The ALICE Inner Tracking System (ITS) has recently been replaced with a full silicon pixel detector constructed entirely with Complementary Metal\textendash Oxide\textendash Semiconductor (CMOS) monolithic active pixel sensors. It consists of three inner layers (50 μm thick sensors) and four outer layers (100 μm thick sensors) covering $10\ \rm m^{2}$ and containing 12.5 billion pixels with a pixel size of 27 μm $\times$ 29 μm. Its increased granularity, the very low material budget ($0.35\% \rm\ X_{0}\rm /layer$ in the inner barrel) as well as a small radius of the innermost layer combined with a thin beam pipe, will result in a significant improvement of impact-parameter resolution and tracking efficiency at low transverse momentum with respect to the previous tracker.



The assembly of the full detector and services finished in December 2019. A comprehensive commissioning phase (on surface) was completed in December 2020, including detector calibration, fake-hit rate determination, data transmission tests, and preliminary evaluation of the detector efficiency and the alignment of the sensors, based on a reconstruction of cosmic-ray tracks. The commissioning of the new ITS within the ALICE apparatus has started in May 2021. After a first phase of standalone tests and detector performance optimization, the detector has been included in the global commissioning activities from July 2021. In this paper, the detector design as well as the first results of the performance studied during the commissioning of the detector will be discussed.