Studies on the upgrade of the ALICE central tracker

When two high-energy lead ions collide, as they currently do inside the “Large Hadron Collider” (LHC) of the “European Organization for Nuclear Research” (CERN), energy densities similar to those shortly (some 1ps to 10μs) after the Big Bang are created. At these energies quarks are loosing their confinement into hadrons and may move around freely, the “quark-gluon plasma” (QGP) is created. Such a picture deserves of course a thorough check and a precise measurement. There are however intrinsic difficulties to overcome: the macroscopic free energy (about 1 mJ) of these collision allow for an infinite number of processes to happen and finally—-due to mass-energy equivalence--a significant number (order of 10,000) of particles is created. The ALICE experiment was designed to be able to cope with this large number of particles, it can measure the properties (species and momentum) of the big majority. This requires a very fine segmentation of the detector. The central part of ALICE is made of a 90 qubic metre time projection chamber (TPC), accompanied by a silicon detector close to the beam pipe (ITS). Completed by a time of flight detector (TOF) they form the “central” tracker. Together they measure the precise three-dimensional track picture of the collision. The measured collision events produce a huge raw data volume of about 700 MByte, which sets tough requirements for the read-out electronics. To measure rare events one needs to record billions of collisions, which asks for high read-out rates (kHz). But also the spatial detector resolutions are required to be very good (a few 10μm) in order to e. g. resolve decays of short-lived particles. Taking the Lambda-c baryon, the lightest charmed baryon, as an example, the precise implications for the detector design are analysed in this thesis. Measuring the Lambda-c is of high importance for the understanding of the quark-gluon plasma. It bears valuable information on the charm sector, in particular on the charm thermalisation and/or energy loss in the plasma. Together with the measurements from the mesonic charm sector (D mesons) one may disentangle the contributions of u and d quarks to the hadronisation process from those caused by c quarks. Till today there is no data from Lambda-c production in central heavy-ion events available. ALICE has however successfully measured the Lambda-c yield in proton–proton (p–p) collisions, which will serve as a valuable cross-check. In the frame of this thesis the following developments were carried out: * A fast analysis procedure for the Λ+c in p–p and Pb–Pb collisions was designed, implemented and successfully tested with existing p–p data (Chap. 3). This did not only prove the method to be working but also helped to improve the currently employed analysis code for p–p. Also were the resonant Lambda-c to Proton, Kaon, Pion channels as well as the Lambda-c to Kaon, Proton and Lambda-c to Lambda, Pion decay modes added to the analysis. * The read-out speed of the existing TPC electronics was analysed systematically and a simulation model as well as an analytic treatment of the obtainable rates were developed. Based on this model a proposal to reorganise the read-out network is made, which can improve the speed by a factor of ten. To prove its feasibility, a prototype card of the main part of the network was designed and realised, which proves the critical part and the technical feasibility of this proposal (Chap. 5). * Based on the experience with the analysis of p–p collisions it was investigated how far an improvement of the ITS will translate into an improvement of the Λ+c measurement. For this purpose a “hybrid” Monte-Carlo method was developed that allows to quickly compare different detector geometries. With this method it could be quantitatively shown how an upgrade of the ITS together with an improved read-out network for the TPC allow to measure the Lambda-c in Pb–Pb (chapter 4). This analysis and the developed methodology are a main contribution to the current ALICE upgrade strategy.