Fast Interaction Trigger for the upgrade of the ALICE experiment at CERN: design and performance

ALICE (A Large Ion Collider Experiment) at the CERN LHC is designed to study the properties of the Quark–Gluon Plasma (QGP) in heavy-ion collisions. In 2019–2020 the upgrade of the LHC will increase the luminosity and the collision rate beyond the design parameters of the current ALICE setup. To be able to benefit from the improved performance of the LHC, ALICE will upgrade several of its key detector systems including the Fast Interaction Trigger (FIT). FIT is designed to provide the functionality of the existing forward detectors while retaining or even improving their performance. It will provide minimum bias (MB) trigger with an efficiency higher than 90% for pp collisions, measure the luminosity for pp and Pb–Pb collisions, and sustain interaction rates up to 1 MHz and 50 kHz, respectively. FIT will determine the collision time with a resolution better than 50 ps and will be used to measure the event multiplicity, the centrality, and the reaction plane. The detector consists of two arrays of Cherenkov radiators with micro-channel plate photo-multiplier (MCP-PMT) sensors, placed on both sides of the interaction point and of a single large-diameter scintillator ring. This article discusses the main design concepts, detector construction, beam test results, Monte Carlo simulations, and the results of detector performance studies.