The High Luminosity Large Hadron Collider (HL-LHC) is an upgrade of the LHC which aims to achieve instantaneous luminosities a factor of five larger than the LHC nominal value, thereby enabling the experiments to enlarge their data sample by one order of magnitude compared with the LHC baseline programme. Following five years of design study and R&D, this challenging project will require about ten years of developments, prototyping, testing and implementation; hence operation is expected to start in the second half of this decade. The timeline of the project is dictated by the fact that, in the coming years, many critical components of the accelerator will reach the end of their lifetime due to radiation damage and will thus need to be replaced. The upgrade phase is therefore crucial not only for the full exploitation of the LHC physics potential, but also to enable operation of the collider beyond 2027.

The HL-LHC will rely on a number of key innovative technologies, including cutting-edge 11-12 Tesla superconducting magnets, compact superconducting crab cavities with ultra-precise phase control for beam rotation, new technology for beam collimation, high-power, loss-less superconducting links, among others. A detailed description of the project and its technological and operational challenges is provided in the HL-LHC Technical Design Report  and the HL-LHC book.