Operation and performance of the upgraded ALICE Inner Tracking System

The ALICE detector underwent significant upgrades during the LHC Long Shutdown 2 from 2019 to 2021. A key upgrade was the installation of the new Inner Tracking System (ITS2), comprising 7 layers with 12.5 billion pixels over 10 m$^{2}$, enhancing its tracking capabilities using the ALPIDE chips that are capable of recording Pb\textendash Pb collisions at an interaction rate of 50 kHz. It offers a significant improvement in impact parameter resolution and tracking efficiency at low transverse momentum, attributed to its increased granularity, low material budget of only 0.36% $\rm\ X_{0}$/layer for the innermost 3 layers and the closer positioning of the first layer to the interaction point. ITS2 was successfully installed and commissioned in ALICE, becoming operational with the start of LHC Run 3. This paper provides an overview of its operation and initial performance results, focusing on calibration, tracking performance in both pp and Pb\textendash Pb collisions, and studies on particle identification.