Opposite-Side Flavour Tagging of B Mesons at the LHCb Experiment

The calibration and performance of the opposite-side flavour tagging algorithms used for the measurements of time-dependent asymmetries at the LHCb experiment are described. The algorithms have been developed using simulated events and optimized and calibrated with B+ → J/ψ K+, B0 → J/ψ K∗0 and B0 → D∗− μ+ νμ decay modes with 0.37 fb^−1 of data collected in pp collisions at √s = 7 TeV during the 2011 physics run. The opposite-side tagging power is determined in the B+ → J/ψ K+ channel to be (2.10 ± 0.08 ± 0.24)%, where the first uncertainty is statistical and the second is systematic.