by Luciano Musa. Published: 05 December 2012

ALICE, the heavy-ion physics dedicated experiment at LHC has so far confirmed the basic picture and observed the creation of hot hadronic matter at unprecedented values of temperatures, densities and volumes. It did so by exceeding the precision and kinematic reach of all significant probes of the QGP that had been measured over the past decade.

The ALICE long-term physics goals require the measurement of heavy-flavour hadrons, quarkonia and low-mass dileptons in the region of low transverse momenta. Measurements of Pb-Pb collisions and the study of QGP in the region of low pT are characterized by a very small signal-over-background ratio which calls for large statistics and also makes triggering techniques very inefficient if not impossible. Despite its advanced tracking and PID capabilities ALICE has yet to be optimized for high precision measurements in the region of low transverse momenta. A significant improvement of the vertexing and tracking efficiency of the ALICE detection system is needed to account for these measurements.

The ITS upgrade plan is part of a bigger plan for the ALICE upgrade which will not only enhance the ALICE vertexing and tracking capabilities but also allow data taking at substantially higher rates corresponding to the higher luminosities at the LHC. Higher luminosities will probably take place during the second long shutdown period of the LHC in 2018. At that time ALICE will be in a position of accumulating 10nb-1 of Pb-Pb collisions recording about 1011 interactions an unprecedented amount of data for the physics that we are studying.

The physics program of ALICE includes the following main topics: i) the study of the thermalization of partons in the QGP with focus on the massive charm and beauty quarks and understanding the behaviour of these heavy quarks in relation to the stroungly-coupled medium of QGP, ii) the study of the mechanisms of energy loss that occur in the medium and the dependencies of energy loss on the parton species, iii) the dissociation of quarkonium states which can be a probe of deconfinement and of the temperature of the medium and finally the production of thermal photons and low-mass dileptons emitted by the QGP which is about assessing the initial temperature and degrees of freedom of the systems as well as the chiral nature of the phase transition. The ITS upgrade plans will open new channels in the study of the Quark Gluon Plasma formed at LHC which are necessary in order to understand the dynamics of this condensed phase of the QCD.

The Upgraded ITS - an animation zooming in the inner tracking system of ALICE

The ITS upgrade plan is mainly based on building a new silicon tracker with greatly improved features in terms of determination of the impact parameter (d0) to the primary vertex, tracking efficiency at low pT and readout rate capabilities. This new silicon tracker will allow ALICE to measure charm and beauty production in Pb-Pb collisions with sufficient statistical accuracy down to very low transverse momentum, measure charm baryons and perform exclusive measurements of beauty production. These measurements are essential in order to understand the energy loss mechanism and thermalization of heavy quarks in the QGP state. In addition, the new ITS will also play a key role for the measurement of thermal photons and low-mass dileptons. Measuring these characteristics of the Quark Gluon plasma will allow us to better understand Quantum Chromodynamics as a genuine multi-particle theory.

Different options explored for the structure of the upgraded Inner Tracking System based on the work of the four working groups of the ITS Upgrade Project.

The ITS upgrade will allow new measurements on charm and beauty production which are needed in order to answer the above physics questions. More specifically, it will allow the study of the process of thermalization of heavy quarks in the medium by measuring heavy flavour charmed and beauty baryons and extending these measurements down to very low pT for the first time (D mesons, Charm and beauty baryons, Λc and Λb. Baryon/meson ratios and finally the elliptic flow of charmed and beauty mesons and baryons). Secondly, it will give us a better understanding of the quark mass dependence of in-medium energy loss by measuring the nuclear modification factors RAA of the pT distributions of D and B mesons. The upgraded ITS will also offer a unique capability of measuring the beauty quarks (via D0 - Κπ , J/ψ – ee). Moreover, it will improve the measurement of single displaced electrons and finally it will improve the beauty decay vertex reconstruction. Finally, the upgraded ITS will give us the chance to characterize the thermal radiation coming from the QGP and the in-medium modification of hadronic spectral functions as related to chiral-symmetry restoration (particularly for the ρ –meson).

The mechanical structure for the upgraded ITS was designed in order to meet the physics requirements of the new detector

The ITS upgrade project requires an extensive R&D effort by our researchers and collaborators all over the world on cutting-edge technologies: silicon sensors, low-power electronics, interconnection and packaging technologies, ultra-light mechanical structures and cooling units. The new technology enhances the ALICE resolution of the charged track impact parameter by almost a factor of three. This factor in the bending plane is the result of lower material budget, reduced radial distance of the first layer and increased spatial resolution. Based on the most recent developments in pixel detector technologies the material budget is substantially reduced by reducing the thickness of the silicon detector components as well as the readout system, mechanical support and cooling system. The reduction of material budget improves the sensitivity to charm by one order of magnitude or more, depending on the transverse momentum range. Moreover, it implies a better signal-to-background ratio. Therefore fully reconstructed rarely produced heavy-flavour hadrons will become accessible thanks to the efforts of the groups that participated in the ITS upgrade project. In summary, the baseline idea for the layout of the ITS upgrade is to replace the existing ITS detector in its entirety with three inner layers of pixel detectors followed by four outer layers of silicon strips, or pixel detectors, or silicon strips, with lower granularity. In addition, R&D on monolithic pixel detectors is trying to reduce the increased S/N ratio, the power density -which is important in reducing the material budget- and finally the integration time (this is where the R&D on the PIXEL chip is mostly focusing).

Substantial effort was place to reduce the material budget and the thickness of the silicon detector components and the mechanical support of the system.

The above plans for an upgraded Inner Tracking System for ALICE have been recently endorsed by the LHC Committee. This success wouldn’t have been possible without the hard work of so many different people coming from various institutions around the globe. In order to deal with the complex physical and technical requirements of the upgrade plans we organized four different working groups which had to collaborate and at the same time come with solutions on very specific areas. I would like to thank the convenors and the members of each working group for showing this strong commitment to our project.

List of the Institutes participating in the project of the ITS Upgrade.

The upgrade plans of ALICE require a long shutdown (LS) and, therefore, will naturally have to be in phase with the installation of upgrades for the other LHC experiments which are planned for 2013/14 and 2017/2018. The ALICE upgrade targets the long LHC shutdown period in 2017/2018 (LS2). Last but not least, the R&D efforts for the ITS upgrade will continue until 2014 and construction will take place in 2015/17 while the final phase of installation and commissioning is planed for 2018.