Online Event Selection at the CMS experiment

Triggering in the high-rate environment of the LHC is a challenging task. The CMS experiment has developed a two-stage trigger system. The Level-1 Trigger is based on custom hardware devices and is designed to reduce the 40 MHz LHC bunch-crossing rate to a maximum event rate of ~100 kHz. The further reduction of the event rate to O(100 Hz), suitable for permanent storage, is performed in the High-Level Trigger (HLT) which is based on a farm of commercial processors. The methods used for object identification and reconstruction are presented. The CMS event selection strategy is discussed. The performance of the HLT is also given.