The muon collider: expected physics, technological challenges, and the prospects for reusing the 21 km UNK tunnel

Abstract

A multi-TeV muon collider is among the main options for the post-LHC energy frontier. Muons radiate about 10^9 times less synchrotron power than electrons of the same energy, so a compact circular collider could combine the energy reach of a 100 TeV-class proton machine with the clean final states of a lepton collider. This review summarizes the expected physics programme, including Higgs couplings and self-coupling, electroweak and top-quark physics, and searches beyond the Standard Model. It also describes the technological challenges: muon production, ionization cooling, rapid acceleration, high-field HTS magnets, the machine-detector interface and the neutrino-flux constraint. None of these technologies has yet been demonstrated at the required performance. Scaling the parameters of the International Muon Collider Collaboration (IMCC), the review examines three ways of reusing the existing 21 km UNK tunnel near Protvino. With the last rapid-cycling synchrotron in the tunnel, the centre-of-mass energy is limited to about 6 TeV. With the collider ring in the tunnel, 11-16 T dipoles give about 14-21 TeV, but the acceleration chain needs a separate 50-73 km tunnel. With both in the tunnel, the reach is about 6-10 TeV at 35-57% of the target luminosity. Non-collider applications of intense muon beams are also discussed.

arXiv comment40 pages, 5 figures, 7 tables. Revised version addressing the referees' comments