Explaining the hints for lepton flavour universality violation with three S2 leptoquark generations

Abstract

Leptoquarks are prime candidates for explaining the intriguing hints for lepton flavour universality violation. In particular, the SU(2)L doublet of scalar leptoquarks S2 is capable of providing an explanation for the tensions between the measurements and the Standard Model predictions in (g − 2)μ, b → sℓ+ℓ− and b → cτν processes, as well as in non-resonant di-electron production. However, in the minimal setup with a single leptoquark generation, a common explanation for all these issues is not possible as this would lead to unacceptably large charged lepton flavour violation. We therefore propose a model with three generations of S2, each coupling exclusively to a single lepton flavour, i.e. a model extending the Standard Model particle content by an electroquark, a muoquark and a tauquark. We show that after taking into account other constraints, such as those originating from electroweak precision observables and ∆F = 2 processes, it is possible to provide a combined explanation for all these hints of lepton flavour universality violation. Moreover, we find that the presence of the tauquark can generate a dimension-six {mathcal{O}}_9^U operator via off-shell photon penguin diagrams, which, together with the muoquark contribution, further improves the global fit to b → sℓ+ℓ− data.