A new algorithm for identifying the flavour of $B_s^0$ mesons at LHCb

Abstract

A new algorithm for the determination of the initial flavour of $B_s^0$ 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 $B_s^0$ meson. The second network combines the kaon charges to assign the $B_s^0$ 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 proton-proton collisions at 7 and 8 TeV centre-of-mass energies. The calibration is performed in two ways: by resolving the $B_s^0$-${\bar{B}}_s^0$ flavour oscillations in $B_s^0\to D_s^{-}\ddot{I}{€}^{+}$ decays, and by analysing flavour-specific $B_{s2}^{\ast }(5840{)}^0\to B^{+}{K}^{-}$ decays. The tagging power measured in $B_s^0\to D_s^{-}\ddot{I}{€}^{+}$ decays is found to be $(1.80\pm 0.19(\mathrm{stat})\pm 0.18(\mathrm{syst}))$\%, which is an improvement of about 50\% compared to a similar algorithm previously used in the LHCb experiment.

All figures and tables, along with any supplementary material and additional information, are available at https://lhcbproject.web.cern.ch/lhcbproject/Publications/LHCbProjectPublic/LHCb-PAPER-2015-056.html