The KOTO II experiment to measure the branching ratio of $K_L\to\pi^0\nu\bar\nu$

by Hajime Nanjo (Osaka University)

Tuesday 22 Sept 2026, 15:30 → 16:30 US/Eastern

Large Seminar Room

abstract: 

The KOTO II experiment is a next-generation rare $K_L$ decay experiment at J-PARC, aiming to measure the branching ratio of $K_L\to\pi^0\nu\bar\nu$  in the 2030s. This seminar will review the physics motivation and the current status of rare kaon decays, and the sensitivity and prospects of the KOTO II experiment.

The rare kaon decay $K_L\to\pi^0\nu\bar\nu$  is one of the best probes for studying new physics beyond the Standard Model (SM), because the SM contribution is highly suppressed and can be theoretically evaluated with high precision; the branching ratio in the SM is $3\times10^{−11}$ with a theoretical uncertainty of 2%. The $s\to d$ transition of this decay could provide unique information on the flavor structure of new physics. Therefore, a measurement of this branching ratio could provide essential new information on new physics.

The decay is being searched for in the KOTO experiment at J-PARC, which has obtained the current best upper limit on the branching ratio $2.2\times 10^{−9}$; a sensitivity to the branching ratio below $10^{−10}$ is a goal by the end of the decade.

To study the decay with greater sensitivity, KOTO II was proposed in 2024, and the KOTO II collaboration was formed with 70 members from Asia, Europe, and the US. The goal of KOTO II is to measure the branching ratio of $K_L\to\pi^0\nu\bar\nu$ with a sensitivity below $10^{−12}$. Discovery of the decay with $5\sigma$ significance is achievable at the SM value of the branching ratio. An indication of new physics with a significance more than 90% is possible if the observed branching ratio differs by 40% or more from the SM value. Another important goal of KOTO II is to measure the branching ratio of the unobserved decays $K_L\to\pi^0\ell^+\ell^-$, which can also provide information on the flavor structure of new physics. Other rare $K_L$ decays and hidden-sector particles are also within the scope of the study.

 

 

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