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Trigger

The art of experiment is to make the equipment answer as accurately as possible the questions we ask, while being as insensitive as possible to what we deem irrelevant. Ability to identify the difference between what can be made irrelevant and what can not depends on the knowledge previously acquired, and is among the experimentalist's most precious qualifications. The purpose of a trigger is to allow the experiment to focus on the selected topic. An hierarchy of trigger conditions is maintained. In its turn, the hardware trigger (i.e. the permission to record the information from the equipment) itself forms the preliminary stage for a more complex off-line analysis. Its negative consequences can not be overcome off-line, but can only be corrected for, in which process the richness and complexity of the above-mentioned hierarchy plays the crucial role.

How important is the issue of a trigger ? The SPS delivers spills of particles with given momentum to the target. A typical duration of a spill is about 5 s, during which about $4\times10^6$ particles cross the target. A typical NA44 run used for single particle analysis lasts for about 100 spills, and records a tape with about $4\times10^4$ events. Having an interaction in the target is therefore roughly two orders of magnitude more frequent an event than writing a selected event to the tape. This means that by virtue of the NA44 trigger, these $4\times10^4$ events can be easily collected so that each of them contains a proton or a kaon, or a pion pair, or at least one charged particle of any identity. With some change in the numbers, but without difficulty, samples of kaon and proton pairs for correlation analysis can be obtained. This section explains how this is possible.