Imprint of a new light particle at KOTO?

Abstract

Recently, the KOTO experiment reported their new preliminary result of searching for the decay K L → π 0 νν. Three candidate events were observed in the signal region, which exceed significantly the expectation based on the standard model. On the other hand, the new NA62 and previous BNL-E949 experiments yielded a consistent result and confirmed the standard model prediction in the charged meson decay K þ → π þ νν. Furthermore, the two decays are bound by a well-motivated relation from an analysis of isospin symmetry that is hard to break by the new physics of heavy particles. In this work, we study the issue by a systematic effective field theory approach with three of the simplest scenarios, in which the K L may decay into a new light neutral particle X, i.e., K L → π 0 X, K L → γγX, or K L → π 0 XX. We assess the feasibility of the scenarios by simulations and by incorporating constraints coming from NA62 and other relevant experiments. Our main conclusion is that the scenario K → πXX for a long-lived scalar X seems more credible than the other two when combining distributions and other experimental constraints while the region below the KOTO's blind box provides a good detection environment to search for all three scenarios for a relatively heavy X.

Cosmological implications of the KOTO excess

“…If the KOTO excess is interpreted at face value, this suggests apparent violation of the GN bound. As has been discussed by several authors [10,[12][13][14][15][16][17][18][19][20][21][22], such a signal at KOTO can be mimicked by a decay of the form K L → π 0 X, where X denotes one or more invisible species. In contrast to most studies, we focus on a new physics scenario where the decay K L → π 0 inv: is realized through a sequence of two-body decays K L → SP → π 0 PP, where S and P are light neutral scalar particles.…”

“…The GN bound is very robust in models where the K → πνν decays are modified by heavy new physics well above the kaon mass. However, in the presence of light new physics, the GN bound can be violated, and the observed events at KOTO may find an explanation [10,[12][13][14][15][16][17][18][19][20][21][22][23][24].…”

New physics searches at kaon and hyperon factories

“…In this way, the GN bound can be violated effectively by orders of magnitude. Detailed studies based on this mechanism have been done in references [34,109,247,265]. Furthermore, such a light long-lived particle [m X = O(100) MeV and τ X = O(0.1) ns] is a good target for the FASER experiment [266].…”

New Physics Searches at Kaon and Hyperon Factories

“…Detailed studies based on this mechanism have been done in Refs. [34,108,234,250]. Furthermore, such a light long-lived particle [m X = O(100) MeV and τ X = O(0.1) ns] is a good target for the FASER experiment [251].…”