New results on the search for rare kaon events with the KOTO detector

TL;DR

The KOTO experiment searches for the rare decay K$^{0}_{L} ightarrow \

Contribution

This paper reports the first results from the 2015 data run, setting a new upper limit on the decay's branching ratio, improving constraints on new physics.

Findings

Set an upper limit of 3 x 10$^{-9}$ on the decay's branching ratio.

Demonstrated improved sensitivity over previous experiments.

Collected data with upgraded hardware in 2015.

Abstract

The KOTO experiment was designed to observe and study the K$^{0}_{L} \rightarrow$ $\pi^{0}\nu\bar{\nu}$ decay at J-PARC. The Standard Model (SM) prediction for the process is (3.0 $\pm$ 0.3) x 10$^{-11}$ with small uncertainties. This unique \emph{golden} decay is an ideal candidate to probe for new physics and can place strict constraints on beyond the standard model (BSM) theories. The previous experimental upper limit of the branching ratio was set by the KEK E391a collaboration as BR(K$^{0}_{L} \rightarrow \pi^{0}\nu\bar{\nu}$) $<$ 2.6 x 10$^{-8}$. The signature of the decay is a pair of photons from the $\pi^{0}$ decay and no other detected particles. For the measurement of the energies and positions of the photons, KOTO uses a Cesium Iodide (CSI) electromagnetic calorimeter as the main detector, and hermetic veto counters to guarantee that there are no other detected particles. KOTO's initial data was collected in 2013 and achieved a similar sensitivity to the E391a result. We completed hardware upgrades and had the first major physics runs in 2015. This proceeding summarizes the presentation of recent results from the 2015 runs on the search for the decay K$^{0}_{L} \rightarrow \pi^{0}\nu\bar{\nu}$ with the data collected in 2015 in the KOTO experiment. The best experimental upper limit on the branching fraction from a direct search was set as BR(K$^{0}_{L} \rightarrow \pi^{0}\nu\bar{\nu}$)$<$ 3 x 10$^{-9}$ at the 90\% confidence level.