Search for Dark Particles in K L 0 → γ X at the KOTO Experiment

Physical Review Letters 2026 Open access

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Authors: T. Wu, Y. -C. Tung, Yee Bob Hsiung, J. K. Ahn, M. Gonzalez, E. J. Kim, T. K. Komatsubara, K. Kotera, S. K. Lee, G. Y. Lim, C. Lin, T. Matsumura
+15 more Hajime Nanjo, Y. Noichi, T. Nomura, T. Nunes, K. Ono, Joseph Redeker, Noritaka Shimizu, Shuichi Shinohara, K. Shiomi, R. Shiraishi, Y. Tajima, Y. W. Wah, H. Watanabe, T. Yamanaka, H. Y. Yoshida
DOI: 10.1103/zyx1-y1bj
Cited by: 0
References: 21

Abstract

We report a search for an invisible particle X in the decay K_{L}^{0}→γX (X→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±4.42_{stat}±2.13_{syst} events. Upper limits on the branching ratio of K_{L}^{0}→γX were set for the X mass range 0≤m_{X}≤425 MeV/c^{2}. For massless X, the upper limit was 3.4×10^{-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 4.1×10^{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}).

Fields of Study & Subjects

Nuclear Physics and Applications Particle physics theoretical and experimental studies Atomic and Subatomic Physics Research

Publication details

Journal / source
Physical Review Letters
Publisher
American Physical Society (APS)
Volume
136
Issue
19
Pages
191802–191802
Publication date
21 April 2026
Language
EN