title: Refining new-physics searches in B→Dτν with lattice QCD.
abstract: The semileptonic decay channel B→Dτν 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→Dτ(-)ν, finding an approximately 2σ disagreement with the standard-model prediction for the ratio R(D)=BR(B→Dτν)/BR(B→Dℓν), where ℓ = 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 f(0)(q(2)). 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 P(L)(D)=0.325(4)(3).
authors.affiliation: Department of Physics and Astronomy, Seoul National University, South Korea.
authors.forename: Jon A
authors.initials: JA
authors.lastname: Bailey
authors.forename: A
authors.initials: A
authors.lastname: Bazavov
authors.forename: C
authors.initials: C
authors.lastname: Bernard
authors.forename: C M
authors.initials: CM
authors.lastname: Bouchard
authors.forename: C
authors.initials: C
authors.lastname: Detar
authors.forename: Daping
authors.initials: D
authors.lastname: Du
authors.forename: A X
authors.initials: AX
authors.lastname: El-Khadra
authors.forename: J
authors.initials: J
authors.lastname: Foley
authors.forename: E D
authors.initials: ED
authors.lastname: Freeland
authors.forename: E
authors.initials: E
authors.lastname: Gámiz
authors.forename: Steven
authors.initials: S
authors.lastname: Gottlieb
authors.forename: U M
authors.initials: UM
authors.lastname: Heller
authors.forename: Jongjeong
authors.initials: J
authors.lastname: Kim
authors.forename: A S
authors.initials: AS
authors.lastname: Kronfeld
authors.forename: J
authors.initials: J
authors.lastname: Laiho
authors.forename: L
authors.initials: L
authors.lastname: Levkova
authors.forename: P B
authors.initials: PB
authors.lastname: Mackenzie
authors.forename: Y
authors.initials: Y
authors.lastname: Meurice
authors.forename: E T
authors.initials: ET
authors.lastname: Neil
authors.forename: M B
authors.initials: MB
authors.lastname: Oktay
authors.forename: Si-Wei
authors.initials: SW
authors.lastname: Qiu
authors.forename: J N
authors.initials: JN
authors.lastname: Simone
authors.forename: R
authors.initials: R
authors.lastname: Sugar
authors.forename: D
authors.initials: D
authors.lastname: Toussaint
authors.forename: R S
authors.initials: RS
authors.lastname: Van de Water
authors.forename: Ran
authors.initials: R
authors.lastname: Zhou