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Search for Dark Particles in $K_{L}^{0} \to γ X$ at the KOTO Experiment

T. Wu¹, Y. C. Tung ^2,*, Y. B. Hsiung¹, J. K. Ahn³, M. Gonzalez^4,†, E. J. Kim⁵, T. K. Komatsubara^6,7, K. Kotera⁴, S. K. Lee⁵ et al. (KOTO Collaboration)

S. K. Lee⁵, G. Y. Lim^6,7, C. Lin⁸, T. Matsumura⁹, H. Nanjo⁴, Y. Noichi⁴, T. Nomura^6,7, T. Nunes⁴, K. Ono⁴, J. Redeker¹⁰, N. Shimizu^4,‡, S. Shinohara^4,§, K. Shiomi^6,7, R. Shiraishi^4,§, Y. Tajima¹¹, Y. W. Wah¹⁰, H. Watanabe^6,7, T. Yamanaka⁴, and H. Y. Yoshida¹¹ (KOTO Collaboration)

¹Department of Physics, National Taiwan University, Taipei 10617, Taiwan, Republic of China
²Department of Physics, National Kaohsiung Normal University, Kaohsiung 824, Taiwan
³Department of Physics, Korea University, Seoul 02841, Republic of Korea
⁴Department of Physics, Osaka University, Toyonaka, Osaka 560-0043, Japan
⁵Division of Science Education, Jeonbuk National University, Jeonju 54896, Republic of Korea
⁶Institute of Particle and Nuclear Studies, High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305-0801, Japan
⁷J-PARC Center, Tokai, Ibaraki 319-1195, Japan
⁸Department of Physics, National Changhua University of Education, Changhua 50007, Taiwan
⁹Department of Applied Physics, National Defense Academy, Kanagawa 239-8686, Japan
¹⁰Enrico Fermi Institute, University of Chicago, Chicago, Illinois 60637, USA
¹¹Department of Physics, Yamagata University, Yamagata 990-8560, Japan

^*Contact author: yctung@mail.nknu.edu.tw
^†Present address: CERN, European Organization for Nuclear Research, CH-1211 Geneva, Switzerland.
^‡Present address: Department of Physics and The International Center for Hadron Astrophysics, Chiba University, Chiba 263-8522, Japan.
^§Present address: Institute of Particle and Nuclear Studies, High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305-0801, Japan.

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Phys. Rev. Lett. 136, 191802 – Published 14 May, 2026

DOI: https://doi.org/10.1103/zyx1-y1bj

Abstract

We report a search for an invisible particle $X$ in the decay $K_{L}^{0} \to γ X$ ( $X \to invisible$ ), where $X$ can be interpreted as a massless or massive dark photon. No evidence for $X$ was found, based on 13 candidate events consistent with a predicted background of $12.66 \pm 4.4 2_{stat} \pm 2.1 3_{syst}$ events. Upper limits on the branching ratio of $K_{L}^{0} \to γ X$ were set for the $X$ mass range $0 \leq m_{X} \leq 425 MeV / c^{2}$ . For massless $X$ , the upper limit was $3.4 \times 10^{- 7}$ at the 90% confidence level, improving the previous indirect bound by over three orders of magnitude. This result yielded a new lower bound of $4.1 \times 10^{6} TeV$ on the probed effective mass scale governing flavor-changing dark-photon couplings, representing approximately two orders of magnitude improvement on existing constraints. For massive $X$ , the upper limits in the searched mass region ranged from $O (10^{- 7})$ to $O (10^{- 3})$ .

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References (21)

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Phys. Rev. Lett. 136, 191802– Published 14 May, 2026

Vol. 136, Iss. 19 — 15 May 2026

Received 12 November 2025
Revised 5 February 2026
Accepted 21 April 2026

DOI: https://doi.org/10.1103/zyx1-y1bj

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. Funded by SCOAP³.

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Search for Dark Particles in $K_{L}^{0} \to γ X$ at the KOTO Experiment

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Search for Dark Particles in KL0→γX at the KOTO Experiment

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Search for Dark Particles in $K_{L}^{0} \to γ X$ at the KOTO Experiment