Proposal of the KOTO II experiment

TL;DR

KOTO II is a proposed experiment at J-PARC aiming to measure the rare decay $K_L\to\pi^0\nu\bar{\nu}$ with unprecedented sensitivity, enabling discovery of new physics beyond the Standard Model and expanding kaon decay studies.

Contribution

It introduces the KOTO II experiment, the only planned dedicated rare kaon decay measurement, with enhanced sensitivity and expanded physics goals beyond previous efforts.

Findings

Single event sensitivity of $8.5\times 10^{-13}$

Potential to observe decay with >5σ significance

Plans to explore additional rare decays and new physics searches

Abstract

The KOTO II experiment is proposed to measure the branching ratio of the decay $K_L\to\pi^0\nu\bar{\nu}$ at J-PARC. With a beamline to extract long-lived neutral kaons at 5 degrees from a production target, the single event sensitivity of the decay is $8.5\times 10^{-13}$, which is much smaller than the Standard Model prediction $3\times 10^{-11}$. This allows searches for new physics beyond the Standard Model and the first discovery of the decay with a significance exceeding $5\sigma$. As the only experiment proposed in the world dedicated to rare kaon decays, KOTO II will be indispensable in the quest for a complete understanding of flavor dynamics in the quark sector. Moreover, by combining efforts from the kaon community worldwide, we plan to develop the KOTO II detector further and expand the physics reach of the experiment to include measurements of the branching ratio of the $K_L\to\pi^0\ell^+\ell^-$ decays, studies of other $K_L$ decays, and searches for dark photons, axions, and axion-like particles. KOTO II will therefore obtain a comprehensive understanding of $K_L$ decays, providing further constraints on new physics scenarios with existing $K^+$ results.