A method to improve b flavour tagging using charged leptons and hard b fragmentation characteristics

The Z0 factory at LEP is a copious source of banti b heavy flavour production. A method to improve b flavour tagging is presented here based on a study of 150 000 Z0 → qanti q events simulated in a realistic detector environment. Exploiting the hard b fragmentation characteristics and the large transverse momentum, PT of the charged lepton from semileptonic decay, we demonstrate that it is possible to improve b tagging by about 40% with good sample purity as compared to using PT alone for tagging. A variable PT/Ejet is suggested where Ejet is the energy of the hadronic jet nearest to the candidate lepton used for b tagging. Compared to the lighter quarks the Ejet distribution is soft for heavier quarks. Events with PT/Ejet > 0.045 lead to a sample purity of 77% for the prompt b decays and 85% for the cascades included. The method allows to select a substantial fraction of low PT (< 1 GeV/c) events which are otherwise lost while rejecting the low PT backgrounds. (orig.)