The basic functions of the ITS are:

1. determination of the primary vertex and of the secondary vertices necessary for the reconstruction of charm and hyperon decays,
2. particle identification and tracking of low-momentum particles,
3. improvement of the momentum and angle measurements of the TPC.

Tracking in ALICE

Track finding in heavy-ion collisions at the LHC presents a big challenge, because of the extremely high track density. In order to achieve a high granularity and a good two-track separation ALICE uses three-dimensional hit information, wherever feasible, with many points on each track and a weak magnetic field. The ionization density of each track is measured for particle identification. The need for a large number of points on each track has led to the choice of a TPC as the main tracking system. In spite of its drawbacks, concerning speed and data volume, only this device can provide reliable performance for a large volume at up to 8000 charged particles per unit of rapidity. The minimum possible inner radius of the TPC (of about 90 cm) is given by the maximum acceptable hit density. The outer radius (of about 250 cm) is determined by the minimum length required for a dE/dx resolution better than 10%. At smaller radii, and hence larger track densities, tracking is taken over by the ITS. The ITS consists of six cylindrical layers of silicon detectors. The number and position of the layers are optimized for efficient track finding and impact parameter resolution. In particular, the outer radius is determined by the track matching with the TPC, and the inner one is the minimum compatible with the radius of the beam pipe (3 cm). The silicon detectors feature the high granularity and excellent spatial precision required. Because of the high particle density, up to 90 per squared centimeter, the four innermost layers (r < 24 cm) must be truly two-dimensional devices. For this task silicon pixel and silicon drift detectors were chosen. The outer two layers at a radius of about 45 cm, where the track densities are below 1 per squared centimeter, will be equipped with double-sided silicon microstrip detectors. With the exception of the two innermost pixel planes, all layers will have analog readout for particle identification via a dE/dx measurement in the non-relativistic region. This will give the inner tracking system a stand-alone capability as a low-pT particle spectrometer.

Physics of the ITS

The ITS will contribute to the track reconstruction by improving the momentum resolution obtained by the TPC. This will be beneficial for practically all physics topics which will be addressed by the ALICE experiment. The global event features will be studied by measuring the multiplicity distributions and the inclusive particle spectra. For the study of resonance production (ρ, ω and φ) and, more importantly, the behaviour of the mass and width of these mesons in the dense medium, the momentum resolution is even more important. We have to achieve a mass precision comparable to, or better than, the natural width of the resonances in order to observe changes of their parameters caused by chiral symmetry restoration. Also the mass resolution for heavy states, like D mesons, J/Ψ and Γ , will be better, thus improving the signal-to-background ratio in the measurement of the open charm production, and in the study of heavy-quarkonia suppression. Improved momentum resolution will enhance the performance in the observation of another hard phenomenon, the jet production and predicted jet quenching, i.e. the energy loss of partons in strongly interacting dense matter. The low-momentum particles (below 100 MeV/c) will be detectable only by the ITS. This is of interest in itself, because it widens the momentum range for the measurement of particle spectra, which allows collective effects associated with the large length scales to be studied. In addition, a low-pT cutoff is essential to suppress the soft γ conversions and the background in the electron-pair spectrum due to Dalitz pairs. Also the PID capabilities of the ITS in the non-relativistic (1/β squared) region will therefore be of great help. In addition to the improved momentum resolution, which is necessary for the identical particle interferometry, especially at low momenta, the ITS will contribute to this study through an excellent doublehit resolution enabling the separation of tracks with close momenta. In order to be able to study particle correlations in the three components of their relative momenta, and hence to get information about the space-time evolution of the system produced in heavy-ion collisions at the LHC, we need sufficient angular resolution in the measurement of the particle's direction. Two of the three components of the relative momentum (the side and longitudinal ones) are crucially dependent on the precision with which the particle direction is known. The angular resolution is determined by the precise ITS measurements of the primary vertex position and of the first points on the tracks. The particle identification at low momenta will enhance the physics capability by allowing the interferometry of individual particle species as well as the study of non-identical particle correlations, the latter giving access to the emission time of different particles. The study of strangeness production is an essential part of the ALICE physics programme. It will allow the level of chemical equilibration and the density of strange quarks in the system to be established. The measurement will be performed by charge kaon identification and hyperon detection, based on the ITS capability to recognize secondary vertices. The observation of multi-strange hyperons (Ξ - and Ω -) is of particular interest, because they are unlikely to be produced during the hadronic rescattering due to the high-energy threshold for their production. In this way we can obtain information about the strangeness density of the earlier stage of the collision. Open charm production in heavy-ion collisions is of great physics interest. Charmed quarks can be produced in the initial hard parton scattering and then only at the very early stages of the collision, while the energy in parton rescattering is above the charm production threshold. The charm yield is not altered later. The excellent performance of the ITS in finding the secondary vertices close to the interaction point gives us the possibility to detect D mesons, by reconstructing the full decay topology.

