by Anil Ananthaswamy

From issue 2671 of New Scientist magazine

27 August 2008, page 31-33
from NewScientist Website
 

Find out what the LHC could discover

AT 27 KILOMETERS long, it is the largest machine in the world. It will accelerate counter-rotating beams of protons to within a whisker of the speed of light and smash them head-on 600 million times a second. The most distinctive feature of the Large Hadron Collider, though, is its temperature. At 1.9 kelvin - a smidgen above absolute zero - the LHC is the coldest ring in the universe, unless an alien civilization has built one that is colder.

Were it not for this searing cold, the LHC might have suffered the same fate as the Superconducting Super Collider (SSC), which was on its way to becoming the most powerful accelerator in the world before the US government canned it in 1993. The SSC's partially completed tunnel, near Waxahachie, Texas, now lies derelict - killed by a ballooning budget and a deficit of innovation.

Keen to avoid a similar debacle, CERN, the European particle physics lab near Geneva, Switzerland, took the momentous decision to cram the LHC into an existing circular tunnel 100 meters underground, which had been built in the 1980s for the Large Electron Positron (LEP) collider. It was a decision that led to a carefully choreographed dance of extreme engineering. Underground rivers were frozen in mid flow, components shipped around the world and superconductors pushed to their limits.

One of the tasks planned for the SSC ring was to search for the Higgs boson, the particle through which the universe is thought to get its mass. It would have meant smashing protons into each other at energies of 40 teraelectronvolts (TeV).

To get the particles traveling at such energies in a ring requires steering so precise that it can only be provided by intense magnetic fields created by superconducting magnets. These operate without loss of power when chilled below a critical temperature. The SSC had not pushed the technological boundaries, though, opting for superconducting magnets cooled by liquid helium to a relatively tepid 4.5 K, which were already being used in other accelerators. This proved to be its undoing.

For a given radius of tunnel, the more energetic the particles, the more powerful the magnets need to be. Since the SSC's magnets were not powerful enough, the tunnel had to be 87 kilometers round. The cost of building a machine this big doomed the SSC.

Anxious not to make the same mistake, LHC's engineers had to be bold. By 1998, they had designed the LHC to fit into the old LEP tunnel, which had been built for a collider with a peak collision energy of 209 gigaelectronvolts. The LHC was aiming for 14 TeV, nearly a 70-fold increase. It needed the next generation of superconducting coils both for the supremely powerful magnets needed to bend its high-energy proton beams around the tunnel's tight curve, and for the radio-frequency cavities used to accelerate protons.

The RF cavities and the magnets had to be compact enough to fit into the small-bore tunnel, while carrying extremely high currents.

The designers went for coils made of niobium-titanium, the only ones that could be made in the large quantities required by the LHC. Generating the extra-strong magnetic fields for the machine meant cooling the coils down to 1.9 K so that they could carry much more current. This, however, came at a price. At that temperature, liquid helium becomes superfluid, with weird quantum properties.

This means it has zero viscosity and can slip through microscopic cracks. So the thousands upon thousands of welds in the plumbing had to be,

"at least as good as those in a nuclear plant", says LHC project leader Lyn Evans.

By the late 1990s, the most pressing concern was the massive cavern needed to house ATLAS, the 7000-tonne detector that will track the particles that fly out from the collisions. Among the most important of these are muons, heavier versions of electrons, and the way to measure their momentum is to bend their paths in a magnetic field.

Because of the LHC's power, these muons will be more energetic and faster moving than anything seen in previous colliders, so the magnetic fields have to be very strong. The stronger the field, the more the particles bend, and the more precisely their properties can be measured.

ATLAS ended up with the world's largest superconducting magnet, by volume. The cavern created to house this 12-storey-high behemoth had to be 35 meters high.

It's so big that the cavern's hydrostatic pressure causes it to rise rather like a bubble in water, albeit extremely slowly.

"It moves about 0.2 millimeters upward every year," says ATLAS spokesman Peter Jenni, so the floor had to be cast 5 meters thick to ensure that it doesn't warp as it rises.

To cross-check the findings from ATLAS, a second catch-all experiment called the Compact Muon Solenoid (CMS) will hunt for the same particles using different technology. It has thrown up its own share of challenges.

As protons collide inside the detectors, the aftermath can slightly disrupt the path of other protons as they race around the ring. To minimize this effect, the detectors have to be as far away from each other as possible. So the CMS has been sited diametrically opposite ATLAS, which puts it at the base of the Jura Mountains.

This spelled trouble.

"It was the worst possible place from a civil engineering point of view," says project engineer John Osborne. "The conditions were terrible."

First, the engineers had to dig two 60-metre-deep shafts, one for elevators and one to lower the detector.

When they got there, they found that the area consisted of loose, gravelly moraine that was permeated by two aquifers, so they borrowed a technique from the mining industry known as ground-freezing. Miners rarely have to contend with the fast-moving water found at the CMS site, though. The workers drilled holes along the periphery of the shafts into which they sank 60-metre pipes.

For six months, they circulated brine at -5 °C through them. Then, for a month, they filled the pipes with liquid nitrogen at -196 °C. This created a 3-metre-thick retaining wall of ice that kept the groundwater at bay, while the workers dug the dry earth within and constructed the shafts.

Meanwhile, the CMS engineers were working on the world's most powerful superconducting magnet - the detector's "pièce de résistance", according to the experiment's spokesman Jim Virdee.

The CMS team decided to outdo ATLAS when it came to the strength of its magnetic field and built one twice as strong. Key to its 10,000-tonne magnet are superconducting coils designed to withstand an outward force of 60 atmospheres generated by the magnet's 4-tesla field - about 100,000 times stronger than Earth's magnetic field. However, the technology was so advanced and varied that no one company, or country, could do the job.

So the magnet's coils were shunted around Europe on a journey that started in Finland and took in Switzerland, France and Italy en route to CERN.

"It took eight years to do that," says Virdee. "The coil was tested in 2006, and it worked perfectly."

 

"The technology was so advanced and varied that no one company, or country, could do the job."

It is crucial that the entire LHC and its detectors work from the word go, as repairing them once the system is up and running will be far from trivial.

"It's like being in outer space," says Evans. "If anything goes wrong, you can't just go in there and touch it."

To repair the LHC, for instance, it would have to be allowed to warm back up to room temperature, which takes about five weeks.

Afterwards, its 40,000 tonnes of magnets would need to be cooled back to 1.9 K, a process that takes another five weeks and requires nearly 10,000 tonnes of liquid nitrogen and 130 tonnes of superfluid helium.

Not surprisingly,

"the quality control has been draconian on this machine", says Evans.

Still, the engineers have done their bit. Now it's the scientists' turn.

"I'm really looking forward to the next few months, and to see the physics coming out," says Evans.

The Large Hadron Collider - find out more about the world's biggest experiment in our cutting-edge special report.