ITS3, the ALICE Vertex Detector for LHC Run 4

During LHC Long Shutdown 3 (2026–2028), the ALICE Collaboration plans to upgrade the innermost layers of the current Inner Tracking System detector (ITS) with a novel vertex detector ITS3. The concept of this new vertex detector is based on ultra-thin (< 50 m), wafer-scale (up to about 10 26 cm^2 large) Monolithic Active Pixel Sensors (MAPS) that are fabricated using the 65 nm CMOS process. Such sensors can be bent to radii as small as 18 mm and can be utilized to construct truly cylindrical-detector shells with an unprecedently low material budget, 0. 05% 𝑋0 per layer only. The development of this detector includes a number of cutting-edge R&D efforts, including the qualification of MAPS in 65 nm, the development of wafer-scale stitched MAPS, the concept of bent MAPS, ultra-light mechanics based on carbon foam supports, and air cooling. This contribution provides an overview of the ALICE ITS3 detector and of the R&D achievements, including: the validation of the 65 nm technology for particle tracking and radiation hardness, the achievements in terms of bending and flexibility, the integration of wafer-scale silicon detectors, and the production of the first MOnolithic Stitched Sensor (MOSS).