Physicists at CERN have unveiled a blueprint for a huge 100 km-circumference particle smasher that would be used to study the Higgs boson in unprecedented detail as well as search for new physics. Today, the conceptual design report has been released for the Future Circular Collider (FCC) – an underground particle collider that would be linked with the existing Large Hadron Collider (LHC) near Geneva.

Since the LHC first switched on in 2008, the 27 km-circumference particle collider has been smashing protons together at energies up to 13 TeV in the hunt for new particles. In 2012, physicists announced they had discovered the Higgs boson with a mass of 125 GeV. This resulted in François Englert and Peter Higgs bagging the 2013 Nobel Prize for Physics for the theoretical prediction work on the particle. However, since then no particles beyond the Standard Model, such as supersymmetric partners, have been found.

While the LHC will still run for a few more decades before it is finally switched off, for more than three decades physicists have been carrying out R&D on linear colliders that could one day be the LHC’s successor. One leading design effort is the International Linear Collider (ILC), which would accelerate electrons and positrons using superconducting cavities. As electrons and positrons are fundamental particles, their collisions are cleaner than proton-proton collision at the LHC, so are ideal to study particles in great detail.

Japan is the only country that has shown interest in hosting the ILC but the Japanese government has dragged its feet in deciding whether to host the machine. This has forced physicists to recently downscale their design for the ILC from 500 GeV to 250 GeV with the Japanese government expected to give a final decision to host the ILC in March.

Yet particle physicists still see advantages in keeping with large circular colliders, not least because they have a lot of experience in building them. From 1989 to 2000, for example, CERN operated the Large Electron–Positron Collider (LEP), which was in the same tunnel that now houses the LHC and carried out precise measurements of the Z and W bosons. And given the Higgs’s relatively low mass, a circular collider would be able to produce higher luminosities without suffering huge losses from synchrotron radiation, which would affect a collider operating at higher energies of 500 GeV.

Precision studies

The FCC project was initiated in 2013 by the European particle-physics community with a meeting held the following year in Geneva to begin work on the report. The new, four-volume conceptual design report looks at the feasibility of building a 100 km circular collider and examines the physics that such a potential machine could carry out. It first calls for the construction of a 100 km underground tunnel that would house an electron-positron collider (FCC-ee). This machine would consist of 80 km of bending magnets to accelerate the beam as well as quadrupole magnets that focus the beam before colliding them at two points in the ring.

The FCC conceptual design report is a remarkable accomplishment. It shows the tremendous potential of the FCC to improve our knowledge of fundamental physics and to advance many technologies with a broad impact on society Fabiola Gianotti

The FCC-ee — estimated to cost around $9bn of which $5bn would be used to build the tunnel — would operate at four energies over a 15-year period. The collider would begin at 91 GeV, producing around 1013 Z bosons over four years before operating at 160 GeV to produce 108 W+ and W- particles for a two-year period. While the W and Z particles have already been measured by the LEP collider, it is estimated that the FCC-ee machine would improve such measurements by an order of magnitude.

By then running at 240 GeV for three years, the FCC-ee would focus on creating a million Higgs particles. This would allow physicists to study the properties of the Higgs boson with an accuracy an order of magnitude better that what is possible today with the LHC. Finally, the collider would then be shut down for a year to prepare it to run at around 360 GeV to produce a million top and anti-top pairs over five years. More precise measurements of such particles could indicate deviations from Standard Model predictions that could point to new physics.

Once the physics programme for the FCC-ee is complete, the same tunnel could then be used to house a proton-proton collider (FCC-hh) much in the same way that LEP made way for the LHC. “The FCC could be an action reply of LEP and the LHC” says theorist John Ellis from Kings College London. “A proton-proton collider would offer the best chance to discover new particles”. Read more China unveils blueprint for huge underground ‘Higgs factory’

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. Yet to produce such collision energies would require the development of new magnets that operate at higher magnetic fields to steer the beam around the collider. The LHC currently works with 8 T superconducting magnets made from niobium-titanium (NbTi) alloys. Superconducting magnets are used as they allow high currents to flow without dissipating energy due to electrical resistance. The FCC-hh, however, with 50 GeV beams, would instead require 16 T magnets made from niobium-tin (Nb 3 Sn) superconductor.

Currently the LHC is undergoing a two-year shutdown to improve its luminosity – a measure of the rate of particle collisions – by a factor of 10. Dubbed the High-Luminosity LHC (HL-LHC) it aims to put this material to the test by using 11 T Nb 3 Sn superconducting dipole magnets. Yet more R&D needs to be carried out before they can be used at 16 T. Given the need for R&D as well as the high construction costs of the magnets, the estimated cost of the FCC-hh would be around $15bn, compared to around $13bn for the total cost of the LHC.

