The ALICE experiment: D+-meson production in heavy-ion collisions and silicon low noise sensors characterization for the ITS Upgrade

This thesis collects my work on two aspects of the ALICE experiment at the Large Hadron Collider: the measurement of D+-meson production in Pb-Pb collisions at √sNN = 2.76 TeV and the characterization of silicon low noise sensors for the Inner Tracking System Upgrade. I worked within the INFN group of Torino that it is involved in the ALICE experiment both in the physics program related to the study of heavy-flavour production and in the project of the ITS Upgrade. ALICE is one of the main experiment of the LHC and it is the only one optimized to study ultra-relativistic heavy-ion collisions. The main goal is to study the properties of the Quark Gluon Plasma (QGP), a phase of matter where quarks and gluons are deconfined. Heavy quarks are a powerful tool to study such properties because they can be created only in hard scat- tering processes at the initial stage of the collision and, subsequently, they interact with the QGP. The measurement of charmed meson production in Pb-Pb collisions allows to assess final state effects due to the formation of the QGP. One of the physical observable studied is the nuclear modification factor (RAA). It quantifies the modification of the D-meson momentum distribution in nucleus-nucleus collisions with respect to the one in proton-proton collisions. The results of the D+-meson RAA, obtained with the Pb-Pb data sample collected in 2011 by ALICE, will be presented in this thesis. However, a more precise measurement, in term of both statistical and systematic uncertainties, would be fundamental for a complete understanding of the properties of the medium and its components. For this reason, an upgrade of the ALICE apparatus and, in particular of the ITS, is planned to be installed during the second long shutdown of LHC in 2019. The goal is to enhance the ALICE physics capabilities and, expecially, the tracking performance for heavy-flavour detec- tion. To overcome the limitations of the present ITS, a different technology has been chosen for the layers of the upgraded detector: Monolithic Active Pixels Sensors. They can offer the granularity and the material budget needed to fulfil the requirements of the new ITS. Several prototypes have been developed to find the best solution, hence an intensive charac- terization campaign has taken place. I contributed in the characterization of small-scall proto- types of MIMOSA sensors, developed at IPHC (Strasbourg), and in the characterization of a full-scale prototype of pALPIDE-v2, developed by a collaboration formed by CCNU (China), CERN, INFN (Italy) and Yonsei (South Corea). The first sensors have been characterized in the framework of the development of a testbeam telescope at the LNF (Frascati, Italy). In the first Chapter of this thesis, an introduction to the physics of heavy-ion collisions will be given. The last section of this chapter will concentrate on the heavy-flavour measurements in high-energy experiments. The second Chapter will be dedicated to the description of the ALICE apparatus, focusing on the sub-systems directly involved in the D+-meson analysis. In the third Chapter, first the procedure used to extract the D+-meson yield in Pb-Pb collisions will be described and, then, the measurement of the D+-meson nuclear modification factor will be presented. The result of the RAA as a function of the D+-meson transverse momentum in the most central Pb-Pb collisions are published in [1], while the result as a function of the centrality of the collisions in [2]. The fourth Chapter will be dedicated to the limitations of the current apparatus, the physics motivations for the ITS upgrade and its main specifications. The last section of this chapter will focus on the features of the Monolithic Active Pixels Sensors, that have been chosen as the baseline for the ITS upgrade. In the fifth Chapter the characterisation of small-scale prototypes of MIMOSA sensors at the DAΦNE Beam Test Facility (BTF) will be presented. The sixth Chapter will be dedicated to the characterization of a full-scale prototypes of pALPIDE-v2 in laboratory and at the PS beam test facility at CERN.