Planning the world's next collider

Physics World, Jul 26, 2012 11 comments
Having led the construction of CERN's Large Hadron Collider (LHC), where researchers recently announced the discovery of a particle that looks like the Higgs boson, Lyn Evans was appointed as the first ever linear collider director by the International Committee for Future Accelerators in June. Hamish Johnston travelled to Geneva to find out more about Evans's new job, which will see him developing the International Linear Collider (ILC) and the Compact Linear Collider (CLIC) – two projects that are vying to be the next big particle-physics facility after the LHC

How will the discovery of the Higgs boson at the LHC affect the design of a future linear collider?

Now that it looks like we have got the Higgs at a low mass, we know the minimum energy – around 250 GeV – at which a linear collider could start to do interesting physics. However, we still need the LHC to operate at its full energy of 14 TeV to guide us towards what else we may need.

How do CLIC and the ILC stack up against each other?

CLIC and the ILC are two separate concepts. Both are designed to accelerate and smash together electrons and positrons. Although there are similarities between the two projects – especially in the detectors – there are big differences in the accelerating structures. The ILC is based on a superconducting technology, involving a series of accelerating cavities that are powered by klystrons. The technology is mature and most of the development effort on the ILC is currently focused on industrializing the technology. In terms of energy, the technology is a bit limited but if we wanted a total collision energy of 500 GeV, the ILC would be perfect. We might eventually be able to push that energy up to around 1 TeV.

So what about CLIC?

CLIC is based on completely new technology, and is still very much in the R&D stage. It has a much higher accelerating gradient and therefore could operate at higher collision energies. CLIC relies on a two-beam concept in which a "drive" beam runs in parallel with the accelerated beam – and energy is transferred from one beam to the other.
CLIC would operate at 11 GHz, whereas the ILC would run at about 1 GHz. This would give CLIC a higher accelerating gradient of 100 MV/m compared with the 31 MV/m of the ILC. This means that, for a given accelerating energy, CLIC would be considerably shorter than the ILC – or to put it another way, CLIC can go to a higher energy, up to 3 TeV, for a given length.

What needs to be done before the winning design is chosen?

An early decision to build a linear collider would imply using ILC technology since it is already mature. In the meantime, without the guarantee of an early decision, we will continue to develop CLIC technology to a level of maturity where we could compare the two options in terms of scientific capability and cost.
The plan is to bring the CLIC and ILC development teams together and give them a common direction. Both technologies will be developed in parallel for three or four years until a final decision is made about what is actually going to be built. The decision will be made in terms of physics and not politics or personal prejudices. My job is to encourage much more dialogue between the CLIC and ILC communities. I also need to ensure that we are in the position to take a collective decision, based on scientific needs about which collider design to choose – without too much emotion.

What are the main differences between a linear collider and the LHC?

A linear collider smashes leptons such as electrons and positrons, which are fundamental particles. As a result the collisions produce a relatively small number of particles. The LHC collides hadrons, which themselves are made of quarks and gluons. In the LHC we want to study the hard collisions between the fundamental components, but there are lots of other ways that protons can collide. It's a bit like smashing two oranges together just to watch the pips collide – it is very messy. The LHC is a beautiful machine for discovery but is less good at precision measurement than a linear collider. There are also fundamental processes that only a lepton collider can address.

What do you make of suggestions that a linear collider might be built in stages?

A staged approach looks attractive in terms of keeping the initial cost down. We could start at a low energy and boost the energy by simply making the collider longer over the years – something that cannot be done with a circular collider. Somewhere around 250 GeV would be a good place to start and that would bring the cost down a lot.

Will the linear collider be built in Japan?

Japan is taking the matter very seriously. It made a big contribution to the construction of the LHC and may now be prepared to host a new international facility. There are two sites in Japan that have been financed for geological surveys and it would not surprise me if the Japanese make a proposal to build a linear collider in the next few years.

But where will the collider development effort be based?

Like the LHC it is very much an international effort. I will be based at CERN and the CLIC team is also here in Geneva with collaborating institutes mainly in Europe, but also in the US, Australia and Japan. The ILC project is dispersed all over the world. There is work going on at KEK in Japan, DESY in Germany and at several labs in the US including Fermilab and Brookhaven. There is a station at Fermilab where ILC modules are tested. The first module is there and the second one is being built. However, the funding situation in the US is very uncertain at the moment. At DESY they are building a free-electron laser using technology that is very similar to the ILC and this will be an important testbed. There is also ILC development work going on in Japan.

As for the machine you masterminded, can we expect an upgrade to the LHC beyond its 14 TeV design energy?

An upgrade to the LHC is a no-brainer – it is a beautiful machine and can do much better than its original design. An upgrade programme has to be the main effort of CERN over the next 15 years. We will definitely be increasing the energy from 8 TeV to 14 TeV after the 2013–2014 technical stop and there is also a plan to boost the LHC's luminosity over five years, but doubling the energy to around 30 TeV would require 16 T magnets. When we started planning the LHC we couldn't make its current 8 T magnets so it is possible that R&D could deliver the technology. However, it is important to realize that a higher-energy LHC would essentially be a new collider, whereas luminosity can be increased incrementally. Everyone agrees that the top priority for CERN is to exploit the LHC to its full potential while contributing to the worldwide effort towards the next linear collider.

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