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Search for $K_{L} \to π^{0} π^{0} γ γ$ and $K_{L} \to π^{0} π^{0} X$ where $X \to γ γ$ at the KOTO Experiment

J. Redeker ¹, C. Lin², Y. W. Wah ¹, J. K. Ahn³, M. Gonzalez^4,*, K. Hanai⁴, Y. B. Hsiung ⁵, T. Kato⁴, E. J. Kim ⁶ et al.

T. K. Komatsubara ^7,8, K. Kotera⁴, S. K. Lee⁶, G. Y. Lim^7,8, T. Matsumura ⁹, H. Nanjo ⁴, T. Nomura ^7,8, T. Nunes⁴, K. Ono⁴, K. Shiomi ^7,8, R. Shiraishi ⁷, Y. Tajima¹⁰, Y. C. Tung ¹¹, H. Watanabe^7,8, T. Wu⁵, T. Yamanaka ⁴, and H. Y. Yoshida¹⁰

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

^*Present address: CERN, European Organization for Nuclear Research, CH-1211 Geneva, Switzerland.

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Phys. Rev. Lett. 137, 051804 – Published 29 July, 2026

DOI: https://doi.org/10.1103/pt2h-99by

Abstract

We performed searches for $K_{L} \to π^{0} π^{0} X$ where $X$ may be an axionlike particle that promptly decays to two photons, and the first search for $K_{L} \to π^{0} π^{0} γ γ$ at the KOTO experiment using data taken in 2021. The search is performed for $X$ mass in the range of $160 - 220 MeV / c^{2}$ . Three events were observed in the signal region, with two events near an $X$ mass of $177 MeV / c^{2}$ . This result led to a range of upper limits on the branching ratio, $BR (K_{L} \to π^{0} π^{0} X)$ $< (1 - 20) \times 10^{- 7}$ at the 95% confidence level (CL). No events were observed for the analysis of $K_{L} \to π^{0} π^{0} γ γ$ , setting an upper limit on the branching ratio, $BR (K_{L} \to π^{0} π^{0} γ γ)$ $< 1.69 \times 10^{- 6}$ at the 95% CL.

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Phys. Rev. Lett. 137, 051804– Published 29 July, 2026

Vol. 137, Iss. 5 — 31 July 2026

Received 22 March 2026
Revised 20 May 2026
Accepted 23 June 2026

DOI: https://doi.org/10.1103/pt2h-99by

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 $K_{L} \to π^{0} π^{0} γ γ$ and $K_{L} \to π^{0} π^{0} X$ where $X \to γ γ$ at the KOTO Experiment

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Search for KL→π0π0γγ and KL→π0π0X where X→γγ at the KOTO Experiment

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Search for $K_{L} \to π^{0} π^{0} γ γ$ and $K_{L} \to π^{0} π^{0} X$ where $X \to γ γ$ at the KOTO Experiment