Pileup Mitigation at CMS Level-1 Trigger for the HL-LHC

The high luminosity operation of the LHC will deliver collisions with a flux of about 10 times higher than the original design value of the instantaneous luminosity planned during early 1990s. This poses a big challenge for trigger and data acquisition in real-time due to nearly 200 overlapping collisions, called pileup. The CMS experiment will revamp its trigger structure as part of the required upgrade, to have the tracking information and more granular calorimeter data available for the first layer (Level 1, L1) of the trigger deployed in custom hardware including high-end FPGAs, SoM etc. The correlator units at L1 will further process the information from each sub-detector to make a global event description through the particle-flow (PF) approach. Disentanglement of the contributions from the pileup particles from those of interesting physics processes is achieved by implementing the Pile-Up Per Particle Identification (PUPPI) algorithm at L1. We present the strategy for implementation of PUPPI and PF at the L1, focusing on the Hadron Forward Calorimeter detector of CMS.