by Giuseppe Bruno. Published: 05 December 2012

The mandate of the working group 2 “Detector Specifications and Performance Studies” was to define the detector specifications starting from the physics requirements and also to simulate the detector performance based on the detector design and implementation studies.

With respect to the first task we have always referred to the first working group. Our group often held joint meetings with the WG1 and we have been working together since the very first day of the ITS upgrade project. On the other front, in order to determine the performance of the detectors we collaborated with our colleagues from the third and the fourth working groups.

The functional requirements of the detector have been studied based on the physics performance that the WG1 determined for a number of benchmark studies. These functional requirements were taken into account from the other working groups in order to define their strategy for the technical implementation and the mechanical layout. At the same time the available options and technical solutions were posing a limit to these functional requirements. More specifically, the detector functional requirements should account for the properties of the beam pipe, the particle load, the detector acceptance, the number and radius of the layers, their material budget, the detector segmentation, the timing properties (i.e. the time resolution and/or the read-out time) and finally the radiation environment. In other words there are numerous design and technical parameters that constrain our functional requirements.

Based on this input one of the important tasks of our group was to describe the tracking efficiency, the momentum resolution and impact parameter resolution as a function of the transverse momentum and rapidity of the different particle species. Moreover, another important task of the second working group was to determine the particle identification capabilities of the upgraded Inner Tracking System. .

The Need for the ITS upgrade

The main objective of the upgraded ITS will be to significantly improve the physics performance of ALICE for heavy flavour hadrons (i.e. charm and beauty) in Pb-Pb collisions. The new ITS will give us the opportunity to study for the first time the D mesons (including Ds) reaching a region of low transverse momentum down to zero pt. In the same region of low pT we will also be able to see charm and beauty baryons (Λc and Λb) and exclusive decay channels of B mesons. These observations will in turn allow the study of baryon/meson ratios for charm and beauty hadrons and also the elliptic flow of charmed and beauty mesons and baryons down to low transverse momentum.

The detection of charmed hadrons is based on the topological reconstruction of their week decays. This is particularly challenging as they have a cτ of the orders of few hundred microns only.

The actual challenge of the upgraded ITS is to extend our reconstruction capabilities to the region of low pt. As we move to lower pt regions we have higher multiple scattering in the material and also smaller average spatial separation between the primary vertex and the secondary decay vertex of the heavy flavour hadron.

In order to obtain an excellent impact parameter resolution, which is needed to resolve the secondary vertexes from the primary one we came up with a number of requirements for the new ITS detector. First of all, it should have very high segmentation since it determines the intrinsic precision of the reconstructed space point associated to the track. We are also opting for very low material budget in order to minimize the multiple scattering. Finally we asked for the new ITS to be placed as close as possible to the interaction vertex in order to minimize any errors related to the track-trajectory back-extrapolation to the interaction region.

The three basic requirements that we suggested were discussed with the third and the fourth working group in order to account for the technologies that will be used but also for the mechanical structure that would support the new detectors – especially given the requirement to keep it very close to the interaction point – and provide the cooling that is needed.

Last but not least, the new detector should have improved readout rate capability to allow the recordings of all Pb-Pb collisions at the increased interaction rate of approximately 50 kHZ, which is expected from LHC after the Long Shutdown 2 period and the increased luminosities at the LHC.

Figure 1. Pointing resolution to the vertex of charged pions as a function of the transverse momentum for the current ITS and the upgraded ITS. Upper panel: ITS stand-alone tracking; lower panel: ITS-TPC combined tracking. Notice that for the present ALICE set-up the ITS-TPC combined tracking provides at high pT a sizeable improvement with respect to the ITS stand-alone tracking. Conversely, in the case of the upgraded ITS, adding the information from the TPC does not yield any further improvement.

Being the “trait d’union” between the first working group from one side and working groups 3 and 4 from the other we had to deal with many different aspect of the project and understand the difficulties and requirements related both to the analysis and to hardware.

However, perhaps the greatest difficulty that we encountered was the fact that the members of all groups had to work hard at the same time for the present detectors and for the “actual” ALICE experiment to keep running. People were often busy doing analysis of current data, working for the upcoming lead-ion run or for doing their shift at ALICE P2. It was a great effort and we had to invest a lot of our personal time in order to get the ITS upgrade project ready on time.

Future plans

In a sense, the original mandate of the working group 2 has been accomplished with the preparation of the Conceptual Design Report of the ITS. We mainly contributed to the third chapter of this document and we are pleased that the LHC Committee endorsed our plans. A good amount of work has been done to achieve this goal but much more is ahead, from now up to 2019 when the new detector should reconstruct its first tracks produced in Pb-Pb collisions at the LHC (and we hope that the very first would come from the decay of a heavy flavoured baryon!).

However, I think that the best way to proceed is one step after the other so our next objective is the preparation of the Technical Design Report. In this document we have to define the technical implementation of the new detector and describe much more accurately its physical performance. In order to achieve this goal, a reorganization of the project may be functional, and so, presumably, the working groups will soon evolve and be reorganized in a different way.

I would like to name all the members of the second working group: Stefan Rossegger, Stefania Bufalino , Annalisa Mastroserio , Leonardo Milano, Stefano Piano, Francesco Prino, Sergey Senyukov, Ruben Shahoyan and Cristina Terrevoli. They detain all the merit for the nice work that has been done so far.