Closing the window on single leptoquark solutions to the B-physics anomalies

TLDR: In this paper, the authors examined various scenarios in which the Standard Model is extended by a light leptoquark state to solve for one or both B-physics anomalies, viz.

Abstract: We examine various scenarios in which the Standard Model is extended by a light leptoquark state to solve for one or both B-physics anomalies, viz. $$ {R}_{D^{\left(*\right)}}^{\exp }>{R}_{D^{\left(*\right)}}^{\mathrm{SM}} $$ or/and $$ {R}_{K^{\left(*\right)}}^{\exp }>{R}_{K^{\left(*\right)}}^{\mathrm{SM}} $$ . To do so we combine the constraints arising both from the low-energy observables and from direct searches at the LHC. We find that none of the scalar leptoquarks of mass mLQ ≃ 1 TeV can alone accommodate the above mentioned anomalies. The only single leptoquark scenario which can provide a viable solution for mLQ ≃ 1÷2 TeV is a vector leptoquark, known as U1, which we re-examine in its minimal form (letting only left-handed couplings to have non-zero values). We find that the limits deduced from direct searches are complementary to the low-energy physics constraints. In particular, we find a rather stable lower bound on the lepton flavor violating b → sl 1 ± l 2 ∓ modes, such as ℬ(B → Kμτ). Improving the experimental upper bound on ℬ(B → Kμτ) by two orders of magnitude could compromise the viability of the minimal U1 model as well.