The CERN Council has today approved an update to the European Strategy for Particle Physics that recommends further work on a huge 100 km collider – dubbed the Future Circular Collider (FCC) – that would be built in Geneva. But with no formal decision having been made to go ahead with the FCC, the strategy also calls for Europe to back a Japanese-led linear collider if it receives the go-ahead from the Japanese government.
The report, released this morning, sets out a plan for the future of particle physics in Europe to the mid-2020s and beyond. It especially concerns planning the next collider that would succeed the Large Hadron Collider, which first switched on in 2008. The 27 km-circumference LHC has been smashing protons together at energies up to 13 TeV in the hunt for new particles and in 2012 physicists announced they had discovered the Higgs boson with a mass of 125 GeV.
The LHC is currently undergoing a major £1.1bn “high luminosity” upgrade – dubbed HL-LHC – that will increase the collider’s luminosity by a factor of 10 over the original machine. The strategy indicates that the completion and exploitation of the HL-LHC should remain “the focal point of European particle physics”.
The strategy update, however, gives the green light for further study into the FCC, which would cost around £20bn. In January 2019, CERN released a four-volume conceptual design report for the FCC, which first called for the construction of a 100 km underground tunnel that would house an electron–positron collider (FCC-ee). The FCC-ee would focus on creating a million Higgs particles to allow physicists to study its properties with an accuracy an order of magnitude better that what is possible today with the LHC.
Once the physics programme for the FCC-ee is complete, the same tunnel could then be used to house a proton-proton collider, dubbed FCC-hh. The FCC-hh would use the LHC and its pre-injector accelerators to feed the collider that could reach a top energy of 100 TeV – seven times greater than the LHC. CERN will now carry out a more detailed costing of the FCC as well as continue research and development into the magnet technology that will be required for such a machine at higher energies.
Eyes on Japan
The strategy also approves European participation in the ¥800bn ($7.5bn) International Linear Collider (ILC) if it receives support from the Japanese government. First mooted over a decade ago, the ILC would accelerate and smash together electrons with positrons at 125 GeV in a 20 km tunnel to study the Higgs boson and other particles in precise detail.
In March 2019, officials in Japan said that their government has formally “expressed an interest” in the particle smasher but did not decide whether to host the machine. The final go-ahead will only be given if enough international support and funding can be found to construct the machine and there is a consensus within the Japanese scientific community that the project is worth pursuing.
The European strategy is to prepare a Higgs factory, followed by a future hadron collider.
Halina Abramowicz
Yet backing the FCC does not contradict supporting the ILC as the two could be complementary. If the ILC is given the green light, then CERN could opt to bypass the FCC-ee and build the FCC-hh after the LHC programme is complete. “The European strategy is to prepare a Higgs factory, followed by a future hadron collider with sensitivity to energy scales an order of magnitude higher than those at the LHC,” noted Halina Abramowicz, who is secretary of the European particle physics strategy update.
“What this [decision] is about is prioritizing the R&D for what comes after the LHC, it’s not proposing a new machine right now,” physicist John Butterworth from Univeristy College London, who sits on the European strategy group, told BBC Radio 4. “It’s looking at what we have learned, looking at the technologies that we think we might need, where should we be prioritizing our resources for what we might want to do after the LHC”.
End of CLIC?
However, the decision to prioritise the FCC and back the ILC if Japan gives the go-ahead, puts the CERN-led Compact Linear Collider – another linear collider proposal – as a “plan B”. CLIC is not as technologically mature as the ILC, but could run at higher energies. It is now only likely to go ahead if the FCC turns out to be too costly and the ILC is not given the go ahead by Japan.
An electron-positron collider Higgs factory as highest priority for our field is clearly the way forward
Philip Burrows
Philip Burrows from the University of Oxford, who is CLIC’s spokesperson, says he congratulates the European Strategy group on their “careful deliberations and clear recommendations”. “An electron–positron collider Higgs factory as highest priority for our field is clearly the way forward,” Burrows told Physics World. “The options for CLIC as a CERN-based Higgs factory, and subsequent energy-frontier exploration collider, are clearly articulated, and the door is opened wide for Europe to make major contributions to ILC should Japan go ahead and realize ILC as a global project. Either way we can plan to realise a linear collider Higgs factory.”
A call for unity within particle physics
Particle theorist John Ellis from King’s College London told Physics World that he is “happy” with the proposal to support a Higgs factory and prepare for a high-energy collider at CERN. “CLIC is de-emphasized, though not explicitly dropped,” he says, adding that the ILC is mentioned “only as something with which the European particle physics community would wish to collaborate, without any commitment by CERN”.
CLIC is de-emphasized, though not explicitly dropped.
John Ellis, King's College London
The FCC, CLIC and the ILC are not the only proposals for a future high-energy collider. Physicists in China unveiled the conceptual design for its own 100 km tunnel in September 2018. It would first house an electron–positron machine before hosting a proton–proton collider operating at 100 TeV. If it gets the go-ahead, construction of the Chinese collider could start before the the FCC.
The European strategy was originally due to be announced in May, but this was postponed due to the COVID-19 pandemic.
