Commissioning and Performance of the New ALICE Inner Tracking System in the First Phase of LHC Run 3

ALICE is one of the four experiments at the CERN Large Hadron Collider (LHC) specifically designed to study nuclear matter at extreme conditions of temperature and pressure. The LHC Run 3 started officially in July 2022 with proton-proton (pp) collisions at a centre-of-mass energy $\sqrt{s}$ = 13.6 TeV after a shutdown of more than two years to allow for the upgrade of both the accelerator and the experiments. To fulfill the requirements of the ALICE physics program for Run 3, new timing, tracking and multiplicity detectors have been installed in the ALICE central barrel both at mid- and forward rapidities, the readout electronics of the majority of the detectors has been upgraded and the ALICE Offline and Online computing farm has been completely renewed. A key detector of the apparatus is the new, ultra-light, high-resolution Inner Tracking System (ITS2) that significantly enhances the resolution on the determination of the distance of closest approach to the primary vertex, the tracking efficiency at low transverse momenta, and the read-out rate capabilities with respect to what was achieved in the LHC Run 2 with the older tracker. The new tracker consists of seven layers, azimuthally segmented into Staves, equipped with silicon Monolithic Active Pixel Sensors with a pixel size of 27$\times$29 $\mu$m$^{2}$ covering a sensitive area of about 10 m$^2$. A long effort allowed the construction, characterization, installation and commissioning of the ITS2 before starting Run 3. The in-pixel discriminator threshold calibration represents a challenging procedure due to the huge number of channels and it is carried out by exploiting the power of the ALICE Offline and Online farm. The detector took data for the full 2022 and its first performance results represent the second key part of this article.