Measurement of the Ratio of Branching Fractions B(¯B0→D*+τ−¯ντ)/B(¯B0→D*+μ−¯νμ)

Abstract

The branching fraction ratio R ( D ∗ ) ≡ B ( ¯ B 0 → D * + τ − ¯ ν τ ) / B ( ¯ B 0 → D * + μ − ¯ ν μ ) is measured using a sample of proton-proton collision data corresponding to 3.0 fb − 1 of integrated luminosity recorded by the LHCb experiment during 2011 and 2012. The tau lepton is identified in the decay mode τ − → μ − ¯ ν μ ν τ . The semitauonic decay is sensitive to contributions from non-standard-model particles that preferentially couple to the third generation of fermions, in particular, Higgs-like charged scalars. A multidimensional fit to kinematic distributions of the candidate ¯ B 0 decays gives R ( D ∗ ) = 0.336 ± 0.027 ( stat ) ± 0.030 ( syst ) . This result, which is the first measurement of this quantity at a hadron collider, is 2.1 standard deviations larger than the value expected from lepton universality in the standard model.