Opposite-side flavour tagging of B mesons at the LHCb experiment

The calibration and performance of the opposites-ide 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 ⁺ , B ⁰ →J/ψK* ⁰ and B ⁰ →D* ⁻ μ ⁺ ν _μ 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.