Abstract
ALICE (A Large Ion Collider Experiment) is studying heavy-ion collisions at the CERN LHC, with the aim of forming, under extreme conditions of temperature and energy density, a Quark-Gluon Plasma (QGP) and studying its properties. The ALICE Collaboration is preparing a major upgrade of the experimental apparatus, planned for installation in the second long LHC shutdown in the years 2018–2019. A key element of the ALICE upgrade is the construction of a new, ultra-light, high-resolution Inner Tracking System (ITS) . The primary focus of the new ITS is on improving the performance for detection of heavy-flavour hadrons, and of thermal photons and low-mass di-electrons emitted by the QGP . With respect to the current detector, the new ITS will significantly enhance the determination of the distance of closest approach of a track to the primary vertex, the tracking efficiency at low transverse momenta, and the read-out rate capabilities. This will be achieved by seven concentric detector layers based on a 50 μm thick CMOS pixel sensor with a pixel pitch of about 30× 30 μm2. A key feature of the new ITS, which is optimized for high tracking accuracy at low transverse momenta, is the very low mass of the three innermost layers, which feature a material thickness of 0.3% X0 per layer. This contribution describes the design goals and layout of the new ALICE ITS, a summary of the R&D activities, with focus on the technical implementation of the main detector components, and the projected detector performance.
Highlights
ALICE (A Large Ion Collider Experiment) is studying heavy-ion collisions at the CERN LHC, with the aim of forming, under extreme conditions of temperature and energy density, a Quark-Gluon Plasma (QGP) and studying its properties
In combination with a slightly increased number of seven instead of the present six silicon tracking layers, this leads to an improvement in the tracking efficiency and momentum resolution
The tracker will cover the pseudo-rapidity range of |η| < 1.22 for the 90% most luminous region; the radial positions of the different layers have been optimised in order to achieve the best combined performance in terms of pointing resolution, tracking efficiency and transverse momentum resolution
Summary
The central part of the upgraded ALICE ITS is the pixel chip. Table 1 summarises the requirements for the chip that arise from the physics programme and the operation environment. The ALPIDE architecture and the ASTRAL/MISTRAL architectures, are under study for the use in the upgraded ALICE ITS, with the goal to select a common design in the first half of 2015 In both architectures several small scale prototypes have been produced and characterised in order to optimise the sensor pixel and the front-end. These prototypes have demonstrated that the requirements in terms of performance and radiation hardness can be met [3]. The spatial resolution, shown in figure 6(b) is slightly above the required 5 μm for the inner layers, taking into account that the values plotted here still include the telescope resolution, which is estimated to be in the order of 3 μm
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