Constraints on scalar leptoquarks from lepton and kaon physics

TL;DR

This paper analyzes how hypothetical scalar leptoquarks could affect lepton and kaon processes, deriving constraints on their interactions and exploring potential signals in low-energy experiments.

Contribution

It provides the most general effective Lagrangian for scalar leptoquark interactions and derives current experimental constraints on their couplings from lepton and kaon data.

Findings

Current bounds on scalar leptoquark Yukawa couplings are established.

Differential distributions for kaon transitions with leptoquark interference are derived.

Loop-induced transitions and interference effects are significant for future constraints.

Abstract

We present a comprehensive analysis of low-energy signals of hypothetical scalar leptoquark interactions in lepton and kaon transitions. We derive the most general effective four-fermion Lagrangian induced by tree-level scalar leptoquark exchange and identify the Wilson coefficients predicted by the five possible types of scalar leptoquarks. The current constraints on the leptoquark Yukawa couplings arising from lepton and kaon processes are worked out, including also loop-induced transitions with only leptons (or quarks) as external states. In the presence of scalar leptoquark interactions, we also derive the differential distributions for flavour-changing neutral-current transitions in semileptonic kaon modes, including all known effects within the Standard Model. Their interference with the new physics contributions could play a significant role in future improvements of those constraints that are currently hampered by poorly-determined non-perturbative parameters.