Status of the Fast Interaction Trigger detector for the ALICE upgrade

During the second Long Shutdown (LS2) of the LHC, ALICE has installed three new detectors and implemented continuous data readout with online reconstruction and data compression for several other subsystems. The changes are needed to benefit from the increased luminosity of the LHC during Run 3 and 4. The ALICE interaction rate will increase, with respect to Run 1 and 2, by up to two orders of magnitude, reaching 50 kHz for Pb-Pb and up to 1 MHz for pp collisions. One of the new ALICE detectors is the Fast Interaction Trigger (FIT). Its main functionality includes generating minimum-latency interaction triggers ($<425$ ns), luminosity monitoring with online feedback to the LHC, a precise measurement of the collision time with a resolution better than 40 ps, determination of the centrality and event plane for heavy-ion collisions, and tagging of diffractive and ultra-peripheral events. The FIT detector consists of three subsystems: two fast Cherenkov arrays with 2 cm thick quartz radiators coupled to modified MCP-PMT photosensors (FT0), a large-area scintillator disc (FV0) implementing a novel light collection system, and the forward diffractive detector (FDD). FDD comprises two plastic scintillator arrays with fast wavelength shifting bars, optical fibre bundles, and PMTs. The FDD arrays are located ~$\pm 20$ m away from the nominal interaction point, along the beam line. A brief description of the detector and its functionalities is given together with the installation and commissioning status.