Trigger performance of the ALICE Silicon Pixel Detector

The ALICE Silicon Pixel Detector (SPD) forms the two innermost layers of the ALICE Inner Tracking System (ITS). It consists of two-layer barrel of hybrid silicon pixel detectors. The SPD contains 1200 pixel readout chips and about 107 pixel cells. The unique feature of the SPD among the vertex detectors of LHC experiments is the prompt trigger capability, called Fast-Or, which allows it to contribute to the experimental first level trigger decision. The SPD data stream includes 1200 digital signals (Fast-Or) promptly asserted by each readout chip on the presence of at least one pixel hit in the matrix. The off-detector Pixel Trigger system elaborates in real time the 1200 Fast-Or signals continuously received from the detector in order to generate a pre-processed signal for the ALICE first level trigger. All the Fast-Or signals are written in the data and available for offline analysis. In this contribution it is presented the methodology used to evaluate the performance of the Fast-Or. The methods are entirely based on offline data analysis. Results are given for a sample of data of proton–proton collisions at s = 7 TeV .