Optimization and calibration of the LHCb tagging performances using 2010 data

We describe the flavour tagging algorithm of LHCb, its optimization and calibration using the decays $B_d \to D^{*-}\ mu^+\nu, B^+ \to J/\psi K^+$ and $B_d \to J/\psi K^*$, with 35 pb$^{-1}$ of data from the 2010 LHCb physics run. After a presentation of the flavour tagging principles, we describe the algorithm and its data-driven optimization and calibration. We measure an effective tagging efficiency for the opposite side taggers and same side pion combination of 2.87$\pm$0.32$\%$ in the $B_d \to D^{*-}\ mu^+\nu$ channel, 2.38$\pm$0.33$\%$ in the $B^+ \to J/\psi K^+$ and 2.52$\pm$0.82$\%$ for $B_d \to J/\psi K^*$. The per event mistag is calibrated using the $B^+ \to J/\psi K^+$ channel and verified in $B_d \to J/\psi K^*$. The precision of the calibration parameters is limited by statistics: it amounts to $\pm 0.012$ on the mean value and $\pm 0.12$ on the scale.