The CMS Level-1 Tau algorithm for the LHC Run II

The CMS experiment relies on a two-level online selection system that achieves a rejection factor of nearly $10^5$. The first level (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 centre-of-mass energy of the LHC collisions will be increased to 13 or 14 TeV, and progressively an instantaneous luminosity of $10^{34} \mbox{cm}^{-2}\mbox{s}^{-1}$ will be reached. To guarantee a successful and ambitious physics program in this intense environment, the CMS Trigger and Data Acquisition system must be upgraded. The L1 Calorimeter Trigger hardware and architecture will be upgraded in particular, allowing sophisticated algorithms to be deployed. These algorithms will better exploit the calorimeter granularity and will open the possibility of making correlations between different parts of the detector. In this context, an optimised tau algorithm, implementing an innovative dynamic clustering technique, has been developed for the selection of hadronically decaying tau leptons, which represents a real challenge for an electronics trigger system. The performance of this tau trigger will be demonstrated, both in terms of efficiency and rate reduction. The different handles to control trigger rates in different pile-up scenarios will be described.