Department of Physics

T0 detector in ALICE experiment at CERN


T0 is the fast timing and trigger detector for ALICE experiment at CERN. It will give the key trigger- and timing signals, measure on-line vertex position and give rough centrality. Among the many technical challenges is 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. It must be ready and operational from the day one. Data from T0 will be crucial not only for extraction of the precise interaction time but also for normalization between proton-proton and heavy ion runs.


Physics Objectives

T0 detector is required to fulfil the following functions:
  • To supply several signals to the ALICE 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.

The trigger functions requested from T0 are as follows:
  • To measure the approximate vertex position
  • To give a rough estimate of event multiplicity
  • To inform that at least one of the arms of T0 detector has registered a valid pulse

Photograph of the 1:1 prototype of T0-C.

The diameter of each PMT is 30 mm.

The tube in the center represents the beam line.


The vertex position approximation is crucial in discriminating against beam gas interactions. With 50 ps time resolution one should obtain +- 1.5 cm accuracy in vertex determination.


The second feature (multiplicity determination) will be an important back-up option for V0 that covers a considerably larger pseudo-rapidity range. For T0 that range is only 2.9 < η < 3.3 and -5 < η <- 4.5. The measured multiplicity will be analysed against 2 pre-set values to generate one of the three possible trigger signals: T0(minimum bias), T0(semi-central), or T0(central) corresponding to low, intermediate, and high multiplicities. There will be only two threshold values because the minimum bias signal is identical with T0-vertex (sufficient multiplicity to have triggered both halves of the T0 detector). Since T0 detector generates the earliest L0 trigger signals, all these signals should be generated strictly “on-line” without the possibility of any off-line corrections.


The early “wake-up” signal to the Transition Radiation Detector has also to be produced strictly on-line. Due to the substantial cable length between the T0 detector and the main T0 electronics rack outside of the L3 magnet, the wake-up signal for the TRD must be generated by a special electronics “shoebox” located somewhere on the of baby space frame inside of L3 magnet.


The T0 detector is the only ALICE sub-detector capable of delivering high-precision start signal for 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 TOF detector that 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 improved 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 (see the photograph of the prototype of T0-C).


T0 Databases


T0 electronics





The main institutes participating in the design, construction, and operation of the T0 detector are:
  • HIP — University of Jyväskylä, Department of Physics and Helsinki Institute of Physics, Jyväskylä, Finland;
  • MEPhI — Moscow Engineering Physics Institute, Moscow, Russia
      o Vladislav Grigoriev (Leader of the Russian Team)
      o Alexey Bogdanov (Trigger, Offline)
      o Vladimir Kaplin (Electronics)
      o Alexandr Karakash (PMT, test measurements)
      o Vitaly Loginov (Electronics)
  • INR — Academy of Science, Institute of Nuclear Research, Moscow, Russia
      o Alexei Kurepin (INR Group Leader)
      o Fedor Guber (Mechanics)
      o Tatyana Karavicheva (DCS, readout)
      o Oleg Karavichev (Electronics)
      o Alexander Kurepin (DCS)
      o Victor Marin (CFD)
      o Alla Mayevskaya (Simulations)
      o Andrei Reshetin (PMT shielding)
  • KI — Russian Research Center “Kurchatov Institute”, Moscow, Russia
      o Evgeni Meleshko (T0 Vertex, Multiplicity)
      o Anatoly Klimov (Technical Project)
  • WUT — Warsaw University of Technology, Warsaw, Poland
      o Radomir Kupczak (WUT Group Leader, Electronics - ELMB + DAC for CFD)
      o Marcin Zajac (Software PVSS ELMB+DAC for CFD)
      o Mateusz Szymanski (Software PVSS ELMB+DAC for CFD)
      o Marcin Zaremba (Software PVSS ELMB+DAC for CFD)


The Greek group from Athens (Marta Spyropoulu-Stasinakhi) has also expressed serious interest in participating in the T0 DAQ. The Greek group would contribute both in terms of manpower and core costs.