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Trigger detector of charged particle multiplicity (T0)

Nuclei can collide at various impact parameters. To study the physics of phaser transitions, the maximal volume is desired. Consequently, experiments select collisions with small impact parameters, i.e. ``central'' or head-on collisions of nuclei. For higher centrality (or smaller impact parameter), the number of nucleons interacting increases, as does the multiplicity of produced particles. Selecting events with high particle multiplicity selects more central collisions.

To characterize event multiplicity and to implement multiplicity trigger, NA44 has a plastic scintillator counter positioned 3 cm downstream of the target and known as T0. It consists of two parts, each in shape of a rectangular parallelepiped $3\times12\times15$ mm, placed symmetrically around a vertical plane passing along the beam line trough the center of the target, with a 3 mm gap between them. T0 covers $1.4 \le \eta \le 3.7$ for an $\eta$-dependent fraction of azimuthal angle, $0.22 \le \Delta \phi / 2\pi \le 0.84$ respectively. Each half of the detector is equipped with two photomultiplier tubes (PMTs) for optical readout of scintillations. Signals from both halves of T0 are required to pass a discriminator threshold to form a centrality trigger. Normally, the discriminator thresholds are set so that the probability of a centrality trigger for an interaction event did not exceed 15%. In such events, there are over three hundred charged particles in the T0 acceptance.