The Next Big Collider

The particle physics community is presently considering what to do for its next big accelerator. The LHC at CERN, which began operation in 2008, has served well in providing the discovery of the Higgs boson, but that was 15 years ago and since then LHC experiments have turned up no major indications of new physics.

Theoretical expectations from supersymmetry theory have proved to be wrong, in that no expected SUSY particles have been found. The Standard Model of Particle Physics, a paste-up job with far too many arbitrary empirical parameters, remains unchallenged as the theoretical best we can do for now. In the near term, particle physics is in for an extended period in which experimental effort goes into refining measurements instead of making new discoveries. In the long term, the best possibility for breaking out of this "crisis" is to construct at least one new and more powerful particle collider.

Every ten years a group called the Particle Physics Project Prioritization Panel (P5) is convened to advise the two principal science funding agencies of the USA, the National Science Foundation (NSF) and the Department of Energy (DOE), on particle physics issues. P5 sets the particle physics research agendas for the coming decade and recommends funding for major particle physics projects. In P5's 2013 report, they recommended the Deep Underground Neutrino Experiment (DUNE), which is presently being constructed in the Homestake mine in Lead, North Dakota (and is rather late and over budget).

The 2023 P5 report is set to come out this Fall. In the area of new colliders, the panel must review alternatives, choosing which particles to collide (protons, electrons, or muons) using which geometry (circular or linear), with about six multi-billion-dollar collider projects in the running.

Circular proton colliders like the LHC offer high collision energy and luminosity (beam intensity) and bring the same particles to a collision vertex many times as they orbit the machine. Further, there can be multiple collision points around the ring, providing collisions for several large detectors at separate locations. However, circular machines have some disadvantages: (1) protons are 3-quark composite particles, so the colliding quarks have only about 1/3 of the beam energy, (2) p-p collisions are messy and complicated, limiting measurement precision, and (3) accelerating electrons and positrons with circular machines has intrinsic energy limits imposed by synchrotron radiation. Electrons have a small mass, and the sideways acceleration of bending their path in a magnetic field induces electromagnetic radiation that constitutes a beam "energy leak". For a given circular collider radius, this sets a limit on the energy to which electrons can be accelerated.

In principle, muons (mu leptons) are a much better choice for acceleration in a circular collider. They have 200 times more mass than an electron, effectively eliminating the synchrotron radiation problem, and the muon is a fundamental particle, not a composite like the proton. Unfortunately, muons have the problems that (1) they are not stable and must be produced in collisions and then "cooled" before they can be accelerated, (2) at rest they have a mean lifetime of about 2.2 microseconds, so there is not much time available to create, cool, and accelerate them before they decay, (3) in a collider they will only orbit for a few thousand turns before they decay and are lost, and (4) because of their decays in the collider, they create their own strong beam-induced background, making severe background problems for the designers of the large experiments using the muon collider.

Linear electron-positron colliders offer better precision, but two linear accelerators must be built (one for electrons and the other for positrons) and these must be very long (10 to 50 km each) to achieve the collision energies that the experiments need. Further, the accelerated particles pass only once through the collision vertex. This limits the luminosity that linear colliders can achieve. It should also be noted that circular proton machines can only be upgraded in collision energy by boosting the magnetic field, which cannot go much above 15 tesla. A linear collider can be upgraded in collision energy by increasing the accelerating electric field or by building out the length of the linear accelerator itself.

Somewhat independent of what P5 recommends, Europe's CERN Laboratory will set its own priorities. CERN's LHC, with upgrades, will dominate the particle physics field until about 2040. CERN has done the initial planning for its successor, the Future Circular Collider (FCC), a large circular accelerator ring (circumference 97.75 km) that would first collide electrons and positrons with low-field magnets, and later be rebuilt with high-field two-channel superconducting magnets to collide protons and heavy nuclei. (Note that the cancelled SSC project in the USA would have had a ring circumference of 87.1 km.)

The CERN laboratory has plans to continue this trajectory by constructing the Future Circular Collider (FCC). Like the earlier LEP/LHC projects constructed in the same tunnel, they would dig a new 97.75 km circumference tunnel and construct FCC-ee, using low field magnets and producing electron-positron collisions at up to 365 GeV (the energy region where top-quark pairs would be created). Later, high-field two-channel superconducting magnets with fields up to 16 tesla would be added, and the facility (FCC-hh) would collide protons in the same energy region.

CERN is presently occupied with upgrading the LHC to higher energies and luminosities, so the FCC project has a rather long timeline. First operation of FCC-ee is projected to occur around the year 2048, a quarter of a century from now, and operation of FCC-hh as a proton collider is projected to occur around 2075, over half a century. As you might imagine, the experimentalists who would like to use the machines are dismayed by these long waits.

CLIC, the other CERN collider project, has a more compressed timeline. CLIC is a linear collider that uses the trick of producing one beam to drive a second beam, producing extremely high acceleration fields on the order of 100 million volts per meter. CLIC could go into operation with beams of 365 GeV as early as 2035 (i.e., in 12 years) with a total length roughly equal to the diameter of the LHC (11 km), and later could be lengthened to 50 km to reach beam energies of 1.5 TeV. The CLIC technology is less well established, and the expected luminosity is lower, but if it works, it promises to be implemented faster and offers the possibility of even higher collision energies than the FCC.

What new collider (if any) will the E5 Committee choose? I do not have a crystal ball to provide an answer, although by the time you read this, the choice should have been made. If I were on the E5 Committee, I would recommend CLIC (~2035) as a fast-implementable and expandable Higgs factory and the Muon Collider (~2044) for new physics opportunities. We'll see.

Note added after publication:  In January-2024 the E5 Report became available.  The panel recommended that the US agencies undertake the initial stages of the Muon Collider project and encouraged CERN's  FCC project, but they did not recommend CLIC or the other projects described above.