The novel, truly cylindrical, ultra-thin silicon detector for the ALICE Inner Tracker System

The ALICE collaboration is planning to replace its innermost tracking layers during the Large Hadron Collider (LHC) Long Shutdown 3 with a novel detector that will be as close as 18 mm to the interaction point and as thin as <0.05%X $_{0}$ per layer. To achieve these numbers, a wafer-scale Monolithic Active Pixel Sensor (MAPS) in 65 nm technology is being developed. This sensor, fabricated on 300 mm wafers, will reach dimensions of up to 280 mm by 94 mm. Sensors are subsequently thinned down to values between 20 μm to 40 μm, where they become flexible and are bent into truly cylindrical half-barrels.Following the publication of a Letter of Intent by ALICE and the endorsement by the Large Hadron Collider Committee (LHCC) end of 2019, a very active research and development programme on bent silicon detectors has started.This contribution reviews the detector concept, the physics motivations, and lays out the R&D; path. Mechanical integration tests with ultra-thin silicon wafers as well as electrical test of bent MAPS, including beam test results, will be shown and demonstrate the feasibility of this new class of tracking detectors.