ALICE FIT data processing and performance during LHC Run 3

During the upcoming Run 3 and Run 4 at the LHC the upgraded ALICE (A Large Ion Collider Experiment) will operate at a significantly higher luminosity and will collect two orders of magnitude more events than in Run 1 and Run 2. A part of the ALICE upgrade is the new Fast Interaction Trigger (FIT). This thoroughly redesigned detector combines, in one system, the functionality of the four forward detectors used by ALICE during the LHC Run 2: T0, V0, FMD and AD. The FIT will monitor luminosity and background, provide feedback to the LHC, and generate minimum bias, vertex and centrality triggers, in real time. During the offline analysis FIT data will be used to extract the precise collision time needed for time-of-flight (TOF) particle identification. During the heavy-ion collisions, FIT will also determine multiplicity, centrality, and event plane. The FIT electronics designed to function both in the continuous and the triggered readout mode. In these proceedings the FIT simulation, software, and raw data processing are briefly described. However, the main focus is on the detector performance, trigger efficiencies, collision time, and centrality resolution.

increased luminosity and interaction rate. The LHC will deliver Pb-Pb collisions at up to luminosity 6 · 10 27 cm −2 s −1 , corresponding to an interaction rate of 50 kHz. The goal of ALICE is to integrate a luminosity of 13 nb −1 for Pb-Pb collisions at √ s N N = 5.5 TeV, together with dedicated p-Pb and pp reference runs. Data from pp collisions will also be collected at the nominal LHC energy √ s = 14 TeV [2]. Run 3 at the CERN LHC is scheduled to start in 2022.
The ALICE upgrade includes: and centrality triggers. Offline, FIT provides the precise collision time for the TOF-based particle identification, determines the centrality and event plane, and measures the cross section of diffractive processes. In addition, FIT can reject beam-gas events and provide vetoes for ultra-peripheral collisions.

Construction of FIT subdetectors
The

FIT electronics
The FT0, FV0, and FDD utilize the same electronics scheme based on two customdesigned modules: the processing Module (PM) and the Trigger and Clock Module (TCM).
The PM processes and digitizes input signals, packs the data for readout (in continuous or triggered mode), and makes the first stage calculations for trigger decision. The TCM processes data from PMs, makes the final trigger decisions, provides accurate clock reference, and serves as the slow control interface to the connected PMs. The FT0 can produce trigger signals every 25 ns that is for each LHC bunch crossing.
However, due to the limited acceptance, the efficiency has to be verified by simulations.
Pythia8 [7] was used to simulate particles from pp collisions. Twenty thousand pp collisions were generated and transported through the ALICE setup. Cherenkov photons from relativistic charged particles traversing quartz radiators were utilized to produce digitized signals taking into account the detector response and possible pile-up. The signals were used to evaluate the efficiency of the following FT0 triggers: FT0A -signal only from the A side; FT0C -signal only from the C; Vertex -signals from both sides and vertex within given range.

FT0 performance for Pb-Pb collisions
When the impact parameter of two colliding ions is larger than the sum their radii, hadron collisions are replaced by electromagnetic interactions corresponding to photon-photon and photon-nuclear collisions. The main source of background comes from pair production (e + e − ), having orders of magnitude larger cross section than the hadronic processes. For instance, according to PYTHIA8, the Pb-Pb hadronic cross-section at √ s N N = 5.5 TeV is 8 b while the cross section for electromagnetic collisions, used by the QED generator, developed especially for ALICE, is around 180 kb. Fortunately, QED events have a very low charged particle multiplicity. They can be rejected by setting a threshold value for the sum of the FT0A and FT0C amplitudes. Centrality determination with a good resolution is an important functionality of the FIT detector. Fig. 5 shows the centrality resolution for Pb-Pb collision at √ s N N = 5.5 TeV calculated for FT0A, FT0C, and FV0 separately, and the combined resolution (FT0A+FT0C+FV0).

CONCLUSIONS
Our analysis has demonstrated that the simulated performance of the FT0 subdetector of FIT satisfies the design requirements of the ALICE experiment: • The minimum bias trigger efficiency matches that of the VZERO detector operated during the Run 1 and 2 of the LHC; • The collision time resolution is better than that of the T0 during the Run 1 and Run 2; • The vertex trigger has a 100% efficiency for semi-central and central events.