New: The Compact Linear ##e^+e^−## Collider (CLIC) -- 2018 Summary Report CLIC is a proposed accelerator, potentially at CERN, that would collide electrons and positrons at energies far above what previous colliders achieved. Its overall energy would still be below the LHC but (unlike at the LHC) all the collision energy is available for the creation of new particles - it would combine the approximate energy reach of the LHC with the cleanliness of elementary particle collisions. To achieve this energy the collider has to be linear (synchrotron radiation doesn't allow circular designs), to keep the tunnel length reasonable it needs very high accelerating gradients. State of the art are superconducting RF cavities achieve ~35 MeV/m - but that would need more than 100 km of tunnel to reach the proposed 3 TeV collision energy. To get higher gradients CLIC uses two beams: A low energy high current beam creates strong electromagnetic waves that drive a high energy low current beam. That should lead to an accelerating gradient of at least 70 MeV/m, potentially as high as 100 MeV/m. The full accelerator would still need a tunnel length of 50 km with 70 MeV/m - acceleration, focusing, pumps and other infrastructure. The focusing system around the collision point alone needs two kilometers for each beam. You don't start with a 50 km tunnel, so the CLIC proposal has three stages with collision energies of 0.38 TeV, 1.5 TeV and 3 TeV with tunnel lengths of 11 km, 29 km, 50 km. The first stage would produce Higgs bosons (##e^-e^+ \to Z^* \to HZ##) and top quark pairs close to the threshold. A great way to study them in more detail, including much better mass measurements. W and Z bosons would be produced often as well. The second stage gives more Higgs bosons and more production modes (including (##e^-e^+ \to t\bar t H##). It would also give events with two Higgs bosons and some chance to produce new particles that escaped detection at the LHC. The third stage gives access to Higgs self-coupling (something very challenging to measure at the LHC) and a larger energy to look for undiscovered particles. In addition most interesting processes that happen at lower energies will be more common. A nice physics program significantly beyond what the ILC proposal with its lower energy and the Chinese proposal with its much lower energy (just enough for the ZH process) would do. Unfortunately it looks more expensive, too. The paper estimates $6B for the first stage, $5B upgrade cost to the second stage and further $7B for stage three (all in 2018 CHF = 2018 dollars). That would make it even more expensive than the LHC. Unlike the LHC the CLIC design only allows one detector at the single collision point.