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.
Highlights
We present a comprehensive analysis of low-energy signals of hypothetical scalar leptoquark interactions in lepton and kaon transitions
In this work we have presented a detailed catalog of upper limits on scalar leptoquark interactions with SM fermions, arising from various decay modes of charged leptons and kaons
We have first derived the most general low-energy four-fermion effective Lagrangian induced by tree-level scalar leptoquark exchange, and have worked out the particular values of its Wilson coefficients for the five possible types of leptoquarks
Summary
From the representations of quark and lepton fields under the SM gauge group SU(3)C ⊗ SU(2)L ⊗ U(1)Y , the leptoquarks can be classified in five different categories. Using the freedom to rotate the different equal-charge fermion fields in flavour space, we adopt a ‘down’ basis where the down-type-quark and charged-lepton Yukawas are diagonal. In this basis, the transformation from the fermion interaction eigenstates to mass eigenstates is given by uL → V †uL and νL → U νL, where V is the quark Cabibbo-. As we neglect the diquark couplings, a baryon and lepton number can be assigned to all leptoquarks in a consistent way. In the subsequent sections we explore the constraints that arise on the arbitrary Yukawa matrices YLQ and ZLQ from various lepton and kaon decay modes
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