Light Scalars and the KOTO Anomaly

TL;DR

This paper proposes that weakly-coupled scalars produced from Kaon decays could explain the KOTO anomaly, fitting the excess events while complying with other experimental bounds, and explores two specific models.

Contribution

It introduces two concrete scalar models, the Higgs portal and hadrophilic scalar, as explanations for the KOTO excess, analyzing their compatibility with existing constraints.

Findings

Scalar models can account for the KOTO excess

Hadronic beam-dump experiments impose significant constraints

Models remain consistent with flavor and beam-dump bounds

Abstract

The KOTO experiment recently presented a significant excess of events in their search for the rare SM process $K_L \to \pi^0\nu\bar{\nu}$, well above both Standard Model signal and background predictions. We show that this excess may be due to weakly-coupled scalars that are produced from Kaon decays and escape KOTO undetected. We study two concrete realizations, the minimal Higgs portal and a hadrophilic scalar model, and demonstrate that they can explain the observed events while satisfying bounds from other flavor and beam-dump experiments. Hadronic beam-dump experiments provide particularly interesting constraints on these types of models, and we discuss in detail the normally underestimated uncertainties associated with them. The simplicity of the models which can explain the excess, and their possible relations with interesting UV constructions, provides strong theoretical motivation for a new physics interpretation of the KOTO data.