Level-1 trigger selection of electrons and photons with CMS for LHC Run-II.

The CMS experiment has a sophisticated two-level online selection system that achieves a rejection factor of nearly $10^5$. The first, hardware-level trigger (L1) is based on coarse information coming from the calorimeters and the muon detectors while the High-Level Trigger combines fine-grain information from all subdetectors. During Run II, the LHC will increase its center of mass energy to 13 or 14 TeV, and progressively reach an instantaneous luminosity of $2\times10^{34} \mathrm{cm}^{-2}\mathrm{s}^{-1}$. In order to guarantee a successful and ambitious physics programme in this intense environment, the CMS trigger and data acquisition system must be upgraded. The L1 calorimeter trigger hardware and architecture in particular has been redesigned to maintain the current thresholds even in presence of more demanding conditions (e.g., for electrons and photons) and improve the performance for the selection of $\tau$ leptons. This design benefits from recent $\mu$TCA technology, allowing sophisticated algorithms to be deployed, better exploiting the calorimeter granularity and opening the possibility of making correlations between different parts of the detector. The electron/photon selection algorithm uses an innovative dynamic clustering procedure along with an online pile-up correction technique which represents a real challenge for an electronics trigger system. The performance of this electron/photon trigger will be demonstrated, both in terms of efficiency and rate reduction. The new features to control rates in different pile-up scenarios will be described. Finally, the plans for the commissioning with the first Run II data will be presented and the expected impact on the physics potential assessed.