Evidence for an excess of B → D(*)τν decays
The full data sample from the BaBar experiment is used to measure the ratios of branching fractions and , where ℓ refers to an electron or muon. These ratios are sensitive to the presence of new physics in charged current decays and are sensitive probes for charged Higgs bosons. These ratios are predicted in the standard model with small theoretical uncertainties, and many experimental uncertainties cancel in the ratio. The measured values, R(D) = 0.440 ± 0.058 ± 0.042 and R(D*) = 0.332 ± 0.024 ± 0.018, exceed standard model expectations by 2.0σ and 2.7σ, respectively. Accounting for correlations, the combined departure from the standard model is 3.4σ. The measured values are not compatible with a charged Higgs boson in type-II two-Higgs-doublet models, but can be accommodated in more general models involving charged mediators.