Anomalous Broadening of Jet-Peak Shapes in Pb-Pb Collisions and Characterization of Monolithic Active Pixel Sensors for the ALICE Inner Tracking System Upgrade

Abstract

Two-particle angular correlations are presented from Pb-Pb and pp collisions at 2.76 TeV. These measurements are powerful tools to study jets in a transverse momentum regime where they cannot be reconstructed over the large fluctuating background produced in heavy-ion collisions. In these studies, the distribution of azimuthal angle (Δφ) and pseudorapidity (Δη) differences of particle pairs is calculated. Jets manifest themselves as a peak around (Δφ, Δη) = (0, 0) and as a structure elongated in Δη at Δφ = π. The jet peak is studied as a function of transverse momentum and centrality in Pb-Pb collisions, and it is compared to pp collisions and to results from A Multi-Phase Transport model (AMPT). In Pb-Pb collisions, an asymmetric broadening of the jet peak towards central collisions at low transverse momentum is found, and in addition an unexpected depletion around (Δφ, Δη) = (0, 0) is observed. The comparison to AMPT suggests that the two effects are connected to large radial and longitudinal flow, therefore they can arise from an interplay of the jets with the flowing medium produced in the collisions of heavy-ions. The measurements are done by the ALICE detector, which will be upgraded during the second long shutdown of the LHC in 2019-2020. During this upgrade, the Inner Tracking System of ALICE will be replaced by a detector consisting of seven concentric layers of Monolithic Active Pixel Sensors. The prototypes of this detector and the setup used for their characterization are discussed in details. Tests at test beam facilities are carried out on unirradiated sensors and irradiated sensors are tested as well to study the long term functionality of the detector. The results of these measurements are presented, and it is found that the prototypes fulfill the requirements of the upgrade in terms of detection efficiency, spatial resolution and noise occupancy both before and after irradiation, making them suitable for the operation in the environment in the ALICE experiment.