Readout system of the ALICE Fast Interaction Trigger

The Fast Interaction Trigger (FIT) detector will be essential for the operation of the ALICE experiment at CERN during Run 3 and 4 of the LHC . FIT will serve as an interaction trigger, luminometer, the first indicator of the vertex position, and the forward multiplicity counter. It will also provide the precise collision time for the TOF-based particle identification, yield the centrality and interaction plane for flow measurements, and measure cross sections of diffractive processes. In order to cope with an increased interaction rate of up to 1 MHz in proton-proton (pp) collisions and up to 50 kHz in Pb-Pb collisions, a new readout system for the ALICE FIT detector has been designed and implemented. FIT readout system is compatible both with the triggered and continuous (triggers-less) ALICE readout modes. The GBT-FPGA based FIT readout and trigger systems allow to stream data up to 5.5 MHz event rate with 3.2 Gbps data rate. The trigger processing time is only 225 ns. With the additional 200 ns delay along the connecting signal cables, the total latency of the FIT trigger is 425 ns.