Excitation Spectra of the <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><mmultiscripts><mrow><mi mathvariant="normal">C</mi></mrow><mprescripts></mprescripts><none></none><mrow><mn>12</mn></mrow></mmultiscripts><mo stretchy="false">(</mo><mi>p</mi><mo>,</mo><mi>d</mi><mo stretchy="false">)</mo></mrow></math> Reaction near the <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><msup><mrow><mi>η</mi></mrow><mrow><mo>′</mo></mrow></msup></mrow></math>-Meson Emission Threshold Measured in Coincidence with High-Momentum Protons

math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><mi>η</mi></mrow></math>-PRiME Collaboration and Super-FRS Experiment Collaboration; R. Sekiya; K. Itahashi; Y. K. Tanaka; S. Hirenzaki; N. Ikeno; V. Metag; M. Nanova; J. Yamagata-Sekihara; V. Drozd; H. Ekawa; H. Geissel; E. Haettner; A. Kasagi; E. Liu; M. Nakagawa; S. Purushothaman; C. Rappold; T. R. Saito; H. Alibrahim Alfaki; F. Amjad; M. Armstrong; K.-H. Behr; J. Benlliure; Z. Brencic; T. Dickel; S. Dubey; S. Escrig; M. Feijoo-Fontán; H. Fujioka; Y. Gao; F. Goldenbaum; A. Graña González; M. N. Harakeh; Y. He; H. Heggen; C. Hornung; N. Hubbard; M. Iwasaki; N. Kalantar-Nayestanaki; M. Kavatsyuk; E. Kazantseva; A. Khreptak; B. Kindler; H. Kollmus; D. Kostyleva; S. Kraft-Bermuth; N. Kurz; B. Lommel; S. Minami; D. J. Morrissey; P. Moskal; I. Mukha; C. Nociforo; H. J. Ong; S. Pietri; E. Rocco; J. L. Rodríguez-Sánchez; P. Roy; R. Ruber; S. Schadmand; C. Scheidenberger; P. Schwarz; V. Serdyuk; M. Skurzok; B. Streicher; K. Suzuki; B. Szczepanczyk; X. Tang; N. Tortorelli; M. Vencelj; T. Weber; H. Weick; M. Will; K. Wimmer; A. Yamamoto; A. Yanai; J. Zhao

doi:10.1103/6vsl-ng7x

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Excitation Spectra of the ${}^{12}C (p, d)$ Reaction near the $η^{'}$ -Meson Emission Threshold Measured in Coincidence with High-Momentum Protons

R. Sekiya ^1,2,3,*, K. Itahashi ^2,3,4,†, Y. K. Tanaka ^5,6,‡, S. Hirenzaki ⁷, N. Ikeno ⁸, V. Metag ⁹, M. Nanova ⁹, J. Yamagata-Sekihara ¹⁰, V. Drozd^6,11 et al. ( $η$ -PRiME Collaboration and Super-FRS Experiment Collaboration)

V. Drozd^6,11, H. Ekawa ⁵, H. Geissel^6,9,§, E. Haettner ⁶, A. Kasagi ^5,12, E. Liu^5,13, M. Nakagawa⁵, S. Purushothaman ⁶, C. Rappold ¹⁴, T. R. Saito^5,6,15,16, H. Alibrahim Alfaki⁶, F. Amjad ⁶, M. Armstrong⁶, K.-H. Behr⁶, J. Benlliure ^17,¶, Z. Brencic ^18,19, T. Dickel ^6,9, S. Dubey ^6,**, S. Escrig ¹⁴, M. Feijoo-Fontán ¹⁷, H. Fujioka ²⁰, Y. Gao^5,13, F. Goldenbaum ^21,††, A. Graña González ^17,§§, M. N. Harakeh ¹¹, Y. He^5,15, H. Heggen⁶, C. Hornung⁶, N. Hubbard ^6,22, M. Iwasaki^2,3, N. Kalantar-Nayestanaki ¹¹, M. Kavatsyuk¹¹, E. Kazantseva⁶, A. Khreptak ^23,24, B. Kindler ⁶, H. Kollmus⁶, D. Kostyleva⁶, S. Kraft-Bermuth ²⁵, N. Kurz ⁶, B. Lommel ⁶, S. Minami ⁶, D. J. Morrissey ²⁶, P. Moskal ²³, I. Mukha ⁶, C. Nociforo⁶, H. J. Ong¹³, S. Pietri ⁶, E. Rocco ⁶, J. L. Rodríguez-Sánchez ^17,27, P. Roy ^6,28, R. Ruber ²⁹, S. Schadmand ⁶, C. Scheidenberger^6,9,30, P. Schwarz⁶, V. Serdyuk²¹, M. Skurzok ²³, B. Streicher⁶, K. Suzuki ^6,21, B. Szczepanczyk⁶, X. Tang ¹³, N. Tortorelli⁶, M. Vencelj ^18,19, T. Weber⁶, H. Weick ⁶, M. Will ⁶, K. Wimmer ⁶, A. Yamamoto³¹, A. Yanai^5,16, and J. Zhao ^6,32,‡‡ ( $η$ -PRiME Collaboration and Super-FRS Experiment Collaboration)

