The liquid argon jet trigger of the H1 Experiment at HERA

We report on a novel calorimeter trigger for the H1 Experiment at HERA, called the “Jet Trigger”. This trigger was running at level 1 and implemented a real-time cluster algorithm. Within only 800 ns, the Jet Trigger finds local energy maxima in the calorimeter, sums their immediate neighbors, sorts the resulting “jets” by energy, and applies topological conditions for the final level 1 trigger decision. The Jet Trigger has operated since the year 2006 until the end of the HERA running in the summer of 2007. With the Jet Trigger it was possible to substantially reduce the thresholds for triggering on electrons and jets.