The ZEUS calorimeter first level trigger

In order to separate the interesting physics events from the background without unmanageable deadtime, the ZEUS experiment uses a pipelined trigger and data acquisition system. One of the chief elements of the first level trigger is the Calorimeter First Level Trigger (CFLT). It provides three different categories of information to the Global First Level Trigger (GFLT): numbers of isolated electrons and muons in the calorimeter, patterns of energy deposition derived from local energy sums, and total energy, transverse energy, and missing energy deposited in the calorimeter. The CFLT has to make each step of these calculations, including digitization of the photomultiplier-tube pulse, at a rate of 96 ns in order to keep up with the pipeline. However, in order to reduce the amount of electronics, cables, and crate backplane required, the data are multiplexed in time at several points so that many elements of the system run at faster rates, up to 83 MHz. The CFLT passes trigger information to the GFLT within 2 μs