Design considerations

The following factors were taken into consideration for the design of the ITS:

Acceptance

In order to be able to analyse particle ratios, pT spectra and particle correlations on an event-by-event basis, the tracking system must have a sufficiently large rapidity acceptance. The rapidity coverage of the tracking system (|η| < 0.9) is large enough to detect several thousand particles per heavy-ion collision at the currently predicted particle production multiplicity. This rapidity window is also necessary for a good efficiency for detecting the decay of large mass, low transverse momentum particles. An efficient rejection of low-mass Dalitz decays can only be implemented if the detector provides full azimuthal coverage. The first pixel layer has a wider pseudorapidity coverage (|η| < 1.75) to extend the rapidity coverage of the multiplicity measurement.

dE/dx measurement

The ITS contributes to particle identification through the measurement of specific energy loss. To apply a truncated-mean method, a minimum of four measurements are necessary, so four out of the six planes need analog readout. As explained in detail in Section 3.3.2 of the ITS-TDR, we require the dynamic range of the analog readout to be large enough to provide dE/dx information for low-momentum, highly ionizing particles, down to the minimum momentum for which the tracks have a reasonable (> 20%) reconstruction probability.

Material budget

The momentum and impact parameter resolution for particles with small transverse momenta are dominated by multiple scattering effects in any existing tracking detector. Therefore the amount of material in the active volume has to be reduced as much as possible. However, the thickness of silicon detectors used to measure ionization densities must be approximately 300 um to guarantee the required signal-to-noise ratio. In addition the detectors must overlap in order to reach full coverage within the acceptance window. Taking also into account the incidence angles of tracks the detectors represent a thickness of 0.4% of X0. The aim set in the ALICE technical proposal was to reduce the thickness of the additional material in the active volume, i.e. electronics, cabling, support structure and cooling system, to a comparable effective thickness. The current design tries to meet this challenge. As shown in Chapter 5, the resulting relative momentum resolution is better than 2% for pion momenta between 100 MeV/c and 3 GeV/c.

Spatial precision and granularity

The granularity of the detectors in the ITS is dictated by the track densities expected. The system is designed for a maximum track density of 8000 tracks per unit of rapidity, the upper limit of the current theoretical predictions. Therefore up to 15 000 tracks will have to be detected simultaneously in the ITS. Keeping the occupancy of the system at the level of a few per cent requires several million effective cells in each layer of the ITS. The resolution of the impact parameter measurement is determined by the spatial resolution of the ITS detectors. For charmed particles the impact parameter resolution must be better than 100 μm in the rφ direction. Therefore the ITS detectors have a spatial resolution of the order of a few tens of um, with the best precision (12 μm) for the detectors closest to the primary vertex. In addition, for momenta larger than 3 GeV/c, relevant for the detection of the decay products of charmed mesons and high-mass quarkonia, the spatial precision of the ITS becomes an essential element of the momentum resolution. This requirement is met by all layers of the ITS with a point resolution in the bending plane about one order of magnitude better than that of the TPC, which in turn provides many more points.

Radiation levels

The ionizing radiation dose received by the detector components was calculated using Monte Carlo techniques based on HIJING and GEANT. The total dose received during the lifetime of the experiment varies from a few krad for the outer parts of the ITS to about 150 krad for the inner parts as shown in Table 1.1. Detailed calculations can be found in Chapter 5 and in Ref. [2]. Each of the sub-detectors is designed to withstand the ionizing radiation doses expected during ten years of operation. The neutron fluence is approximately 3 times 10 ^11 per squared centimeter throughout the ITS, which does not cause significant damage to the detectors or the associated electronics. Where necessary, the components used in the ITS design were tested for their radiation hardness up to the expected doses.

Readout rate

The ALICE system will be used in two basically different readout configurations, operated simultaneously with two different triggers. The centrality trigger activates the readout of the whole of ALICE, in particular all layers of the ITS, while the trigger of the muon arm activates the readout of a subset of fast readout detectors, including the two inner layers of the ITS. Therefore the readout time for the pixel detectors is set at 400 μs.