ALICE ITS3: A truly cylindrical vertex detector based on bent, wafer-scale stitched CMOS sensors

The new inner tracking system (ITS2) of the ALICE experiment at the LHC, upgraded during the LHC Long Shutdown 2 (2019–2021) with CMOS monolithic active pixel sensors (ALPIDE), is currently taking data and demonstrating excellent performance in the LHC Run 3. A replacement of the three innermost layers of the ITS2, called ITS3, is foreseen during the LHC Long Shutdown 3 (2026–2028) to further improve its tracking precision and efficiency, particularly at very low transverse momentum down to pT = 0.1 GeV/c. The ITS3 is a cylindrically bent silicon vertex detector based on stitched wafer-scale monolithic active pixel sensors with 65 nm CMOS technology. The large stitched sensors are 26.6 cm in length and can be thinned down to below 50 μm, where the sensors are flexible to be bent to form truly cylindrical detector half-barrels. An extremely low material budget of 0.05% X/X0 per layer can be achieved in combination with air-cooling and lightweight carbon foam spacers as support structures. In this article, an overview of the ALICE ITS3 and the R&D achievements including detector concept, sensor design and technology qualification, sensor bending, detector mechanics and cooling, and system integration will be given.