Probing lepton number violating interactions in rare kaon decays

TL;DR

This paper explores how rare kaon decays can be used to detect lepton number violation, setting new limits on the associated physics scale and highlighting the importance of dedicated experimental searches.

Contribution

It provides the first detailed analysis of LNV effects in $K o ext{ extpi} u u$ decays within the Standard Model effective field theory, including current experimental constraints and implications for high-scale leptogenesis.

Findings

Kaon decays probe LNV scales of 15-20 TeV.

Current experiments constrain LNV operators more tightly than neutrinoless double beta decay.

Observation of LNV in kaons could challenge high-scale leptogenesis.

Abstract

We investigate the possibility to probe lepton number violating (LNV) operators in the rare kaon decay $K\to\pi\nu\nu$. Performing the analysis in the Standard Model effective field theory with only light active Majorana neutrinos, we determine the current limits on the corresponding LNV physics scale from the past E949 experiment at BNL as well as the currently operating experiments NA62 at CERN and KOTO at J-PARC. We focus on the specific signature of scalar currents in $K\to\pi\nu\nu$ arising from the LNV nature of the operators and study the effect on the experimental sensitivity, stressing the need for dedicated searches for beyond the SM currents. We find that the rare kaon decays probe high operator scales $\Lambda_\text{LNV} \approx 15$ to $20$ TeV in different quark and neutrino flavours compared to neutrinoless double beta decay. Furthermore, we comment that the observation of LNV in kaon decays can put high-scale leptogenesis under tension. Finally, we discuss the connection with small radiatively generated neutrino masses and show how the severe constraints therefrom can be evaded in a minimal ultraviolet-complete scenario featuring leptoquarks.