Measurement of the Ratio of Branching FractionsB(B¯0→D

Abstract

The branching fraction ratio R(D^{*})≡B(B[over ¯]^{0}→D^{*+}τ^{-}ν[over ¯]_{τ})/B(B[over ¯]^{0}→D^{*+}μ^{-}ν[over ¯]_{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[over ¯]_{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[over ¯]^{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.

Signature of lepton flavor universality violation in Bs→Dsτν semileptonic decays

“…Very recently, in Ref [7], the authors predicted the value of R D Ã to be 0.260 AE 0.008. Comparing with the current world averages of R D ¼ 0.407 AE 0.039 AE 0.024 and R D Ã ¼ 0.304 AE 0.013 AE 0.007 from BABAR [8], Belle [9][10][11], and LHCb [12], the combined deviation currently stands at about 4.1σ. For definiteness, we report in Table I the current status of the experimentally measured ratio of branching ratios R D and R D Ã [13].…”

Test of lepton flavour universality with semi-tauonic decays of b-hadrons at LHCb

“…Similar parametrizations are used for semitauonic D * * decay modes. The contribution ofB 0 → D * * + µ −ν background amounts to about 12% of the normalization mode in the fit to signal sample [14].…”

“…In the B rest frame, the following quantities allow us to distinguish signal from muonic decay: the missing mass squared m 2 miss = (p 14] .…”

“…where is α is the angle between unit vector and z axis (beam pipe) [14] . The main background sources for the signal and normalization channel consist of partially reconstructed B decays.…”

“…Hence, it is necessary to use an approximation. In this analysis, the hadron momentum in the B rest frame is calculated using the B momentum direction, determined from the unit vector to B decay vertex from the associated primary vertex and assuming that the proper velocity β γ of visible part of semileptonic decay along z-axis is equal to B proper velocity along the same axis [14] …”