Importance of Loop Effects in Explaining the Accumulated Evidence for New Physics in B Decays with a Vector Leptoquark

Abstract

In recent years experiments revealed intriguing hints for new physics (NP) in B decays involving b→cτν and b→sℓ+ℓ− transitions at the 4σ and 5σ level, respectively. In addition, there are slight disagreements in b→uτν and b→dμ+μ− observables. While not significant on their own, they point in the same direction. Furthermore, Vus extracted from τ decays shows a slight tension (≈2.5σ) with its value determined from Cabibbo-Kobayashi-Maskawa unitarity, and an analysis of BELLE data found an excess in Bd→τ+τ−. Concerning NP explanations, the vector leptoquark SU(2) singlet is of special interest since it is the only single particle extension of the standard model which can (in principle) address all the anomalies described above. For this purpose, large couplings to τ leptons are necessary and loop effects, which we calculate herein, become important. Including them in our phenomenological analysis, we find that neither the tension in Vus nor the excess in Bd→τ+τ− can be fully explained without violating bounds from K→πν¯ν. However, one can account for b→cτν and b→uτν data finding intriguing correlations with Bq→τ+τ− and K→πν¯ν. Furthermore, the explanation of b→cτν predicts a positive shift in C7 and a negative one in C9, being nicely in agreement with the global fit to b→sℓ+ℓ− data. Finally, we point out that one can fully account for b→cτν and b→sℓ+ℓ− without violating bounds from τ→ϕμ, Υ→τμ, or b→sτμ processes.