Refining new-physics searches in B -> D tau nu decay with lattice QCD

TL;DR

This paper presents a lattice QCD calculation of the ratio R(D) in B -> D tau nu decay, reducing theoretical uncertainties and providing a Standard-Model benchmark that helps interpret experimental deviations potentially indicating new physics.

Contribution

First ab initio lattice QCD calculation of R(D) with reduced uncertainties, improving the theoretical precision for testing the Standard Model and new physics models.

Findings

R(D) = 0.316(12)(7), smaller error than previous estimates.

Result is about 1-sigma higher than previous estimates, reducing tension with experiment.

Provides Standard-Model prediction for polarization ratio P_L (D)= 0.325(4)(3).

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).