ITS3: the ALICE Inner Tracking System upgrade

The ALICE experiment at CERN is developing an upgrade to its vertexing detector in order to improve the impact parameter resolution at low transverse momenta, with a target timeline for installation during the third long shutdown of the LHC (2026–28). The concept consists in replacing the three innermost layers with wafer-scale monolithic active pixel sensors bent into truly cylindrical sensors wrapped around the beam pipe. The dramatic reduction in material budget (0.35% X$_0$ to 0.07% X$_0$ per layer), together with a thinner beam pipe (18 to 16.5 mm in radius) and a first layer closer to the interaction point (23 to 19 mm), are key features to increase the pointing resolution by at least a factor of 2 over all momenta. This contribution provides an overview on the plans, challenges and status of the design and validation of the first wafer-scale silicon detector for high energy physics.