A new algorithm for identifying the flavour of B⁰ _s mesons at LHCb

A new algorithm for the determination of the initial flavour of B⁰ _s mesons is presented. The algorithm is based on two neural networks and exploits the b hadron production mechanism at a hadron collider. The first network is trained to select charged kaons produced in association with the B0 s meson. The second network combines the kaon charges to assign the B0 s flavour and estimates the probability of a wrong assignment. The algorithm is calibrated using data corresponding to an integrated luminosity of 3 fb^-1 collected by the LHCb experiment in protonproton collisions at 7 and 8 TeV centre-of-mass energies. The calibration is performed in two ways: by resolving the B⁰ _sB⁰ _s flavour oscillations in B⁰ _s →D^- _s φ⁺ decays, and by analysing flavour-specific B∗ _s2 (5840)⁰→ B⁺K^- decays. The tagging power measured in B⁰ _s → D^- _s φ⁺ decays is found to be (1:80 ± 0:19 (stat) ± 0:18 (syst))%, which is an improvement of about 50% compared to a similar algorithm previously used in the LHCb experiment.