Flavor anomalies in leptoquark model with gauged U(1)Lμ−Lτ

In addition to anomalies associated with the muon g−2 and the branching ratio (BR) of B→D(*)τν¯, Belle II recently observed an unexpectedly large BR in the B+→K+νν¯ decay. To resolve the anomalous excesses in these observables, we propose a framework involving a leptoquark, denoted by S1, which has the feature that down-type quarks merely couple to neutrinos but not the charged leptons, avoiding the strict constraint from the Bs→μ−μ+ decay. With the introduction of the U(1)Lμ−Lτ gauge symmetry, the Z′ with light mass not only resolves the muon g−2 anomaly, but also ensures that S1 couples exclusively to the third-generation leptons so that only τν¯τ and τ-neutrino modes are involved in the processes b→cℓν¯ and b→sνν¯, respectively. Under the dominant constraints from |ΔF|=2 processes, we find that the S1 contributions to the BRs of B→K(*)νν¯ and KL→π0νν¯ can be factorized into the same multiplicative factor multiplying the standard model predictions, and the enhancement can be up to a factor of 2. In particular, B(K+→π+νν¯) can reach the upper 1σ error of the experimental value, i.e., ≃15.4×10−11. We also show that the model can fit the new world averages of R(D(*)) and contribute significantly to the τ polarization in the B→Dτν¯ decay and B(Bc→τν¯). Published by the American Physical Society 2024