Jon A. Bailey, A. Bazavov, C. Bernard, C. M. Bouchard, C. DeTar, Daping Du, A. X. El-Khadra, J. Foley, E. D. Freeland, E. Gamiz, Steven Gottlieb, U. M. Heller, Jongjeong Kim, A. S. Kronfeld, J. Laiho, L. Levkova, P. B. Mackenzie, Y. Meurice, E. T. Neil, M. B. Oktay, Si-Wei Qiu, J. N. Simone, R. Sugar, D. Toussaint, R. S. Van de Water, Ran Zhou
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 discrepancy with the Standard-Model prediction for the ratio R(D)=BR(B -> D tau nu)/BR(B -> 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. It agrees with previous theoretical estimates, but the errors are smaller primarily due to the reduced uncertainty in the scalar form factor f_0(q^2). We also compute R(D) in models with electrically charged scalar exchange, such as the type II two-Higgs doublet model (2HDM). Our result disagrees significantly with previous estimates for large values of the scalar coupling (e.g., large tan(beta)/M_{H^+} in the 2HDM), and therefore impacts the interpretation of experimental measurements of R(D) as constraints on new-physics models. 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).
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http://arxiv.org/abs/1206.4992
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