Commissioning of the upgraded ALICE Inner Tracking System

The ALICE Inner Tracking System (ITS) detector is undergoing a major upgrade in order to cope with the increased data rates and to meet the requirements as set out by the physics goals of the experiment after the LHC Long Shutdown 2 (LS2). The new ITS will be completely made up of monolithic active pixel sensors (MAPS), called ALPIDE, based on the CMOS 180 nm process. A single sensor measures 30 mm × 15 mm and contains half a million pixels, with a pitch of 27 μm×29 μ m, distributed over 512 rows and 1024 columns. The ALPIDE sensors, thinned to 50 μ m and 100 μ m for the Inner and Outer Barrels, respectively, are mounted on ultra-lightweight carbon composite support structures with an embedded cooling system. This results in a considerable reduction of the material budget and a significant improvement of the impact parameter resolution and tracking efficiency for low-transverse momentum particles. The construction of the ITS detector has been completed and the commissioning on surface is ongoing. The detector will be installed in ALICE in 2020. This paper will give a brief overview of the motivation for the upgrade and will present the first results of the detector performance obtained during the commissioning.