To run this endeavour as a global collaboration is truly important. This opens up the possibility of substantial in-kind contributions Michael Benedikt

The FCC-hh would have a total integrated luminosity of around 15-20 ab-1 – a factor of 5-10 more than that produced at the HL-LHC – and corresponding to 1010 Higgs bosons being produced. It would also be used to search for new particles at higher masses than possible at the LHC as well as discover or rule-out the existence of thermal dark-matter particles known as WIMPs. As with the LHC, the FCC-hh could also be used as a heavy-ion collider, smashing together lead ions at 39 TeV to study effects such as a quark-gluon plasma. It is estimated that the collider would be operational for at least 25 years to “provide a research tool until the end of the 21st century”.

“The FCC conceptual design report is a remarkable accomplishment. It shows the tremendous potential of the FCC to improve our knowledge of fundamental physics and to advance many technologies with a broad impact on society,” says CERN director-general Fabiola Gianotti. “While presenting new, daunting challenges, the FCC would greatly benefit from CERN’s expertise, accelerator complex and infrastructures, which have been developed over more than half a century.”

Show me the money

Given the huge costs of building the FCC, it would need wide support from the community and so officials at CERN have been busy building a collaboration in recent years that now consists of 135 institutions in 34 countries. “To run this endeavour as a global collaboration is truly important,” says CERN physicist Michael Benedikt, who leads the FCC project. “This opens up the possibility of substantial in-kind contributions from parties who are experts in building parts of such a machine.”

Even if physicists get financial support to build the FCC, there is the question of when to start building the machine. One option is to start by doubling the energy of the LHC to around 30 TeV with a high-energy upgrade (HE-LHC). Yet Benedikt thinks that it may be possible to bypass the HE-LHC and go straight to the FCC instead. In this case, the HL-LHC programme would run in parallel with the construction of the FCC tunnel before stopping around 2037. The FCC-ee would then start operation around 2040. Read more Building the next collider

Yet CERN is not the only one developing new circular collider designs. In November, physicists in China unveiled the conceptual design for its own 100 km tunnel, which would first house an electron-positron machine before hosting a proton-proton collider operating at 100 TeV. Although construction of the Chinese collider could start earlier than the FCC, Benedikt says that there are many similarities between the two designs. “That is a good thing,” adds Benedikt. “The considerable effort by China confirms that this is a valid option and there is a wide interest in such a machine.”

Analysis: Let’s go round again? It is a simple enough question, but the answer is proving rather tricky: is a circular or linear collider the best way forward to carry out precise measurements on the Higgs boson? While CERN’s Large Hadron Collider (LHC) has been producing copious amount of Higgs bosons since the particle was discovered in 2012, proton-proton collisions are not the best way to study a particle’s precise properties. This is because protons are not elementary particles and so their collisions produce debris that affects the accuracy of the measurements. That is not the case, however, when smashing together electrons with positrons and that is why particle physicists want to build such a machine to study the Higgs boson and try to spot any tiny deviations that could give hints of physics beyond the Standard Model. For years, physicists have been designing linear colliders that would operate on the TeV scale. One such leading design is the International Linear Collier (ILC), which Japan has shown interest in hosting, albeit in a cheaper incarnation running at 250 GeV. Due to the need to overcome energy losses from synchrotron radiation as electrons are accelerated around the ring, linear colliders offer a higher luminosity – a measure of the rate of particle collisions – compared to their circular counterparts for collision energies over 400 GeV. Yet at energies below this threshold, circular colliders have better luminosities than linear colliders — and can also host multiple detectors around the ring. If the mass of the Higgs boson was around 500 GeV or more, most would agree that a linear collider offers the best way forward. But with the Higgs mass being 125 GeV, a rather large luminosity curveball has been thrown into proceedings. This has put circular colliders firmly back on the drawing board and for the past five years physicists have been designing possible alternatives. This has resulted in two recent proposals – the Future Circular Collider (see main text) and China’s Circular Electron Positron Collider, the design of which was released last November. While circular designs must bear the cost of building a huge underground tunnel, they more than make up in terms of versatility and the fact that physicists have decades of experience in building them. For example, the same 100 km tunnel could also be used for a proton-proton machine operating at 100 TeV that would be used to hunt for new particles. The technology for both an ILC and a 100 km electron-positron collider is ready, but given the eye-watering price tags for both, all designs would need a large amount of international collaboration. Indeed, it is widely understood that Japan would only offer to fund half the cost of the $7.5bn ILC. If only one machine gets built, as looks probable, the question is which one? The battle lines have been drawn.