LHC illustration

Finding a successor to the world's largest science experiment

Just six years after the LHC came online, particle physicists are looking for a successor. The scale of the engineering challenge will be unlike any before, but the first task is choosing which road to follow.

Just over a year ago scientists investigating data from the Large Hadron Collider confirmed the discovery of the Higgs Boson – the holy grail of the Standard Model of particle physics that underpins scientists' understanding of the fundamental workings of our universe.

One month earlier, the LHC – the European Organization for Nuclear Research's (Cern) 27km particle collider buried beneath the Franco-Swiss border near Geneva – shut down for upgrades. When it fires back up in early 2015, its maximum collision energy will almost double, opening the door to new discoveries that could either provide further proof for the Standard Model or turn the theory on its head.

But despite the untapped resources of the largest science experiment in the world, physicists and engineers are already looking to a successor. The Cern Council had released its plans for future colliders in its European Strategy for Particle Physics by July 2006, more than two years before it fired the first protons around the LHC. Such a long-term approach makes sense when you consider that it took the LHC from the early 1980s to September 2008 to get to the first test run.

Exactly what sort of collider should come next is a matter of debate. "It's impossible to say," says experimental physicist Professor Terry Wyatt, of the Particle Physics Group at the University of Manchester, who works on the ATLAS detector at the LHC, one of two that detected the Higgs. "It's extremely hard to have any objective and absolute hard reason to believe one thing or another."

Options for a successor range in size and feasibility, but a central question is what the machine should be colliding. The LHC collides opposing beams of either protons or lead nuclei, which are made up of neutrons. Protons and neutrons are hadrons – composites of smaller particles, called quarks. The LHC's predecessor, the Large Electron Positron Collider (LEP), was designed to collide leptons – fundamental and indivisible particles – specifically electrons and their antimatter counterpart positrons.

As leptons are elementary they annihilate completely in collisions and nearly all the energy goes into creating new particles, making precision measurements much easier, but they are extremely light, making them difficult to accelerate to high collision energies. Hadrons are much heavier, making them capable of higher energy collisions, but their composite nature makes collisions messy. "Not all the energy goes into producing new particles, most goes into the crap that fills your detector," says Prof Wyatt.

Making a decision about what kind of collider to build depends on what you expect to find. The Higgs Boson was discovered to have a mass of 125GeV (gigaelectronvolts), well within the 1TeV (teraelectronvolt) collision energy scope of proposed lepton colliders. But if new physics beyond the Standard Model such as supersymmetry and string theory exists, as many theorists expect, collision energies beyond the refitted LHC's 14TeV are widely considered essential.

The LHC's next experiments from 2015 to 2018 may help guide a decision. Regardless of what the LHC throws up, the time it took to get that project off the ground demonstrates the need to press ahead now.

The most technologically mature option is the International Linear Collider (ILC) – an electron-positron collider with an initial collision energy of 500GeV and the potential to upgrade to 1TeV. The technology is well established, with a smaller version running at the DESY research centre in Hamburg. Proponents are impatient to get started.

"It's a question of swings and balances, but if we were forever waiting for some new phenomenon to show up then we would never have built the LHC," says Professor Brian Foster, of the University of Oxford and European regional director for the ILC.

The ILC will use two 12km-long main linear accelerators, each composed of 8,000 metre-long superconducting radio frequency (RF) cavities made of pure niobium end to end in cryomodules that cool them to -271°C. The RF cavities are shaped to create a standing wave when an electromagnetic field is applied to them, with a frequency to give the bunches of electrons and positrons an accelerating push, sending them towards each other at nearly the speed of light.

Building on past colliders

As a lepton collider, the ILC follows in the footsteps of LEP, but by abandoning a circular design it overcomes a weakness in electron-positron colliders. Forcing very light particles such as electrons to accelerate radially as they are bent round corners causes them to emit synchrotron radiation and the harder they are pushed the more energy they radiate. This makes reaching high collision energies hugely inefficient.

Unlike in circular colliders where beams interact thousands of times a second, in linear colliders they pass once before dissipating. This means particle bunches have to be crushed to a tiny volume to collide with the required probability to provide a reasonable luminosity – the number of collisions that can be produced in a detector per cm2 per second. The electron and positron beams will be fed into 7km-circumference damping rings where they will pass through a series of 'wigglers' – magnetic structures with a periodically alternating vertical field that wiggle the beam to emit photons. Upon exiting the damping rings, the bunches will be just a few millimetres long and thinner than a human hair.

Prof Foster says they are in a position to build due to a step change in the performance of superconducting RF cavities, which can generate the 31.5MV/m acceleration gradient necessary to reach the collider's target collision energy within a reasonable distance. "Over the last 20 years the costs have gone down by a factor of 10, and the gradient has gone up by a factor of 10."

More importantly, Japan seems willing to host the project and put forward the bulk of the $8bn costs. About 30 per cent of Japan's parliament has signed up to an ILC promotion body and the high-energy physics community has earmarked the Kitakami Mountains as a potential site. According to Prof Foster, the site could take the ILC beyond the proposed 1TeV collision energy, but even at lower energies the case for a new lepton collider remains convincing.

Advances and developments

A potential challenge to the ILC could come from the Compact Linear Collider (CLIC), another linear electron-positron collider that could reach up to 3TeV. Unlike the ILC, the machine would rely on normal conducting RF cavities as it requires an accelerating gradient of about 100MV/m – superconducting cavities are fundamentally limited to accelerating gradients of approximately 60MV/m.

