T0 is the fast timing and trigger detector of ALICE. It gives the key trigger and timing signals, measures on-line vertex position and gives rough centrality. There are many technical challenges, such as the dead time below 25 ns needed to cope with the bunch crossing rate in p-p collisions, sustained count rate of 0.2 MHz, required time resolution below 50 ps, radiation hardness of up to 500 krad, operation in the 0.5 T magnetic field, compact design, high reliability, and maintenance-free operation during the entire life-time of ALICE. Data from T0 are crucial not only for extraction of the precise interaction time but also for the normalisation between proton-proton and heavy ion runs.
T0 detector is made of two arrays of Cherenkov radiators (quartz) equipped with fast photomultipliers placed on the opposite sides of the Interaction Point.
The T0 detector is required to fulfil the following functions:
- To supply main signals to the ALICE L0 trigger.
- To deliver an early (prior to L0 trigger) wake-up to TRD.
- To give a precise start signal for time-of-flight (TOF) particle identification
Finally, T0 is the first of the ALICE detectors to be turned on as it also measures luminosity – the parameter determining the quality of the colliding beams.
The T0 detector consists of two arrays (T0-A and T0-C) of Cherenkov counters with the IP in–between. The distance from the IP to T0-C is 70cm while on the opposite side the distance from T0-A to the IP is about 3.6m. T0-C covers the pseudorapidity range 2.9 < η < 3.3 and T0-A of 4.5 < η < 5.
The T0 detector is the only ALICE sub-detector capable of delivering high-precision start signal for the TOF detector. This T0 signal must correspond to the real time of the collision (plus a fixed time delay) and be independent of the position of the vertex. The required precision of the T0 signal must be better, or at least equal, to that of the TOF detector, which is about 50 ps (sigma). The function of generating T0 start will not be doubled by any other detector in ALICE, so the quality of T0 time resolution will directly influence the quality of TOF identification. In favourable cases, mostly for HI collisions, one may expect some further improvement of T0 time resolution in off-line analysis. For that purpose, it is important to read out and store the time and amplitude from each PMT of the T0 array.
Click here to read the Technical Design Report for the ALICE Forward Detectors.