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Direct Measurement of the In-Medium $η^{'}$ Mass Spectrum through the $γ γ$ Decay Channel

Y. Matsumura ¹, N. Muramatsu², T. A. Hashimoto³, M. Miyabe¹, H. Shimizu¹, A. O. Tokiyasu¹, J. K. Ahn⁴, W. C. Chang⁵, J. Y. Chen⁶ et al. (LEPS2/BGOegg Collaboration)

J. Y. Chen⁶, M. L. Chu⁵, S. Daté³, T. Gogami⁷, H. Hamano³, Q. H. He⁸, K. Hicks⁹, T. Hiraiwa¹⁰, Y. Honda¹, T. Hotta³, Y. Inoue¹, T. Ishikawa³, I. Jaegle¹¹, Y. Kasamatsu³, H. Katsuragawa³, S. Kido¹, R. Kobayakawa³, Y. Kon³, S. Masumoto¹², K. Mizutani³, T. Z. Mo⁸, T. Nakamura¹³, T. Nakano³, T. Nam¹⁴, M. Niiyama¹⁵, Y. Nozawa¹⁶, Y. Ohashi³, H. Ohnishi¹, T. Ohta¹⁶, M. Okabe¹, K. Ozawa¹⁷, C. Rangacharyulu¹⁸, S. Y. Ryu³, Y. Sada¹, T. Shibukawa¹², R. Shirai¹, K. Shiraishi¹, E. A. Strokovsky¹⁹, Y. Sugaya³, M. Sumihama^13,3, S. Suzuki^20,3, S. Tanaka³, Y. Taniguchi¹, N. Tomida⁷, Y. Tsuchikawa²¹, T. Ueda¹, T. F. Wang²², H. Yamazaki²³, R. Yamazaki¹, Y. Yanai³, T. Yorita³, C. Yoshida²⁴, and M. Yosoi³ (LEPS2/BGOegg Collaboration)

¹Research Center for Accelerator and Radioisotope Science, Tohoku University, Sendai, Miyagi 982-0826, Japan
²Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou 730000, China
³Research Center for Nuclear Physics, Osaka University, Ibaraki, Osaka 567-0047, Japan
⁴Department of Physics, Korea University, Seoul 02841, Republic of Korea
⁵Institute of Physics, Academia Sinica, Taipei 11529, Taiwan
⁶National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan
⁷Department of Physics, Kyoto University, Kyoto 606-8502, Japan
⁸Department of Nuclear Science and Technology, College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China
⁹Department of Physics and Astronomy, Ohio University, Athens, Ohio 45701, USA
¹⁰RIKEN SPring-8 Center, Sayo, Hyogo 679-5148, Japan
¹¹Thomas Jefferson National Accelerator Facility, Newport News, Virginia 23606, USA
¹²Department of Physics, University of Tokyo, Tokyo 113-0033, Japan
¹³Department of Education, Gifu University, Gifu 501-1193, Japan
¹⁴Dalat Nuclear Research Institute, Dalat, Lam Dong, Vietnam
¹⁵Department of Physics, Kyoto Sangyo University, Kyoto 603-8555, Japan
¹⁶Department of Radiology, The University of Tokyo Hospital, Tokyo 113-8655, Japan
¹⁷Institute of Particle and Nuclear Studies, High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305-0801, Japan
¹⁸Department of Physics and Engineering Physics, University of Saskatchewan, Saskatoon, Saskatchewan S7N 5E2, Canada
¹⁹Laboratory of High Energy Physics, Joint Institute for Nuclear Research, Dubna, Moscow Region 142281, Russia
²⁰Japan Synchrotron Radiation Research Institute (SPring-8), Sayo, Hyogo 679-5198, Japan
²¹J-PARC Center, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japan
²²School of Physics, Beihang University, Beijing 100191, China
²³Radiation Science Center, High Energy Accelerator Research Organization (KEK), Tokai, Ibaraki 319-1195, Japan
²⁴Instrumentation Technology Development Center, High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305-0801, Japan

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Phys. Rev. Lett. 137, 051901 – Published 27 July, 2026

DOI: https://doi.org/10.1103/x414-n6g9

Abstract

We measured the invariant mass of the $η^{'} (958)$ meson through the $η^{'} \to γ γ$ decay channel to search for possible character change of $η^{'}$ in a nuclear medium. The measured invariant-mass spectra were analyzed by fitting realistic spectral functions. An enhancement was observed in the lower tail of the $η^{'}$ mass peak for low-momentum ( $P_{γ γ} < 1 GeV / c$ ) events. Three statistical methods were applied to assess the significance of the enhancement. All methods yield a statistical significance above $3.7 σ$ . The spectral fitting indicates a mass reduction of ${57.5}_{- 27.8}^{+ 5.7} {MeV / c}^{2}$ . Our result provides the first evidence for an in-medium spectral modification of the $η^{'}$ meson obtained through a direct measurement of its invariant mass.

