Credit: Mark Evans
Like some medieval cathedral, the Large Hadron Collider (LHC) has already outlasted some of its builders and will continue in daily use long after most are retired or dead.
And just like those vaulted temples of worship, the LHC commands silent awe from its visitors. As it should. The LHC is the most complicated and expensive scientific experiment ever, with the audacious goal of explaining the ultimate nature of matter.
Just to build the apparatus is costing upwards of a$5 billion (US$3.6 billion). Planning started 20 years ago, the first applied research began 15 years ago, and more than a thousand engineers, technicians and scientists have already been working on this project for at least a decade.
Visiting this cathedral to science, however, isn’t that simple. It’s hidden from most eyes, buried 80 to 100 metres deep as an oval tunnel 27 kilometres in circumference beneath the Swiss and French countryside outside Geneva, enclosing an area big enough to squeeze in Bermuda, Monaco and four Vatican Cities.
There are occasional surface outcroppings – anonymous metal-sided sheds set well back from the roads. And there is a large jumble of nondescript off-white buildings that house the designers, builders and eventual users in a campus-like setting near the France-Switzerland border. But mostly there are cultivated fields, cud-chewing cows and placid villages, with the LHC’s tunnel passing below unseen and largely ignored..
Yet in mid-2007, if all goes according to plan, packets of high-energy protons will be accelerated in opposite directions around the 27 km tunnel, attaining velocities within a whisker of the speed of light, and then be deliberately smashed into each other at four separate locations.
Resembling the chapels in a cathedral, those four locations hold detection experiments. Fitting in with the religious analogy, two are multi-part detectors specifically designed to spot the ‘God Particle’: an elusive and theorised atomic fragment so called because physicists hope it will finally make clear how the universe works at the most basic level.
The God Particle’s real name is the Higgs boson, named after British theoretical physicist Peter Higgs who first proposed its existence. It is believed to be the last missing piece to the puzzle of the so-called Standard Model – the 20 fundamental forces and particles that, in various permutations and combinations, account for everything around us – light, magnetism, gravity and all forms of matter.
The Higgs is what supposedly gives mass to fundamental particles, such as quarks and leptons, which in turn constitute neutrons, protons and electrons, which in turn make up atoms and molecules and eventually this page, you and the entire universe. The hadron of the Large Hadron Collider is the classification for neutrons and protons, from the Greek ‘hadros’ for strong, because they are held together in the nucleus of an atom by the strong nuclear force.
Experiments at the collider are also intended to provide the ultimate test of Albert Einstein’s famous formula, E=mc2; to yield a long shot at identifying the mysterious dark matter that supposedly permeates the cosmos; to take a stab at recreating the quark-gluon goo or gas that existed for an instant at the Big Bang; and to roll the dice for a peek into extra dimensions of space-time.
Not to mention the real possibility of confirming supersymmetry. Also known as SUSY, supersymmetry is an arcane concept of paired elementary particles that lies at the heart of the theory that all the forces of nature are interconnected – the ‘theory of everything’ pursued by Albert Einstein for the latter third of his life.
“Simply to be a visitor here is tremendously exciting,” says Stuart Tovey, a pioneer in high-energy physics in Australia.
The 66-year-old Tovey is much more than an ordinary visitor at CERN, the campus-like setting close to the France-Switzerland border. Now formally called the European Organisation for Nuclear Research, CERN is better known by the acronym for its former French name of Centre Européen pour la Recherche Nucléaire and operated collectively by 20 European countries.
Some know of CERN’s existence either because this is where the World Wide Web was born (so researchers could exchange data quickly) or because The Da Vinci Code author Dan Brown fancifully invoked an anti-matter experiment that threatened world peace here in his earlier book, Angels and Demons.
Less well known by the general public is that CERN is currently the top spot in the world for high-energy physics – much to the dismay of many Americans – and home to the Large Hadron Collider.
Tovey’s twice-yearly visits from Melbourne are a kind of homecoming. After earning his PhD from the University of Bristol, he did full-time research at CERN from 1966 to 1973. Tovey then moved to the University of Melbourne in 1974 but came back here on four occasions to spend a year as a CERN scientific associate. Still a principal fellow in Melbourne’s school of physics, Tovey now returns to CERN every April and October; both to indulge his love of all things French, and to participate in key planning meetings concerning ATLAS, one of CERN’s two Higgs boson detectors.
