The LHCb Trigger System

The LHCb detector has been conceived to study with high precision CP violation and rare decays of b-flavoured hadrons produced at the LHC. The LHCb trigger is of crucial importance in selecting among the bulk of collisions those that are of interest for b-physics studies. The trigger is based on a two-level system. The first level, Level-0, is implemented in hardware and uses information from the calorimeter, muon and pile-up systems to select events containing particles with relatively large transverse momentum, typically above 1-2 GeV. The Level-0 trigger accepts events at a rate of 1 MHz. All the detector information is then read out and fed into the High Level Trigger. This software trigger runs in the event filter farm composed of about 1800 CPU nodes. Events are selected at a rate of 2kHz and sent for mass storage and subsequent offline reconstruction and analysis. The current status and expected performance of the trigger system are described.