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

The CMS experiment implements a sophisticated two-level online selection system that achieves a rejection factor of nearly 10e5. 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 sub-detectors. During Run II, the centre of mass energy of the LHC collisions will be increased up to 13/14 TeV and the instantaneous luminosity will eventually reach 2e34 cm-2s-1. To guarantee a successful and ambitious physics program under this intense environment, the CMS Trigger and Data acquisition system must be consolidated. In particular, the L1 calorimeter Trigger hardware and architecture will be upgraded, benefiting from the recent microTCA 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. Given the enhanced granularity provided by the new system, an optimized tau algorithm has been developed. This algorithm is based on an innovative dynamic clustering technique which is also used to identify electrons and photons at the trigger level. The selection of a hadronically decaying taus, giving multiple decay channels, represents a real challenge for an electronics trigger system. In addition, to satisfy both physics performance, and hardware constraints, the level-1 tau algorithm requires a specific adaptation of the electron/photon algorithm. The performance of this tau trigger will be demonstrated, both in terms of efficiency and rate reduction. The different handles to control the rate 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.