA 11 School of Physics and Astronomy, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK 12 Center for Fundamental Physics, Zewail City of Science and Technology, 6 October City, Giza 12588, Egypt 13 Faculty of Physics, Sofia University, 5 J. Bourchier Blvd., 1164 Sofia, Bulgaria 14 Institute for Nuclear Research (Atomki), P.O. Box 51, Debrecen 4001, Hungary 15 INFN-Laboratori Nazionali di Legnaro, Legnaro, PD, Italy 16 Department of Physics, University of Washington, Seattle, WA 98195, USA 17 School of Physics and Astronomy, University of Southampton, Southampton SO17 1BJ, UK 18 Istituto Nazionale di Fisica Nucleare, Laboratori Nazionali di Frascati, C.P. 13, 00044 Frascati, Italy 19 Institute of Experimental and Applied Physics, Czech Technical University in Prague, Prague, Czech Republic 20 Paul Scherrer Institute, Würenlingen, Switzerland 21 Institute of Experimental Particle Physics, Karlsruhe Institute of Technology, Karlsruhe, Germany 22 Physics Division, Oak Ridge National Laboratory, Oak Ridge, USA 23 Facility for Rare Isotope Beams, Michigan State University, East Lansing, MI 48824, USA Received: 15 June 2022 / Accepted: 23 January 2023 © The Author(s) 2023 Abstract The workshop “Shedding light on X17” brings together scientists looking for the existence of a possible new light particle, often referred to as X17. This hypothetical particle can explain the resonant structure observed at ∼ 17 MeV in the invariant mass of electron-positron pairs, produced after excitation of nuclei such as 8 Be and 4 He by means of proton beams at the Atomki Laboratory in Debrecen. The purpose of the workshop is to discuss implications of this anomaly, in particular theoretical interpretations as well as present and future experiments aiming at confirming the result and/or at providing experimental evidence for its interpretation. 1 Executive summary The Standard Model (SM) of particle physics has been highly successful in describing the fundamental particles and their interactions in experiments performed in the last decades. Nevertheless, the SM leaves unanswered questions on many aspects of fundamental physics, like dark matter, the origin of matter over anti-matter asymmetry in the Universe, the strong CP problem. It has been considered a low energy limit of a more complete theory implying a huge experimental and theoretical effort to search for new phenomena. One of the most unambiguous signals of physics beyond the SM would be the observation of new particles, which in a e-mail: (corresponding author) 0123456789().: V,-vol 123 230 Page 2 of 66 Eur. Phys. J. C (2023) 83:230 fact are searched for in a variety of experiments over a wide energy mass range. 1.1 Summary of the experimental status of X17 searches In 2016, an experiment conducted at the Atomki laboratory (Debrecen, Hungary) [1] studied the nuclear reaction 7 Li(p, e+ e− )8 Be. The experiment consists in exciting a target nucleus using the proton capture process, and measuring the distribution of θe+ e− , the relative angle between positron and electron (e+ e− ) produced in the Internal Pair Creation (IPC) in the transition from the exited to the ground state of 8 Be. Due to the very low energy of the emerging e+ e− pair, just a few MeV, the measurement of their angles is not a trivial task. This experiment was using a set of multiwire proportional counters placed in front of the ΔE and E detectors to determine the θe+ e− angle. The very thin ΔE detectors were made of plastic scintillators and chosen to provide excellent γ suppression while the much thicker E detectors were used to determine the energy of the electron and positron separately (Figs. 1 and 2). A detailed description of the experimental setup can be found in [2]. The Atomki collaboration observed an enhancement around 140o in the distribution of θe+ e− in contrast with the expectations that are based on the Rose theory [3]. The study of the IPC angular distributions has in fact a long tradition in nuclear physics. It has been used for more than 30 years to study the multi-polarity of nuclear transitions exploiting the different kinematics of magnetic and electric transitions at large angles. After 2016 the Atomki collaboration improved the measurements in many ways [4–6] but the anomaly did not disappear. Moreover no nuclear physics model could explain it. This led to a fascinating explanation of physics beyond the SM, such as the creation of a light boson that decays in the observed e+ e− pair. This particle is now referred to as X17, because of the value of the observed e+ e− invariant mass excess. It should be noticed that 8 Be along with 4 He, is somehow a peculiar nucleus. They both exhibit states with excitation energy larger than 15 MeV with respect to the ground state, very uncommon in nuclear physics. This characteristic makes them particularly suited to search for new particles, being the accessible mass region quite large with respect to other studies of IPC performed on different nuclei. In the first experiment [1] the 7 Li(p, e+ e− )8 Be reaction was induced with a 1 µA proton beam impinging on 15 µg/cm2 thick LiF2 or 700 µg/cm2 Li2 O targets. The proton beam energy was adjustable, so that it was possible to populate both the 18.15 MeV and 17.6 MeV 8 Be excited states. At t (...truncated)