Proton colliders at the energy frontier

Since the first proton collisions at the CERN Intersecting Storage Rings (ISR) (Johnsen, 1973; Myers, 2010) [1, 2], hadron colliders have defined the energy frontier (Scandale, 2014) 3. Noteworthy are the conversion of the Super Proton Synchrotron (SPS) (Hatton, 1991; Evans, 1988) [4, 5] into a proton–antiproton collider, the Tevatron proton–antiproton collider (Lebedev and Shiltsev, 2014) 6, as well as the abandoned SSC in the United States (Jackson et al., 1986; Wienands, 1997) [7, 8], and early forward-looking studies of even higher-energy colliders (Keil, 1992; Keil, 1997; Barletta and Leutz, 1994; The VLHC Design Study Grup (Ambrosio et al.) 2001) [9, 10, 11, 12]. Hadron colliders are likely to determine the pace of particle-physics progress also during the next hundred years. Discoveries at past hadron colliders were essential for establishing the so-called Standard Model of particle physics. The world’s present flagship collider, the Large Hadron Collider (LHC) (Brüning et al., 2004) 13, including its high-luminosity upgrade (HL-LHC) (Apollinari et al., 2017) 14, is set to operate through the second half of the 2030’s. Further increases of the energy reach during the 21st century require another, still more powerful hadron collider. Three options for a next hadron collider are presently under investigation. The Future Circular Collider (FCC) study, hosted by CERN, is designing a 100 TeV collider, to be installed inside a new 100 km tunnel in the Lake Geneva basin. A similar 100-km collider, called Super proton–proton Collider (SppC), is being pursued by CAS-IHEP in China. In either machine, for the first time in hadron storage rings, synchrotron radiation damping will be significant, with a damping time of the order of 1 h. In parallel, the synchrotron-radiation power emitted inside the cold magnets becomes an important design constraint. One important difference between FCC and SppC is the magnet technology. FCC uses 16 T magnets based on Nb3Sn superconductor, while SppC magnets shall be realized with cables made from iron-based high-temperature superconductor. Initially the SppC magnets are assumed to provide a more moderate dipole field of 12 T, but they can later be pushed to a final ultimate field of 24 T. A third collider presently under study is the High-Energy LHC (HE-LHC), which is a higher energy collider in the existing LHC tunnel, exploiting the FCC magnet technology in order to essentially double the LHC energy at significantly higher luminosity.