¹Department of Physics, Kyoto University, 606–8502 Kyoto, Japan
²Meson Science Laboratory, RIKEN, Wako, 351–0198 Saitama, Japan
³Nishina Center for Accelerator-Based Science, RIKEN, Wako, 351–0198 Saitama, Japan
⁴Department of Physics, The University of Osaka, Toyonaka, 560–0043 Osaka, Japan
⁵High Energy Nuclear Physics Laboratory, RIKEN, Wako, 351–0198 Saitama, Japan
⁶GSI Helmholtzzentrum für Schwerionenforschung GmbH, 64291 Darmstadt, Germany
⁷Department of Physics, Nara Women’s University, 630–8506 Nara, Japan
⁸Graduate School of Maritime Sciences, Kobe University, 658–0022 Hyogo, Japan
⁹II. Physikalisches Institut, Justus-Liebig-Universität Gießen, 35392 Gießen, Germany
¹⁰Department of Physics, Kyoto Sangyo University, 603–8555 Kyoto, Japan
¹¹ESRIG, University of Groningen, 9747 AA Groningen, The Netherlands
¹²Department of Engineering, Gifu University, 501–1193 Gifu, Japan
¹³Institute of Modern Physics, Chinese Academy of Sciences, 730000 Lanzhou, China
¹⁴Instituto de Estructura de la Materia—CSIC, 28006 Madrid, Spain
¹⁵Lanzhou University, 730000 Lanzhou, China
¹⁶Department of Physics, Saitama University, 338–8570 Saitama, Japan
¹⁷IGFAE, Universidade de Santiago de Compostela, 15782 Santiago de Compostela, Spain
¹⁸Jožef Stefan Institute, 1000 Ljubljana, Slovenia
¹⁹Faculty of Mathematics and Physics, University of Ljubljana, 1000 Ljubljana, Slovenia
²⁰Institute of Science Tokyo, Meguro, 152–8550 Tokyo, Japan
²¹Institut für Kernphysik, Forschungszentrum Jülich, 52425 Jülich, Germany
²²Institut für Kernphysik, Technische Universität Darmstadt, 64289 Darmstadt, Germany
²³Jagiellonian University, 30–348 Kraków, Poland
²⁴INFN, Laboratori Nazionali di Frascati, 00044 Frascati, Roma, Italy
²⁵Institute for Medical Physics and Radiation Protection, TH Mittelhessen University of Applied Sciences, 35390 Gießen, Germany
²⁶National Superconducting Cyclotron Laboratory, Michigan State University, Michigan 48824 East Lansing, USA
²⁷CITENI, Campus Industrial de Ferrol, Universidade da Coruña, 15403 Ferrol, Spain
²⁸Variable Energy Cyclotron Centre, 1/AF–Bidhan Nagar, 700064 Kolkata, India
²⁹Uppsala University, 75220 Uppsala, Sweden
³⁰Helmholtz Forschungsakademie Hessen für FAIR (HFHF), GSI Helmholtzzentrum für Schwerionenforschung, Campus Gießen, 35392 Gießen, Germany
³¹High Energy Accelerator Research Organization (KEK), Tsukuba, 305–0801 Ibaraki, Japan
³²Peking University, 100871 Beijing, China

^*Present address: Department of Physics, The University of Osaka, Toyonaka, 560-0043 Osaka, Japan.
^†Contact author: itahashi@phys.sci.osaka-u.ac.jp
^‡Contact author: yoshiki.tanaka@a.riken.jp
^§Deceased
^¶Present address: Instituto de Física Corpuscular (CSIC—Universitat de València), 46980 Valencia, Spain.
^**Present address: Department of Mathematics, University of Alabama, 35487 Tuscaloosa, USA.
^††Present address: GSI Helmholtzzentrum für Schwerionenforschung GmbH, 64291 Darmstadt, Germany and Faculty of Mathematics and Physics, Bergische Universität Wuppertal, 42119 Wuppertal, Germany.
^‡‡Present address: Beihang University, 100191, Beijing, China.
^§§Present address: CEA, DAM, DIF, F-91297, Arpajon, France.

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Phys. Rev. Lett. 136, 142501 – Published 7 April, 2026

DOI: https://doi.org/10.1103/6vsl-ng7x

Abstract

The missing mass of the ${}^{12}C (p, d)$ reaction has been measured near the $η^{'}$ -meson emission threshold in coincidence with a high-momentum proton to selectively collect $η^{'} − {}^{11}C$ mesic nucleus formation events at GSI, Germany. A 2.5 GeV proton excites a carbon nucleus in order to form an $η^{'}$ -mesic nucleus emitting a deuteron forward with an energy of $\sim 1.5 GeV$ . The deuteron is momentum analyzed by the fragment separator used as a high-resolution spectrometer to deduce the excitation energy of the residual system. The large-acceptance detector WASA surrounding the target identifies high-momentum protons emitted in the decay of the $η^{'}$ -mesic nucleus. The measured semiexclusive spectrum exhibits structures below the threshold though the statistical significance is limited. The spectrum is fitted by theoretically calculated spectra varying optical-potential parameters of the $η^{'}$ -nucleus interaction. The analysis results indicate possible formation of $η^{'}$ -mesic nuclei for the real potential depth of $\sim - 61 MeV$ with a local statistical significance of $3.5 σ$ and, taking into account the look-elsewhere effect, a global significance of $2.1 σ$ .

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Hints of a Nucleus Irked by a Meson Houseguest

Published 7 April, 2026

Experiments with a proton beam striking a carbon target have uncovered events that may be due to a short-lived meson residing within a nucleus.

Physical Review Letters

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Excitation Spectra of the ${}^{12}C (p, d)$ Reaction near the $η^{'}$ -Meson Emission Threshold Measured in Coincidence with High-Momentum Protons

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Hints of a Nucleus Irked by a Meson Houseguest

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Excitation Spectra of the C12(p,d) Reaction near the η′-Meson Emission Threshold Measured in Coincidence with High-Momentum Protons

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Hints of a Nucleus Irked by a Meson Houseguest

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Excitation Spectra of the ${}^{12}C (p, d)$ Reaction near the $η^{'}$ -Meson Emission Threshold Measured in Coincidence with High-Momentum Protons