Measurement of the Ratio of Branching Fractions B([bar over B][superscript 0] → D[superscript *+]τ[superscript -][bar over ν][subscript τ])/B([bar over B][superscript 0] → D[superscript *+]μ[superscript -][bar over ν][subscript μ])

Abstract

The branching fraction ratio R(D*) ≡ B([bar over B][superscript 0] → D[superscript *+]τ[superscript -][bar over ν][subscript τ])/B([bar over B][superscript 0] → D[superscript *+]μ[superscript -][bar over ν][subscript μ]) is measured using a sample of proton-proton collision data corresponding to 3.0 fb[superscript -1] of integrated luminosity recorded by the LHCb experiment during 2011 and 2012. The tau lepton is identified in the decay mode τ[superscript -] → μ[superscript -][bar over ν][subscript μ]ν[subscript 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 [bar over B][superscript 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.