Abstract
The semileptonic decay channel is sensitive to the presence of a scalar current, such as that mediated by a charged-Higgs boson. Recently, the BABAR experiment reported the first observation of the exclusive semileptonic decay , finding an approximately disagreement with the standard-model prediction for the ratio , where . We compute this ratio of branching fractions using hadronic form factors computed in unquenched lattice QCD and obtain , where the errors are statistical and total systematic, respectively. This result is the first standard-model calculation of from ab initio full QCD. Its error is smaller than that of previous estimates, primarily due to the reduced uncertainty in the scalar form factor . Our determination of is approximately higher than previous estimates and, thus, reduces the tension with experiment. We also compute in models with electrically charged scalar exchange, such as the type-II two-Higgs-doublet model. Once again, our result is consistent with, but approximately higher than, previous estimates for phenomenologically relevant values of the scalar coupling in the type-II model. As a by-product of our calculation, we also present the standard-model prediction for the longitudinal-polarization ratio .
- Received 25 June 2012
DOI:https://doi.org/10.1103/PhysRevLett.109.071802
© 2012 American Physical Society