For well over 20 years, there have been two major linear collider proposals: the International Linear Collider (ILC), based on klystron-driven superconducting radio frequency (SCRF) cavities, and the Compact Linear Collider (CLIC), based on normal-conducting cavities powered by a drive beam. In the last couple of years, many more ideas have come up – from the Cool Copper Collider (C3) to concepts based on energy recovery, continuous-wave operation, or even plasma acceleration. Often, proposals have been discussed in the context of a specific technology mapped to a specific host location: ILC in Japan, CLIC at CERN, C3 at Fermilab, and so on. While this approach is very logical if the approval of a concrete project seems imminent, unfortunately, none of the linear colliders is at this stage at the moment. Thus why not take a step back and remind ourselves that across all the different technologies and siting options, all linear colliders share common physics goals and a common global vision for the future of particle physics?
A linear collider facility with polarised electron and positron beams, colliding at centre-of-mass energies of up to about 1 tera electron volt (TeV), would offer a rich programme to explore the Higgs boson and the top quark in a way highly complementary to a hadron collider. Above 1 TeV, the physics case, beyond pure exploration, is less clear today – but this could of course change with the high-luminosity LHC! While, e.g. the ILC’s Technical Design Report presents a proposal on how to reach 1 TeV with existing technology, a key ingredient of the linear collider programme is upgradeability: many important measurements of the Higgs boson and the top quark can actually be done at lower energies.
Therefore, an initial facility with 250 giga electron volts (GeV) — or even 550 GeV — could deliver unprecedented new clues on the mysteries of our universe at a significantly lower price tag in terms of financial and environmental resources than the full 1TeV-machine. If designed appropriately, the initial facility could then be upgraded to higher energies – and/or higher luminosities – with whatever technology seems the most suitable when the decision needs to be taken.
Here, the International Workshop on Future Linear Collider (LCWS) presented a fresh and much wider view than ever before by exploring upgrades with other technologies – i.e. replacing the full original main linac with a technology allowing for much higher gradients in order to reach higher centre-of-mass energies, even beyond 1 TeV, without extending the tunnel! Improved SCRF with much higher gradients, cool copper cavities, a CLIC-like drive beam scheme or even plasma-acceleration-based accelerators are intriguing candidates. Should future physics results determine that much higher luminosities are more urgent than higher energies, upgrades with energy- and particle recovery would offer a promising path. It is too early to choose a concrete upgrade scenario today, but the flexibility to incorporate advanced accelerator technologies is a key feature of the linear collider approach.
In any case a linear collider facility can be augmented with a second interaction region, to host a second interaction point – for example, for photon collisions – as well as beam dump and beamlines for fixed-target experiments and detector and accelerator R&D. Thus it could host large-scale demonstrators for its own upgrade technologies! On the other hand, a linear collider facility does not restrict in any way the choices for eventually exploring the 10-TeV-parton energy scale, for which, as of today, we don’t have the technology at hand – nor a very clear picture of what the required energy actually is. Whether in the end, a hadron collider, a muon collider, a plasma wakefield e+e-collider or any combination of those will be best can be chosen independently depending on R&D progress and physics developments.
Under the nickname of “A Global Linear Collider Vision’’, or short LCVision, these ideas were discussed throughout LCWS2024, starting with a dedicated session in the afternoon of the very first day and culminating in a presentation in the closing plenary. As I perceived it, the topic also came up in numerous parallel sessions and coffee-break conversations, inspiring all participants. It became very clear that these ideas should be pursued and put into a coherent picture for the upcoming Update of the European Strategy for Particle Physics. Meanwhile, an LCVision organisation has been setup, and work towards substanciating the various upgrade ideas, as well as towards proposing such a linear collider facility as next project for CERN, has started. The results will be presented at an LCVision workshop to be held Jan 8-10, 2025 at CERN. Registration is open until December 15. Everybody interested to follow the developments is cordially invited to sign up at LCVision-General !
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