The CMS trigger system for the HL-LHC

The High-Luminosity LHC will open an unprecedented window on the weak-scale nature of the Universe, providing data to perform high-precision measurements of the Standard Model, as well as searches for new physics beyond the Standard Model. Such precision measurements and searches require information-rich datasets with a statistical power that matches the high luminosity provided by the upgrade of the LHC. Efficiently collecting these datasets will be a challenging task, given the harsh environment of up to 200 proton-proton interactions per LHC bunch crossing. For this purpose, CMS is designing an efficient two-level trigger system: the Level 1 Trigger (L1T), implemented in advanced hardware, and the High Level Trigger (HLT), a streamlined version of the CMS reconstruction software running on a computer farm. The L1T will include tracking information and high-granularity calorimeter information for the first time. The current conceptual system design is expected to take full advantage of FPGA and link technologies over the coming years, providing a high-performance, low-latency computing platform for large throughput and sophisticated data correlation across diverse sources. The envisaged L1T system will closely replicate the full offline object reconstruction, to perform more sophisticated and optimized selection. The higher luminosity, event complexity and input rate of 750 kHz present an unprecedented challenge to the HLT, which aims to achieve a similar efficiency and rejection factor as today, despite the higher pileup, and a purer preselection. The introduction of a heterogenous platform combining CPUs and GPUs is described. The expected performance of the upgraded trigger system is presented.