Selecting long-lived particles in the first trigger level at the LHC

Abstract: The LHCb experiment is starting to take data in Run 3 with a new DAQ system, capable of performing complete event reconstruction at the full LHC collision rate. One novel opportunity offered by this system is triggering on long-lived particles (LLPs) at the very first stage of the trigger. This could potentially increase trigger efficiency for LLPs, typically suffering from low online detection efficiency at hadron collider experiments, because of their decay signatures. We investigated the feasibility and effectiveness of an early LLP-triggering approach in LHCb with the implementation of two LLP-dedicated selections in the first trigger level (HLT1), targeting the presence of either one, or two, $K^0_S$ decays. Selection tuning is performed on simulation, targeting some benchmark channels with $K^0_S$ particles in the final state, as $D^0 \rightarrow K^0_S K^0_S$ and $B^0 \rightarrow K^0_S K^0_S$. Tests ran on simulated samples predict a large increase in selection efficiency, up to 2.6x for the $D^0 \rightarrow K^0_S K^0_S$ channel, at the price of a very modest increase of HLT1 computational load and trigger rate. These selections were implemented in the GPU-based HLT1 trigger sequence, and took data during the physics data-taking LHCb run in year 2022. In this document, we present results obtained from these first data, yielding good quality $K^0_S$ and $K^0_S$-pair samples even from a very limited integrated luminosity. We conclude with a discussion of the physics prospects opened by these new triggers, and their planned extension to tracks decaying outside the volume of the VELO subdector ("downstream tracks") to further extend their acceptance.