The Level 0 trigger must make decisions in < 1 microsecond based on total event multiplicity, coarse variations in multiplicity topology, event vertex location and remnant nucleon multiplicities. The Level 0 trigger receives ADC inputs from the various trigger detectors which include the Central Trigger Barrel (CTB), Multiwire Proportional Counter (MWC ), Vertex Position Detector (VPD) and the Veto Calorimeter (VTC). Detector response simulations have been developed for each of these subsystems (excluding VTC). Multiplicity analysis at Level 0 is based entirely on the results from the CTB and MWC.
The CTB is a series of scintillator slats placed cylindrically about the beam interaction point on the exterior of the STAR TPC. The detector has complete coverage in phi and 2 units of coverage in pseudorapidity from -1 < eta < 1. The CTB consists of 240 slats (4 in eta and 60 in phi), where each slat supplies an 8 bit ADC output. The MWC utilizes the 8000 anode wires in the endcaps of the TPC as a multi-wire proportional counter. This detector also covers phi completely and 2 units in eta: 2 < |eta| < 1. Original design plans had each encap subdivided into 12 sectors in phi and further divided in 4 sections in eta, thus yielding 48 sections per endcap. By charge integrating over the wires, each of these sections would provide a 10 bit ADC output. Recently the focus has changed to performing hit counting with the wires in the endcaps. By setting a bit if the charge collected on a given wire breaks some threshold we are then able to merely count the number of wires hit to get a determination of the particle multiplicity in the coverage range of the detector. This methodology has the advantage that in using the 8000 wires individually there is the possibility of having a much finer pixelation in the multiplicity distributions in the endcaps, as well as greater flexibility in variation of the coarse eta-phi pixel array which is passed to Level 1 trigger analysis .