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References (39)

S. Leupold, V. Metag, and U. Mosel, Int. J. Mod. Phys. E 19, 147 (2010).
R. S. Hayano and T. Hatsuda, Rev. Mod. Phys. 82, 2949 (2010).
V. Metag, M. Nanova, and E. Paryev, Prog. Part. Nucl. Phys. 97, 199 (2017).
Y. Nambu, Phys. Rev. 117, 648 (1960).
Y. Nambu, Phys. Rev. Lett. 4, 380 (1960).
Y. Nambu and G. Jona-Lasinio, Phys. Rev. 122, 345 (1961).
S. Weinberg, Phys. Rev. D 11, 3583 (1975).
E. Witten, Nucl. Phys. B156, 269 (1979).
G. Veneziano, Nucl. Phys. B159, 213 (1979).
T. Hatsuda and S. H. Lee, Phys. Rev. C 46, R34 (1992).
P. Costa, M. Ruivo, and Y. Kalinovsky, Phys. Lett. B 560, 171 (2003).
P. Costa, M. C. Ruivo, C. A. de Sousa, and Y. L. Kalinovsky, Phys. Rev. D 71, 116002 (2005).
H. Nagahiro, M. Takizawa, and S. Hirenzaki, Phys. Rev. C 74, 045203 (2006).
S. Sakai and D. Jido, Phys. Rev. C 88, 064906 (2013).
K. Saito, K. Tsushima, and A. Thomas, Prog. Part. Nucl. Phys. 58, 1 (2007).
S. D. Bass and A. W. Thomas, Phys. Lett. B 634, 368 (2006).
S. Kono, D. Jido, Y. Kuroda, and M. Harada, Prog. Theor. Exp. Phys. 2021, 093D02 (2021).
S. H. Lee and T. Hatsuda, Phys. Rev. D 54, R1871 (1996).
D. Jido, H. Nagahiro, and S. Hirenzaki, Phys. Rev. C 85, 032201 (2012).
M. Nanova et al. (CBELSA/TAPS Collaboration), Phys. Lett. B 727, 417 (2013).
M. Nanova et al. (CBELSA/TAPS Collaboration), Phys. Rev. C 94, 025205 (2016).
M. Nanova et al. (CBELSA/TAPS Collaboration), Eur. Phys. J. A 54, 182 (2018).
E. Czerwiński et al., Phys. Rev. Lett. 113, 062004 (2014).
Y. K. Tanaka et al. ( $η^{'}$ -PRiME/Super-FRS Collaborations), Phys. Rev. C 97, 015202 (2018).
N. Tomida et al. (LEPS2/BGOegg Collaboration), Phys. Rev. Lett. 124, 202501 (2020).
T. Csörgő, R. Vértesi, and J. Sziklai, Phys. Rev. Lett. 105, 182301 (2010).
R. Vértesi, T. Csörgő, and J. Sziklai, Phys. Rev. C 83, 054903 (2011).
N. Muramatsu et al. (LEPS Collaboration), Nucl. Instrum. Methods Phys. Res., Sect. A 737, 184 (2014).
T. Ishikawa et al., Nucl. Instrum. Methods Phys. Res., Sect. A 837, 109 (2016).
T. Hashimoto et al. (LEPS2/BGOegg Collaboration), Phys. Rev. C 106, 035201 (2022).
N. Muramatsu et al. (LEPS2/BGOegg Collaboration), Phys. Rev. C 100, 055202 (2019).
See Supplemental Material at http://link.aps.org/supplemental/10.1103/x414-n6g9 for detailed descriptions of background reduction, the momentum cut optimization, the $χ^{2}$ difference tests in the $k_{1} - k_{2}$ space, and the fitting results under different event selection conditions, which includes Refs. [33,34].
M. Williams et al. (CLAS Collaboration), Phys. Rev. C 80, 045213 (2009).
M. Nanova et al. (CBELSA/TAPS Collaboration), Phys. Lett. B 710, 600 (2012).
S. Agostinelli et al., Nucl. Instrum. Methods Phys. Res., Sect. A 506, 250 (2003).
R. Muto et al., Phys. Rev. Lett. 98, 042501 (2007).
S. Navas et al. (Particle Data Group), Phys. Rev. D 110, 030001 (2024).
C. W. Glover, K. W. Kemper, and A. D. Frawley, Phys. Rev. C 22, 522 (1980).
S. Goda and D. Jido, Prog. Theor. Exp. Phys. 2014, 033D03 (2014).

Phys. Rev. Lett. 137, 051901– Published 27 July, 2026

Vol. 137, Iss. 5 — 31 July 2026

Received 5 December 2025
Revised 24 April 2026
Accepted 15 June 2026

DOI: https://doi.org/10.1103/x414-n6g9

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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Direct Measurement of the In-Medium $η^{'}$ Mass Spectrum through the $γ γ$ Decay Channel

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Direct Measurement of the In-Medium η′ Mass Spectrum through the γγ Decay Channel

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Direct Measurement of the In-Medium $η^{'}$ Mass Spectrum through the $γ γ$ Decay Channel