Cosmological implications of the KOTO excess

TL;DR

This paper explores models where the KOTO excess arises from a new decay involving invisible particles, which could also serve as dark matter, with implications for early universe cosmology and future experimental tests.

Contribution

It introduces a class of models with two light scalars explaining the KOTO excess and investigates their potential as dark matter candidates, linking particle physics with cosmology.

Findings

The lighter scalar can account for dark matter via thermal or nonthermal production.

Constraints on the model's structure depend on dark matter production mechanisms.

Reheating temperature in the early universe is constrained to around 10 MeV.

Abstract

The KOTO experiment has reported an excess of $K_L\to\pi^0\nu\bar\nu$ events above the Standard Model prediction, in tension with the Grossman-Nir (GN) bound. The GN bound heavily constrains new physics interpretations of an excess in this channel, but another possibility is that the observed events originate from a different process entirely: a decay of the form $K_L\to\pi^0X$, where $X$ denotes one or more new invisible species. We introduce a class of models to study this scenario with two light scalars playing the role of $X$, and we examine the possibility that the lighter of the two new states may also account for cosmological dark matter (DM). We show that this species can be produced thermally in the presence of additional interactions apart from those needed to account for the KOTO excess. Conversely, in the minimal version of the model, DM must be produced nonthermally. In this case, avoiding overproduction imposes constraints on the structure of the low-energy theory. Moreover, this requirement carries significant implications for the scale of reheating in the early Universe, generically preferring a low but observationally permitted reheating temperature of O(10 MeV). We discuss astrophysical and terrestrial signatures that will allow further tests of this paradigm in the coming years.