ITS3: A truly cylindrical inner tracker for ALICE

After the successful installation and first operation of the new Inner Tracking System (ITS2), which consists of about 10 m$^2$ of monolithic silicon pixel sensors, ALICE is pioneering the usage of bent, wafer-scale pixel sensors for the ITS3 for Run 4 at the LHC in 2029. Sensors larger than typical reticle sizes can be produced using the technique of stitching. At thicknesses of about 30 $\mu$m, the silicon is flexible enough to be bent to radii of the order of 1 cm. By cooling such sensors with a forced air flow, it becomes possible to construct a detector with minimal material budget. The reduction of the material budget and the improved pointing resolution will allow new measurements, in particular of heavy-flavor decays and electromagnetic probes. Mechanical studies have shown the sensors to be unaffected by bending, and bent sensors have been shown to be fully efficient in test beams. New sensor developments for the ITS3 have shown promising results for fluences even beyond those expected for ITS3.