Refining New-Physics Searches in B → Dτν with Lattice QCD

The semileptonic decay channel B -> D tau nu 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 B -> D tau(_)(nu) over bar, finding an approximately 2 disagreement with the standardmodel prediction for the ratio R(D) = BR(B -> D tau nu)/BR(B -> Dlv), where l = e, mu. We compute this ratio of branching fractions using hadronic form factors computed in unquenched lattice QCD and obtain R(D) 0.316(12)(7), where the errors are statistical and total systematic, respectively. This result is the first standard-model calculation of R(D) 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 f(0)(q(2)). Our determination of R(D) is approximately 1 sigma higher than previous estimates and, thus, reduces the tension with experiment. We also compute R(D) in models with electrically charged scalar exchange, such as the type-II twoHiggs- doublet model. Once again, our result is consistent with, but approximately 1 sigma 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 longitudinalpolarization ratio P-L(D) = 0.325(4)(3).