Level 1 of the trigger is an extremely fast and wholly automatic process that looks for simple signs of interesting physics, e.g.
To not confuse particles from two different events, the detectors must have very good time resolution and the signals from the millions of electronic channels must be synchronised so that they can all be identified as being from the same event. New waves of particles are being generated before those from the last event have even left the detector! The solution is to store the data in pipelines that can retain and process information from many interactions at the same time. However, with groups of protons colliding 40 million times per second there are only ever 25 nanoseconds (25 billionths of a second) before the next lot arrive.
We therefore need a “trigger” that can select the potentially interesting events, such as those which will produce the Higgs particle, and reduce the rate to just a few hundred “events” per second, which can be read out and stored on computer disk for subsequent analysis. There is no way that data from all these events could be read out, and even if they could, most would be less likely to reveal new phenomena they might be low-energy glancing collisions for instance, rather than energetic, head-on interactions. When CMS is performing at its peak, about one billion proton-proton interactions will take place every second inside the detector.
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