# Refining New-Physics Searches in $B\rightarrow D_{\tau\nu}$ with Lattice QCD

Bailey, J.A.; Bazavov, A.; Bernard, C.; Bouchard, C.M.; DeTar, C.; Du, D.; El-Khadra, A.X.; Foley, J.; Freeland, E.D.; Gámiz, E.; Gottlieb, S.; Heller, U.M.; Kim, J.; Kronfeld, A.S.; Laiho, J.; Levkova, L.; Mackenzie, P.B.; Meurice, Y.; Neil, E.T.; Oktay, M.B.; Qiu, S.-W.; Simone, J.N.; Sugar, R.; Toussaint, D.; Van de Water, R.S.; Zhou, R.

Date:
2012

Publisher:
American Physical Society

Citation:
Bailey, J. A., Bazavov, A., Bernard, C., Bouchard, C. M., Detar, C., Du, D., . . . Zhou, R. (2012). Refining New-Physics Searches in $B\rightarrow D_{\tau\nu}$ with Lattice QCD. Physical Review Letters, 109(7), 071802. http://dx.doi.org/10.1103/PhysRevLett.109.071802

Rights:
© 2012 American Physical Society

Type:
Article

### Abstract:

The semileptonic decay channel $B\rightarrow D_{\tau\nu}$ is sensitive to the presence of a scalar current, such as that mediated by a charged-Higgs boson. Recently, the $\textit{BABAR}$ experiment reported the first observation of the exclusive semileptonic decay $B\rightarrow D_{\tau}−\overline{\nu}$, finding an approximately $2\sigma$ disagreement with the standard-model prediction for the ratio $R(D)=BR(B\rightarrow D_{\tau\nu}/BR(B\rightarrow D\ell\nu)$, where $\ell=e,\mu$. 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 $\textit{ab initio}$ 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 two-Higgs-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 longitudinal-polarization ratio $P_{L}(D)=0.325(4)(3)$.

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