If none of these three alternatives is affordable or deemed worth the cost, it looks like the only alternative for energy frontier physics is to do what the US has done: give up. The machines and their cost being considered here are similar in scale to the SSC project, which would have been a 40 TeV CM energy 87 km proton-proton collider but was
cancelledin 1993. Note that the capabilities of the SSC would have been roughly comparable to the HE-LHC (it had higher energy, lower luminosity). Since it would have started physics around 2000, and an HE-LHCmight be possible in 2040, one could say that the SSC cancellation set back the field at least 40 years. The worst part of the SSC cancellation was that the project was underway and there was no fallback plan. It’s hard to overemphasize how disastrous this was for US HEP physics. Whatever the Europeans do, they need to be sure that they don’t end up with this kind of failure.
Faced with a difficult choice like this, there’s a temptation to want to avoid it, to believe that surely new technology will provide some more attractive alternative. In this case though, one is running up against basic physical limits. Peter Woit, ” Should the Europeans Give Up?” at Not Even Wrong
He doesn’t want to give up but he makes it clear that the options are narrow and expensive. He provides good links to the back-and-forth among physicists.
Perhaps we are entering a period of decline when cosmology is about the multiverse rather than the Higgs boson.
See also: Experimental Physicist: Particle Theory Is “In A Crisis” And A Bigger Collider IS The Answer!