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 the collisions 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 2 kHz and sent for mass storage and subsequent offline reconstruction and analysis. The current status and expected performance of the trigger system are described.