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Refining New-Physics Searches in BDτν with Lattice QCD

Jon A. Bailey et al. (Fermilab Lattice and MILC Collaborations)
Phys. Rev. Lett. 109, 071802 – Published 16 August 2012

Abstract

The semileptonic decay channel BDτν 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 BDτν¯, finding an approximately 2σ disagreement with the standard-model prediction for the ratio R(D)=BR(BDτν)/BR(BDν), where =e,μ. 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 f0(q2). Our determination of R(D) is approximately 1σ 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 1σ 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 PL(D)=0.325(4)(3).

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  • Received 25 June 2012

DOI:https://doi.org/10.1103/PhysRevLett.109.071802

© 2012 American Physical Society

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Vol. 109, Iss. 7 — 17 August 2012

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