The Hadron Alley of the LHCb High Level Trigger

This note describes the "hadron alley", the list of algorithms that selects B meson decays into hadrons in the first level of the LHCb software trigger (HLT1). The alley aims to obtain a rate reduction of at least a factor 30 with respect to the input rate of the hardware trigger (L0), while keeping the efficiency on offline selected B decays into hadrons as high as possible. The hadron alley implements the "L0 confirmation" strategy, that requires that HLT1 trigger decisions are based on particles that triggered the L0. The performance of the alley is presented and some alternative configurations of the L0 hadron trigger and the hadron alley are discussed.