Triggering on electrons and photons with CMS

Throughout the year 2011, the Large Hadron Collider (LHC) has operated with an instantaneous luminosity that has risen continually to around 4 × 10³³cm^-2s^-1. With this prodigious high-energy proton collisions rate, efficient triggering on electrons and photons has become a major challenge for the LHC experiments. The Compact Muon Solenoid (CMS) experiment implements a sophisticated two-level online selection system that achieves a rejection factor of nearly 10⁶. The first level (L1) is based on coarse information coming from the calorimeters and the muon detectors while the High-Level Trigger (HLT) combines fine-grain information from all sub-detectors. In this intense hadronic environment, the L1 electron/photon trigger provides a powerful tool to select interesting events. It 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 performance as well as the optimization of the electron/photon trigger are presented.