The lack of superconductivity results in lower efficiency. No conventional RF source can provide the necessary power, so a novel two-beam acceleration scheme has been designed. Two 1km accelerators, powered by 326 conventional high-power klystrons, will accelerate high-current low-energy drive beams that serve as an RF power source for the low-current high-energy main beams. The drive beams are led to 24 decelerator modules where 90 per cent of the beam power is extracted by Power Extraction and Transfer Structures (PETS) to power the 21km main beam linear accelerators. The structure operates as a gigantic power transformer.

But the scheme still faces major challenges such as developing PETS that generate the required power and efficiently generating the high-intensity drive beam. A smaller scale version of the technology has been trialled at Cern, but Prof Foster is sceptical about whether it could be scaled up. "It would need huge amounts of energy and it would be almost certainly unaffordable," he says.

While Cern's ideas for a linear collider are still formative, plans for a new circular collider are further advanced. A civil engineering study has confirmed the viability of a new 100km tunnel under Lake Geneva. "In terms of scale it's not so crazy but of course it's still a very big civil engineering project," says Cern director of beams Dr Paul Collier. And there are a variety of ideas about what should fill this enormous tunnel, firstly a gigantic hadron collider that could reach energies of 80 to 100TeV.

While synchrotron radiation has much less of an effect on hadrons, as it is inversely proportional to the mass of the particle, the extra mass makes it much harder to bend them round a circuit. To reach high energies a circular hadron collider needs either very high magnetic fields or a very big tunnel. The LHC's magnets are made of superconducting niobium-titanium composite that can carry very high current density – essential to create a high magnetic field – but as the field increases the material's superconducting properties diminish.

Researchers at Cern are working on magnets using niobium-tin, which can reach higher fields before losing superconductivity. "Niobium-tin technology is capable of about 15T (Tesla) – almost twice the magnetic field of the LHC – with a lot of development work," says Dr Collier. "But it will run out of steam after that. To sort that out we're going to need to use novel materials, and these are only in the infancy of their development."

A 100TeV hadron collider will need magnets capable of 20T upwards, but materials with the potential to provide this kind of field are very brittle. The theory suggests using powders of the ceramic-like materials in curved moulds to create cables before curing them at a high temperature. "It's a tricky technology to master," says Dr Collier, who has a background in applied physics and electrical engineering.

Such developmental challenges mean the 100km tunnel could be ready before the hadron collider technology, so Cern is investigating installing an electron-positron collider – TLEP – in the tunnel first. Using a 100km tunnel would reduce losses from synchrotron radiation and a circular design can provide much higher total luminosity than a linear collider. TLEP would have between two and four interaction points each, with luminosity equal to the ILC's one interaction point. But it would operate at a maximum of 350GeV and would still require vast improvements in the efficiency of the technology, says Dr Collier. A particular area needing attention is high-efficiency RF power conversion from the grid to the beam focusing on amplifier technologies.

A meeting was held in Geneva in February to get the ball rolling, but a decision on a future hadron collider is unlikely before the LHC finishes its next experiments in 2018, just prior to the update of the European Strategy. "It's such a huge machine it takes a long time to refine down to a coherent design; at the moment we're really just exploring the parameters," says Dr Collier.

With so many competing designs one might wonder if there is a danger of spreading the physics community's limited resources too thin, but Prof Wyatt says it is the only way to avoid any developmental cul-de-sacs. "It's perfectly possible that the R&D will make you realise that these technologies won't work at the scale we need for these accelerators," he says.

To this end, some researchers are working on even more speculative projects such as machines that collide muons – highly unstable leptons roughly 200 times the mass of electrons. Their extra mass reduces the effect of synchrotron radiation, offering the opportunity to combine the clean collisions of leptons with multi-TeV collision energies.

The Muon Accelerator Program at US research centre Fermilab is investigating facilities capable of up to 6TeV, but this is still on the drawing board. Muons decay rapidly, making it difficult to get them to collide before morphing into other particles, and the accelerator and detectors would be constantly flooded by high-energy particles.

Other scientists eager to investigate the newly discovered Higgs have suggested colliding photons in the form of high-energy gamma rays as a cost-effective 'Higgs factory'. The SAPPHiRE gamma-gamma collider would be only 10km in circumference and operate at just 125GeV, but Prof Wyatt says its limited physics program means it would have to become very cheap to be considered.

Political decision making

With so many designs contending to become the next big physics project, and all of them coming with a multi-billion pound price tag, science may not be the sole deciding factor. The physics community agrees that both a lepton and a hadron collider could do good physics, but with government science budgets being squeezed the world may not have the resources to back more than one horse in this race. "A certain amount of real-politique comes into the decision," says Prof Wyatt. "If the Japanese will stump up half the funds but want to build the ILC, then probably the ILC will be the one."

With the huge trickle-down of collateral benefits that large-scale scientific projects create, Prof Wyatt hopes those with their fingers on the purse strings think about the bigger picture. "If you think how many trillions of dollars got printed and handed over to the banks in the last five years, spending 10 billion of whatever currency is a very small amount of money," he says.

"The challenge to industry to deliver the technology needed and manufacture at the scale required to build any of these facilities is huge. If you had the choice of that or printing huge amounts of money, I know which makes most sense to me." 

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