The design of a fast Level-1 track trigger for the High Luminosity Upgrade of ATLAS

To increase the number of proton-proton collisions the Large Hadron Collider at CERN aims to increase its instantaneuos luminosity to around five times the nominal value for run four, set to begin in 2026. This will force the experiments, including ATLAS, to adapt to the increased event rate which will require substantial hardware upgrades. The current trigger system will not be able to cope with these rates without raised thresholds wich would mean loosing many of the events. To increase the rejection rate without loosing signal efficiency tracking information could be utilized in the first level hardware trigger. This document presents results from simulating a track trigger seeded by regions of interest. It is shown that with this approach we can reach a five times rejection of background events while keeping the single lepton efficiency above 95%. To reduce the amount of track fits needed per event the L1Track trigger is not only seeded by regions of interest corresponding to 10% of the tracking volume, but also uses a course resolution matching of hits with precomputed patterns. The production and design of these patterns are discussed.