In particle physics, a trigger is a system that uses simple criteria to rapidly decide which events in a particle detector to keep when only a small fraction of the total can be recorded. Trigger systems are necessary due to real-world limitations in data storage capacity and rates. Since experiments are typically searching for "interesting" events (such as decays of rare particles) that occur at a relatively low rate, trigger systems are used to identify the events that should be recorded for later analysis. Current accelerators have event rates greater than 1 MHz and trigger rates that can be below 10 Hz. The ratio of the trigger rate to the event rate is referred to as the selectivity of the trigger. For example, the Large Hadron Collider (LHC) is anticipated to have an event rate of 1 GHz (109 Hz), and the Higgs boson is expected to be produced there at a rate of at least 0.01 Hz. Therefore the minimum selectivity required is ten to the minus eleven (see Wikipedia).
We are working on the software that controles this trigger at the Atlas experiment.