Refining new-physics searches in B -> D tau nu decay with lattice QCD
Abstract
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, finding an approximately 2-sigma disagreement with the Standard-Model prediction for the ratio R(D)=BR(B->D tau nu)/BR(B->D l nu), 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 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 byproduct of our calculation, we also present the Standard-Model prediction for the longitudinal polarization ratio P_L (D)= 0.325(4)(3).
Cite
@article{arxiv.1206.4992,
title = {Refining new-physics searches in B -> D tau nu decay with lattice QCD},
author = {Jon A. Bailey and A. Bazavov and C. Bernard and C. M. Bouchard and C. DeTar and Daping Du and A. X. El-Khadra and J. Foley and E. D. Freeland and E. Gamiz and Steven Gottlieb and U. M. Heller and Jongjeong Kim and A. S. Kronfeld and J. Laiho and L. Levkova and P. B. Mackenzie and Y. Meurice and E. T. Neil and M. B. Oktay and Si-Wei Qiu and J. N. Simone and R. Sugar and D. Toussaint and R. S. Van de Water and Ran Zhou},
journal= {arXiv preprint arXiv:1206.4992},
year = {2013}
}
Comments
6 pages, 3 figures, 1 table. References and text added. Fig. 3 R(D) in 2HDM II corrected and conclusions modified. Standard-Model R(D) unchanged