Cosmological implications of the KOTO excess

Understand

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.