The HyperCP Trigger

A total of 24 triggers are used in the experiment, most for diagnostic and monitoring purposes. The triggers are fast, with single-bucket resolution (19 ns), simple, and provided high yields to tape. Only first-level triggers are employed. Common to all the physics triggers are a left-right coincidence of charged particles in the Same-Sign (as the channeled beam) and Opposite-Sign hodoscopes at the rear of the spectrometer. The addition of the hadronic calorimeter to the Cascade (calling it a Cascade trigger is a bit of a misnomer as it only requires two charged particles of opposite sign, and hence is really a Lambda trigger) and Kaon triggers reduced their rates by about a factor of six, with a conservative minimum energy threshold. The typical Cascade trigger rate was 30 kHz and the overall trigger rate including the Kaon, muon, and monitoring triggers was 75 kHz.

The trigger worked well and provided a high yield to tape. For example, typical Lambda (anti-Lambda) yields were 20% (5%) for the Cascade trigger. The trigger rate was about a factor of two higher than our Monte Carlo predictions, which is not surprising since the simulations did not include gamma conversions and particle production at the collimator exit. Every hodoscope element, sub-trigger, and trigger was scaled and written to disk by the slow DAQ, as well as being latched and written to tape by the fast DAQ.