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Isospin Symmetry Breaking Disclosed in the Decay of Three-Proton Emitter ${}^{20}{Al}$

X.-D. Xu ^1,2,3,*, I. Mukha ^3,†, J. G. Li ^1,2,4, S. M. Wang ^5,6, L. Acosta ^7,8, M. Bajzek^3,9,10, E. Casarejos ¹¹, D. Cortina-Gil ¹², J. M. Espino ¹³ et al.

A. Fomichev ¹⁴, H. Geissel^3,9,‡, J. Gómez-Camacho ¹³, L. V. Grigorenko ^14,15,16, O. Kiselev ³, A. A. Korsheninnikov ¹⁶, D. Kostyleva³, N. Kurz ³, Yu. A. Litvinov ³, I. Martel ¹⁷, C. Nociforo³, M. Pfützner ^18,3, C. Rodríguez-Tajes¹⁹, C. Scheidenberger^3,9,20, M. Stanoiu ²¹, K. Sümmerer ³, H. Weick ³, P. J. Woods²², and M. V. Zhukov ²³

¹Institute of Modern Physics, Chinese Academy of Science, Lanzhou 730000, China
²School of Nuclear Science and Technology, University of Chinese Academy of Sciences, Beijing 100049, China
³GSI Helmholtzzentrum für Schwerionenforschung GmbH, 64291 Darmstadt, Germany
⁴Southern Center for Nuclear-Science Theory (SCNT), Institute of Modern Physics, Chinese Academy of Sciences, Huizhou 516000, China
⁵Key Laboratory of Nuclear Physics and Ion-beam Application (MOE), Institute of Modern Physics, Fudan University, Shanghai 200433, China
⁶Shanghai Research Center for Theoretical Nuclear Physics, NSFC and Fudan University, Shanghai 200438, China
⁷Instituto de Estructura de la Materia, CSIC, 28006 Madrid, Spain
⁸Instituto de Física, Universidad Nacional Autónoma de México, Apartado Postal 20-364, Mexico City 01000, Mexico
⁹II.Physikalisches Institut, Justus-Liebig-Universität, 35392 Gießen, Germany
¹⁰Faculty of Science, University of Zagreb, 10000 Zagreb, Croatia
¹¹CINTECX, Universidade de Vigo, E-36310 Vigo, Spain
¹²Instituto de Física Corpuscular, CSIC - Universidad de Valencia, 46980, Paterna, Valencia, Spain
¹³Department of Atomic, Molecular and Nuclear Physics, University of Seville, 41012 Seville, Spain
¹⁴Flerov Laboratory of Nuclear Reactions, JINR, 141980 Dubna, Russia
¹⁵National Research Nuclear University “MEPhI”, 115409 Moscow, Russia
¹⁶National Research Centre “Kurchatov Institute”, Kurchatov square 1, 123182 Moscow, Russia
¹⁷University of Huelva, 21007 Huelva, Spain
¹⁸Faculty of Physics, University of Warsaw, 02-093 Warszawa, Poland
¹⁹Universidade de Santiago de Compostela, 15782 Santiago de Compostela, Spain
²⁰Helmholtz Research Academy Hesse for FAIR (HFHF), GSI Helmholtz Center for Heavy Ion Research, Campus Gießen, 35392 Gießen, Germany
²¹IFIN-HH, Post Office Box MG-6, Bucharest, Romania
²²University of Edinburgh, EH1 1HT Edinburgh, United Kingdom
²³Department of Physics, Chalmers University of Technology, S-41296 Göteborg, Sweden

^*Contact author: xiaodong.xu@impcas.ac.cn
^†Contact author: I.Mukha@gsi.de
^‡Deceased.

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Phys. Rev. Lett. 135, 022502 – Published 10 July, 2025

DOI: https://doi.org/10.1103/hkmy-yfdk

Abstract

The previously unknown nucleus ${}^{20}{Al}$ has been observed for the first time by detecting its in-flight decays. Tracking trajectories of all decay products with silicon microstrip detectors allowed for a conclusion that ${}^{20}{Al}$ is unbound with respect to three-proton ( $3 p$ ) emission. The $3 p$ -decay energy of the ${}^{20}{Al}$ ground state has been determined to be $1.93 (_{- 0.10}^{+ 0.12}) MeV$ through a detailed study of angular correlations of its decay products, ${}^{17}{Ne} + p + p + p$ . This value is significantly smaller than the predictions inferred from the isospin symmetry by using the known neutron separation energy of its mirror nucleus ${}^{20}N$ , which indicates a possible isospin symmetry breaking in the mirror nuclei ${}^{20}{Al}$ and ${}^{20}N$ . This observed isospin symmetry breaking is supported by the calculations of the continuum embedded theoretical frameworks, describing the observed ${}^{20}{Al}$ ground state as a $1 p$ $s$ -wave state with a spin-parity of $1^{-}$ , which differs from the spin-parity ( $2^{-}$ ) of the ${}^{20}N$ ground state. The ${}^{20}{Al}$ ground state decays by sequential $1 p − 2 p$ emission via the intermediate ground state of ${}^{19}{Mg}$ , which is the first observed case of “daughter” $2 p$ radioactivity following $1 p$ decay of the parent state.

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Phys. Rev. Lett. 135, 022502– Published 10 July, 2025

Vol. 135, Iss. 2 — 11 July 2025

Received 24 December 2024
Revised 20 April 2025
Accepted 31 May 2025

DOI: https://doi.org/10.1103/hkmy-yfdk

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

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Isospin Symmetry Breaking Disclosed in the Decay of Three-Proton Emitter ${}^{20}{Al}$

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Isospin Symmetry Breaking Disclosed in the Decay of Three-Proton Emitter Al20

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Isospin Symmetry Breaking Disclosed in the Decay of Three-Proton Emitter ${}^{20}{Al}$