Main Image


Experimental mini-review on $R(D)$ and $R(D^*)$ measurements at LHCb

C. Bozzi, on behalf of the LHCb collaboration

in 9th International Workshop on the CKM Unitarity Triangle

Contribution: pdf


The large cross section for the production of $b$ quark pairs at the LHC and the large boost of the resulting $b$-hadrons, combined to the excellent vertexing capabilities of the LHCb detector, enable precise measurements of semi-tauonic decays in an hadronic environment. By using a data sample of pp collisions, corresponding to 3$fb^{-1}$ integrated luminosity, LHCb has measured the ratio $R(D^*)$ of branching fractions for the $B^0\to D^{*-} \tau^+ \nu_{\tau}$ and $B^0\to D^{*-} \mu^+ \nu_{\mu}$ decays. The result $R(D^*) = 0.336 \pm 0.027 \pm 0.030$ is in tension with the Standard Model prediction at the 2$\sigma$ level and, when combined with other measurements from the B Factories on decays with $D$ and $D^*$ mesons in the final states, increases the discrepancy with the Standard Model at the 4$\sigma$ level. In this review, the LHCb measurement is described and an outlook on other possible studies in the domain of semitauonic decays of $b$-hadrons is given.