Multi-jet pile-up suppression techniques for ATLAS' HL-LHC Level 0 trigger

The Large Hadron Collider (LHC) will undergo a major improvement from 2026-2029 for the new High Luminosity LHC (HL-LHC). The number of collisions per proton bunch crossing will increase from ~60 to ~200. This will stress the current event selection (trigger) system, and the efficiency of specialized jet triggers in particular. An important challenge lies in classifying jets coming from a single vertex or from multiple ones, and the difficulty in doing this is exacerbated by the increased high energy background jets under high luminosity. Therefore, as part of the ongoing ATLAS detector upgrade, we present pile up suppression and event classification techniques for Level 0 (hardware-based level) at the HL-LHC. Firstly, a multi-vertex jet trigger using Boosted Decision Trees (BDTs) is explored. FPGA implementation of the trigger using fwXmachina is also investigated. Finally, the use of calorimeter cell timing as well as jet timing to suppress out-of-time jets at HL-LHC is evaluated.