The Large Hadron Collider (LHC) in Geneva is the arguably most famous experiment on Earth. It’s also, by many measures, the largest; a particle collider 27km in circumference and 100m underground, it took 16 years to build and cost £6bn.
The LHC was built to answer some of the burning questions facing particle physicists. Within the massive tunnels, protons travelling at near-light-speed collide with each other, and the physicists examine the resulting debris to try to work out what is going on. Hundreds of researchers scour the data for evidence of the Higgs boson, embraced by the media as “the God particle”. The Higgs, if it is found, would explain why everything around us has mass.
Professor Jon Butterworth, head of Physics at UCL, is, more excited by the prospect of the LHC failing to find the Higgs. The Higgs is at the centre of modern physics, and if the Higgs doesn’t exist, he says “there is no kind of neat little theory waiting to slot into place”. Instead, physics could go almost anywhere. “There’s about a thousand flowers blooming” in theorists minds, said Butterworth.
Constant updates from the LHC on how the hunt for the Higgs is progressing fill the middle pages of newspapers, but we rarely consider the nuts and bolts of the LHC. How do you even begin to build such a vast machine?
The man who was charged with making the LHC a physical reality is Dr Lyn Evans, a Welsh physicist who has worked at the LHC’s home, the European Centre for Nuclear Research (Cern), for around 40 years. He is a typical academic; thoughtful, forthright and slightly scruffy.
Funnily enough, Evans – the man at the helm of the foremost physics experiment of our time – didn’t start out as a physicist. He switched from chemistry at university because “physics was more interesting, and easier.”
Forty years on, he stands by that view, and he is now highly respected in academic circles. In 2010 he received the highest honour in British science when he was elected a fellow of the Royal Society. The august institution nominates only a handful of fellows a year. Evans was honoured not only for his work on the LHC, but also on a previous particle accelerator at Cern, the Super Proton Synchotron (SPS). He enabled the SPS to run at ten times the energy it was originally designed for, paving the way for the discovery of some of the particles that make up the Standard Model of physics.
Evans doesn’t regard building the LHC as simply a physics or an engineering challenge. Digging a 27km circle under the Swiss Alps then filling it with high-tech equipment threw up a myriad of surprises. “Building the LHC… you have to be able to handle anything,” says Evans. After 16 years of work, Evans can claim success. He has built a machine that works; now it is up to physicists to do their experiments. Results are flowing in from the collider and detectors, and the building works were only marginally over budget – by less than 25 per cent, which Evans describes as “perfectly acceptable” for a high-tech project. But even though he has fulfilled his brief without major incident, Evans reckons that “If I knew [16 years ago] what I know now I would never have started.”
Evans was in charge of constructing the main tunnel of the LHC, through which protons whizz at 99.9999 per cent of the speed of light. Two beams of protons are fired in opposite directions,and then travel round and round the tunnel for a whole day. In caverns at various points along the tunnel are four giant detectors, where protons collide and measurements take place. Two of these detectors, CMS and Atlas, are charged with finding the Higgs boson. CMS and Atlas were built independently, so that each provides a check on the results of the other. The detectors themselves were designed and constructed by teams of scientists in universities around the world, and then were assembled by Evans’ team in caverns 100m below the ground.
This was no mean feat. One detector in particular, CMS, presented some rather unusual challenges. “When we were preparing that site we came across something that you never want when you’re starting a civil engineering project… Roman ruins,” says Evans. Work stopped for six months while the archaeologists excavated a 4th Century AD Roman villa. While less dramatic than the search for the Higgs, Evans enjoys the nuggets of history the archaeological dig revealed. “The villa is perfectly aligned with the fields today… the land registry in this region was laid out by the Romans and remains to this day.” Also uncovered were coins from Ostia (in Italy), Lyons and London. Even in the 4th Century AD, Cern was an international hub.
But the trouble didn’t end when the dig finished. In a move some might describe as foolhardy, the CMS detector is sited below an underground river. Evans’ engineers knew the river’s course, but digging a cavern through flowing water required some lateral thinking. Rather than going around the river, they decided to simply stop it. “It was much more difficult than we had anticipated,” says Evans. Engineers sunk pipes down to 50m below the surface which were then pumped full of liquid nitrogen at -77oC. The liquid nitrogen “[froze the water in] the ground, making it all ice down to 50m”. Then diggers removed the frozen earth, and CMS was lowered in. “That was quite exciting,” says Evans, with exemplary British reserve.
Freezing a river was certainly a creative way to lower CMS into position, but Evans doesn’t regard it as the biggest technical challenge he faced. The protons in the LHC originate in an old linear collider, and would travel in a straight line were it not for intense magnetic fields from supermagnets in the tunnels. The supermagnets control the direction of the proton’s flight path. Like a Ferrari travelling at 300mph, protons moving at only a few kph less than the speed of light are difficult to bend round corners. Supermagnets have been used for years in MRI scanners, but none of these could produce the high fields needed in the LHC. “When we started we didn’t have a single superconducting magnet that worked,” Evans recalls. “[Not] even a small model, a foot long.”
Sixteen years on, and the LHC uses 2000 supermagnets, each 14 metres long. Supermagnets only work at very low temperatures, and work better the colder they are, so each is filled with liquid helium at -271oC .
Liquid helium is a tricky substance to handle – as Evans puts it, “ nobody in their right mind would deal with that”. A bizarre quantum mechanical effect, superfluidity, means that all the liquid in a pipe can leak from just a tiny crack. A leak underground is therefore a nightmare scenario. There is only one entrance to the 27km tunnel wide enough to lift a magnet out, and manoeuvring them in the dark tunnel is a slow process. Each magnet had to be tested at the operating temperature of -271oC before being lowered into the tunnel. Cooling and re-warming each magnet takes over a week, so magnets were built and tested for 24 hours a day, seven days a week, for 50 weeks of the year. So far thanks to careful welding of joints, only relatively minor setbacks have occurred.
Most notorious among setbacks is the electrical connection that fizzled out shortly after the LHC switch-on in 2008. The tunnel contains around 10,000 electrical connections, each with a one in 10,000 chance of failing. The statistics were uncannily precise – shortly after being fired up for the first time, one of the connections failed, shutting down the LHC for several months.
At least one setback in this ambitious project was probably inevitable. The collider was designed not only to find the Higgs boson, but probe questions around where we came from and why we are here – how did the big bang lead to the Universe? Why is there matter? What are we made of? Physicists have been asking these questions for years. But when construction started, the technology didn’t exist to make the LHC a reality. Evans had to develop the tools he needed. The technology is therefore new, and built-to-purpose.
While the main tunnel was all under Evans’s control the detectors were built in bits by over 150 institutions. “When you’re in the LHC and you don’t like something, you can do something about it, but if an institute in Siberia is not producing the goods, you’ve got no control at all over them, ” said Evans. Despite this fragmented approach, the detectors worked. They slotted into place perfectly.
Industrialists and business people tend to be surprised that this behemoth of engineering has succeeded with academics at the helm. Professor Butterworth recalls showing an industrialist around the nearly-complete Atlas detector. “When he walked in the room and he didn’t say anything for about 15 seconds – which was the quietest he’d been all day.”
Remarkable scientific and technological achievements have been achieved with limited money. According to Evans, the budget of Cern has been constant in real terms for the past forty years. The same philosophy might not work in industry, driven by profit. “The only reason it works is the will of the people, that they really want to do the science from this.”
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