by Vito Manzari. Published: 05 December 2012

A key characteristic of heavy-ion collisions at the LHC energy is the high number of particles produced per event in the central region, more than two orders of magnitude higher than in a typical proton–proton collision. The design of ALICE is optimized for a charged particle multiplicity of around 4000 and has been tested with simulations up to double this number.

The tracking was made particularly safe and robust by using mostly three-dimensional hit information with many points (up to 150) in a moderate field of 0.5 T provided by the large L3 solenoid magnet. This was achieved with a combination of very low material thickness to reduce multiple scattering at low pT and a large tracking lever arm of up to 3.5 m to guarantee a good resolution at high pT.

The current Inner Tracking System of ALICE consists of 6 layers of silicon detectors

Tracking in the central barrel of ALICE is accomplished with the Inner Tracking System (ITS), surrounding the beam pipe, and the Time Projection Chamber (TPC). The main functions of the ITS are the localization of the primary vertex (with a resolution better than 100 μm), the reconstruction of the secondary vertices from the decays of D and B mesons and hyperons, the tracking and identification of particles with momentum below 200 MeV/c, and improving the momentum and angle resolution for particles reconstructed by the TPC.

The present ITS consists of six cylindrical layers of silicon detectors placed coaxially around the beam pipe. They are located at radii between 39 mm and 430 mm and cover the pseudo-rapidity range |η|< 0.9 for vertices located within 60 mm with respect to the nominal interaction point. The number, position and segmentation of the layers were optimized –within the technological boundaries at the time of the design- to achieve a high precision in the determination of the charged particle impact parameter to the primary vertex and efficient track finding in combination with the TPC. The inner radius is the minimum allowed by the radius of the beam pipe, while the outer radius is determined by the necessity to match tracks reconstructed in the ITS with those from the TPC.

The installation of the current Inner Tracking System of ALICE in 2007

The innermost two layers of the ITS consist of two silicon pixel detectors (SPD), followed by two layers of drift detectors (SDD) and two layers of double-sided microstrips (SSD). The SPD operates in a region where the track density could be as high as 50 tracks/cm2 and being the detector closest to the interaction region it plays a key role in the determination of the position of the secondary vertices and the measurement of the impact parameter of secondary tracks originating from the weak decays of charm and beauty particles. While the SPD has a binary readout, the SDD and SSD layers are equipped with analogue readout for independent particle identification via energy loss, dE/dx, in the non-relativistic region, thus providing the current ITS with stand-alone capability as a spectrometer for particles with low transverse momentum, pT. The SPD layers have a more extended pseudo-rapidity coverage (|η|<1.98 for the innermost one) which, together with the Forward Multiplicity Detectors (FMD), provides continuous coverage for the measurement of charged particle multiplicity. The detector elements were carefully optimized to minimize their radiation length, achieving per layer, the lowest value among all the current LHC experiments.

Animation showing the Inner Tracking System of ALICE, created by Stefan Rossegger

The precision of the present ITS in the determination of the track impact parameter is adequate to study the production of charm mesons in exclusive decay channels (e.g. D0 - Kp and D+ - Kpp) at values of transverse momentum above 2 GeV/c. However, when we move to lower transverse momenta regions the statistical significance of our measurements seems to be insufficient. The situation is getting worse when we try to study charm baryons. The most abundantly produced charm baryon (Λc) has a mean proper decay length (cτ) of only 60 μm which means that they cannot be detected by any of the ALICE detectors. The decay length of Λc is lower than the impact parameter resolution of the current ITS in the transverse momentum range for almost all the particles in which it decays. Therefore, charm baryons are presently not accessible by ALICE in central Pb-Pb collisions. For the same reasons the study of beauty mesons, beauty baryons, and of hadrons with more than one heavy quark are also beyond the capability of the current detector. In addition, the upgraded ITS will be extremely important for a detailed measurement of thermal electromagnetic radiation from the hot QGO, which is just in its infancy at the LHC.

Another major limitation of the present ITS detector is related to its readout rate capabilities. The present ITS can run up to a maximum read-out rate of 1 kHz (with 100% dead time) almost irrespective of the detector occupancy. This rate limitation restricts ALICE to use only a small fraction of the full Pb–Pb collision rate of 8 kHz that the LHC presently can deliver and prevents the collection of required reference data in pp collisions. Clearly, the present ITS has to be upgraded in order to cope with the required rate capabilities envisaged for the ALICE long-term plans after the long shutdown scheduled in 2017-18 (LS2). The new detector aims to read the data related to each individual interaction up to a rate of 50 kHz for Pb–Pb collisions and 2MHz for pp collisions.

Finally, the impossibility to access the present ITS detector for maintenance and repair interventions during the yearly LHC shutdowns represents another major limitation in sustaining a high data quality. In the context of an upgrade, the rapid accessibility to the detector will be set as a priority.

A close up photo of the current silicon pixel detectors of the ITS.

The above difficulties were seriously considered when thinking about the ITS upgrade plans and the new capabilities present at ALICE. The upgrade of ALICE detection system combined with the higher luminosities achieved at LHC after the long shutdown will open new channels in the study of the interactions of the fundamental particles of matter.

The upgraded ITS will improve the track position resolution at the primary vertex by a factor of 3 or even larger with respect to the present detector, and will feature a standalone tracking efficiency comparable to what can be presently achieved by combining the information of the ITS and the TPC.

First of all, a smaller radius beampipe will enable the first detection layer to be located at a radius of 22 mm from the interaction point.

Different monolithic pixel detectors developed as part of the R&D effort for the upgrade of the ITS. From top: i) MISTRAL prototype circuit (IPHC), ii) LePIX prototype circuit (CERN) and iii) INMPAS (RAL). These new technologies will boost the tracking performance of the new Inner Tracking System of ALICE.

In addition the new ITS will have an even smaller material budget compared to the current ITS that will improve the tracking performance and momentum resolution. The use of Monolithic Active Pixel Sensors (MAPS) will allow to boost the impact parameter resolution capabilities thanks to the smaller pixel cell size, of the order of 20 μm x 20 μm as compared with the present 50 μm x 425 μm, and the reduced silicon budget (50 μm compared to the 350 μm of the current pixel layers). A reduction of the power density by a factor of 2 could also be achieved, which would allow a further reduction of the material budget in terms of needs for power connections and cooling system. The low material budget (lighter layer) is critical in particular for the first detection layer as it strongly affects the impact parameter resolution in the low pT region in which we are mostly interested. The reason is that the resolution at low pT is mainly determined by multiple Coulomb scattering. It is still under discussion whether the new ITS detector should preserve the PID capabilities of the current detector by measuring the ionization in the silicon layers (dE/dx).

The construction of the present ITS has been an international effort in which more than 20 institutes from 10 different countries participated. The development of the new ITS is part of the current ALICE ITS Project. In addition to the Institutes already members of the ITS Project, several new Institutes have joined the ITS Collaboration to participate in the development and construction of the new detector and a few more are considering to take part to this effort.