The CMS Level-1 Trigger for LHC Run II

The Compact Muon Solenoid (CMS) experiment implements a sophisticated two-level online selection system that achieves a rejection factor of nearly $10^5$. During Run~II, the LHC has increased the centre-of-mass energy of proton-proton collisions up to 13 TeV and may progressively reach an instantaneous luminosity of $2\times10^{34}\mbox{cm}^{-2}\mbox{s}^{-1}$ or higher. In order to guarantee a successful and ambitious physics programme under this intense environment, the CMS Trigger and Data acquisition system has been upgraded. The upgraded CMS Level-1 (L1) trigger benefits from the recent $\mu$TCA technology and is designed to maintain the performance under high instantaneous luminosity conditions. More sophisticated, innovative algorithms are now the core of the first decision layer of CMS: this drastically reduces the trigger rate and improves the trigger efficiency for a wide variety of physics processes. %In this document the structure and the performance of the upgraded CMS L1 trigger system are described. In this document, we present the overall architecture of the upgraded Level-1 trigger system. The performance of single object triggers, measured on collision data recorded during the 2016 running period, are also summarised.