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

The calibration and performance of the oppositeside 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 /ψK + , 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 oppositeside tagging power is determined in the B + → J /ψK + channel to be (2.10 ± 0.08 ± 0.24) %, where the first uncertainty is statistical and the second is systematic.