Next-gen particle accelerator ‘not built’ at CERN

Particles will speed through the ILC's tunnel before before being smashed together at huge energies Rey.Hori/KEK

The successor to the Large Hadron Collider (LHC) – the world’s most powerful particle accelerator – will most likely be based in Japan. But this does not mean the end of CERN, the European Organization for Nuclear Research, near Geneva.

The international electron-positron collider project, known as the International Linear Collider (ILC), already exists on paper.

The blueprint was officially handed over to the International Committee for Future Accelerators (ICFA) on June 12. Three consecutive ceremonies were organised at Tokyo University, CERN and the US Fermi National Accelerator Laboratory (Fermilab) in Chicago, Illinois.

The blueprint’s technical design report was the fruit of more than ten years’ research by more than 1,000 scientists and engineers from over 100 laboratories in 20 countries.

Unlike the LHC, which forces particles around an oval-shaped 26km underground tunnel to get them almost to the speed of light before smashing them together, the ILC will consist of two linear accelerators that face each other. Electrons and their anti-particles, positrons, will be accelerated and collided in the detectors at the centre of the 31-km machine.

At the height of operation, bunches of electrons and positrons will collide roughly 7,000 times per second, creating a surge of new particles that are tracked and registered in the ILC’s detectors. Each bunch will contain 20 billion electrons or positrons concentrated into an area much smaller than that of a human hair.

Slices of cake, cherry stones and black matter

Smashing electrons and positrons together is not new. The Large Electron–Positron Collider (LEP), the predecessor to CERN’s LHC, was operational at the French-Swiss site from 1989-2000. But total collision energy was limited to 209 gigaelectron volts (GeV), while that of the ILC may go up to 1,000 GeV when at full power.

But power is not all. One of the advantages of the electron-positron collider is the clarity of results, since electrons and positrons are basic particles, while the protons used in the LHC are composed of many other smaller particles.

A Japanese researcher once described the LHC collisions as smashing together slices of cherry pie. The collisions produce a mass of cake base, sugar, fruit and sometimes two cherry stones crashing into each other, he explained, whereas with the ILC you just see the cherry stones.

But what are scientists likely to find experimenting with the ILC? Not the famous Higgs Boson, as that was discovered by the LHC in July 2012 at 126GeV. The new particle collider will be able to produce all the known particles but will also allow their interactions to be studied.

But that’s not all

“One of the beauties of the ILC is being able to detect the disintegration of the Higgs Boson into dark matter particles,” explained physicist François Le Diberder from Paris Diderot University, who is a member of the European ILC commission.

“When you launch an electron against a positron, the resulting collision simultaneously produces a Z Boson and a Higgs [particles], which rapidly disintegrate, but you only see the disintegration of the Z Boson. You measure the energy and movement of the resulting particles and from that you can deduce the mass of whatever you are faced with but cannot see. If it’s 126 GeV, it’s proof that the Higgs can disintegrate into invisible particles.”

This would be the start of the answer to one of the greatest enigmas in physics and cosmology. Normal visible matter is thought to form only four per cent of everything that exists in the universe, while 22 per cent is dark matter and 74 per cent is dark energy.

There are strong suspicions that dark matter is made up of particles but, according to Le Diberder, dark energy remains a “total mystery”.

“Dark energy completely escapes any attempt to describe it in terms of particle physics. In theory, the ILC is not designed to resolve this enigma, unless we get a surprise,” he said.

Japan in pole position

Although the LHC was built in the former tunnel of the LEP on the outskirts of Geneva, the ILC will not be based at CERN.

“There is absolutely no chance of the accelerator being built here,” confirmed CERN Director-General Rolf-Dieter Heuer.

“We are already totally busy with the LHC. For the moment the only candidate country that I see actually able to build the thing within a reasonable deadline is Japan. The government seems to be prepared to put up the funds which would come from outside its normal research budget.”

The machine is likely to cost around CHF8 billion ($8.8 billion).

The Japanese ILC support committee proposes to house the accelerator in a tunnel inside the Kitakami mountains, 500km north of Tokyo.

The Japanese are extremely enthusiastic about the project. In a promotional video available on YouTube, they boast that the Big Bang and particle physics are taught at primary school level in Japan.

Nobody disputes the fact that the country has the scientific know-how and technology necessary. The Japanese Productivity Centre has also calculated that the ILC could have a financial impact of over $40 billion over a 30-year period and create 250,000 direct and indirect jobs.

But it’s not a done deal. A final decision is expected in 2015 and other countries are also in the running, such as Germany, Russia and the United States. There is also some confusion over the organisation and structure that will manage the huge collider.

“That will depend on government-level talks which are beyond our competences,” said Heuer. “For the moment, we are still at the bilateral talks stage between Japan and other countries. Will it be a structure similar to CERN or even a branch of CERN? We don’t know. But one thing is clear: it can only be an international organisation.”

CERN remains CERN

The ILC will probably start working between 2025-2030, just when the LHC comes to the end of its lifetime. But it is not an updated version of its predecessor, explained Heuer. They are actually two complementary tools.

“It’s like in astrophysics. You observe the skies with optical, infrared, ultraviolet and radio telescopes. They are all needed to form a complete image. It’s the same with us. Different colliders examine similar questions but from different viewpoints,” he explained.

So, what will happen to CERN without its LHC once the focus is on the ILC and Japan?

“In terms of scientists and engineers, the Geneva site won’t be very different from what it is today,” he said.

“We will participate in the ILC’s experiments from here. We will analyse the results and work on the ILC’s successor. Ever since we started building giant particle colliders, we’ve had the LEP, and then power levels jumped to the Tevatron with the Fermilab before things switched to CERN with the LHC. I believe this constant back-and-forth between continents is something very stimulating and healthy.”

Whatever happens, the community of particle physicists is already thinking about what comes next after the ILC. Its design and future performance will depend not only on how technology evolves, but especially on the results that come out of the LHC over the next ten to 15 years. But to continue research into the mysteries of matter, space and time, a much more powerful collider will probably be needed.

(Translated from French by Simon Bradley)