CMS Level-1 electron/photon trigger performance

Since March 2010 the Large Hadron Collider (LHC) has provided high energy proton collisions with an instantaneous luminosity that has risen by several orders of magnitude to around 4e33 cm-2 s-1 at the end of 2011 corresponding to millions of collisions per second. With this unprecedented collision rate, efficient triggering on electrons and photons has become a major challenge for LHC experiments. The Compact Muon Solenoid (CMS) experiment uses a two-level trigger system. The first level (L1) is based on coarse information coming from calorimeters and muon detectors, accepting up to 100kHz of events per second. A High-Level Trigger (HLT) then combines fine-grain information from all sub-detectors to reduce this rate further to about 200-300Hz. At L1 the electron/photon trigger is based upon information from the Electromagnetic Calorimeter (ECAL), a high resolution detector comprising 75848 lead tungstate (PbWO4) crystals in a "barrel" and two "endcaps". The optimization and performance of this system in terms of electron and photon triggering efficiency are presented.