Development of the ITS3: A bent-silicon vertex detector for ALICE in the LHC Run 4

In parallel with commissioning and installation of the second generation ALICE Inner Tracking System (ITS2), R&D; is underway to replace the innermost tracking layers of the newly installed detector during the LHC long shutdown 3 (2026–28) with a fully cylindrical, bent-silicon tracker (ITS3) to be ready for LHC Run 4 (currently scheduled to start in 2029). By thinning down the Monolithic Active Pixel Sensors (MAPS) to 20–40μm the silicon becomes flexible; using this in conjunction with sensor stitching will allow for truly cylindrical half-barrels to be produced. In addition, the intrinsic stability of the sensors and their low power consumption (≈ 20 mW/cm2) will facilitate the removal of the supporting material for the powering, cooling and data transmission from the active area of the detector. With this design, an unprecedented low material budget of below 0.05% X0 per layer can be achieved and the innermost of three new layers can be positioned 18 mm from the interaction point. A brief motivation for the development of the ALICE ITS3, a description of the detector specifications and highlights from the R&D; are discussed in this contribution.