  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.
