Upgrade of the ALICE Inner Tracking System

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 μm(2). 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% X(0) 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.