“Australia has been involved with ATLAS since day one,” says Tovey. Currently Australia’s financial involvement amounts to a relatively modest a$2.4 million over eight years, a figure that doesn’t include salaries. There are about 20 Australians among the nearly 1,700 scientists, engineers and technicians from 34 countries working on ATLAS.
What Tovey doesn’t tell – but others here confirm – is that his vision and drive played a large part in Australia signing a co-operation agreement with CERN in 1991 and with inspiring a stream of physics graduate students to come here. The active leadership baton has now passed to Geoff Taylor, current head of Melbourne University’s School of Physics.
“High-energy physics is one of the frontier areas that turns on the very best students at the University of Melbourne,” Tovey says.
An underground visit to ATLAS (an acronym for ‘A Toroidal LHC Apparatus’) illustrates why. The God Particle detector resembles a giant’s Meccano set. In an activity akin to building a ship in a bottle, five storeys of complex electronics and metal parts are being assembled with a clockmaker’s precision in a vast rock-hewn cavern that forms one of the LHC’s four detector chapels.
A borehole soars up nearly 100 metres to the surface from where cranes routinely lower pre-assembled sections weighing as much as 280 tonnes. Technicians tethered to safety harnesses inch their way along metal gantries, connecting up thousands of wires and cables as they go. When complete, the device will weigh some 7,000 tonnes and stretch 46 metres – half again as long as an adult blue whale.
While the God Particle and supersymmetry may be fascinating for physicists, what non-scientists may find more intriguing is how to design and build equipment that will both create a particle that no one can be sure exists, and then to confirm the presence of such fleeting phantoms.
“The technology needed to carry out the experiment didn’t exist when we started work a dozen years ago. We went ahead on the assumption that it would be developed,” says Yves Sirois, an ebullient 46-year-old physics professor at the École Polytechnique in Paris, France’s premier high-technology university. “We didn’t even have some of the basic science.”
But the LHC planners had the motivation. After World War II, the U.S. wrested dominance of particle physics away from Europe, thanks to generous government funding and leading scientists who had fled the Nazis. The Americans built the world’s most powerful accelerators for high-energy particle physics; devices that headline writers back then immediately dubbed ‘atom smashers’.
After decades of being bested in the Nobel physics stakes, the old empire struck back, setting up CERN and building an accelerator 10 times as powerful as anything in the U.S.
In 1983, a brash Italian physicist named Carlo Rubbia used CERN’s new machine to collide protons with antiprotons and thus proved the existence of the W and Z bosons, the elementary particles that filled one of the last gaps in the Standard Model. The very next year Rubbia shared the Nobel Prize in Physics with Simon van der Meer, a colleague at CERN who had discovered how to pack antiprotons – an unruly form of antimatter – densely enough for the experiment to succeed.
In its announcement, the Nobel committee noted that CERN’s super-accelerator was the largest piece of apparatus ever to be connected with a Nobel Prize. The moral behind that message did not go unheeded by high-energy physicists: build big and find fame.
The next Holy Grail was the Higgs boson, which both CERN and U.S. researchers tried to find in recent years with yet another new generation of particle accelerators. They had tantalising glimpses of something that ‘could’ have been the Higgs ... but the accelerators weren’t powerful enough to be sure.
So CERN decided to go for broke. It shut down its main accelerator in order to refit the same underground tunnel and create the Large Hadron Collider, which will be seven times more powerful than the Tevatron at the famed Fermi National Accelerator Laboratory near Chicago, better known as Fermilab and the current top dog in the accelerator stakes. Like all particle accelerators, the LHC must somehow meld brute force and ultrasophistication into a seamless package. The brute force comes from the proton beams, and the sophistication from detector experiments like ATLAS.
Getting this balance right is a vital undertaking for Emmanuel Tsesmelis, 40, who studied with Tovey in Melbourne and came to Switzerland a decade ago. “People think that CERN is this place which has all these Einsteins walking around. That’s not quite right. What we’ve got down in the workshops are lots of top engineers and technicians,” says Tsesmelis.
The numbers bear him out. Of CERN’s roughly 2,600 staff members, nearly 800 are technicians, another 225 are craftsworkers and 950 are engineers or scientists working on the operations side. Only 74 are identified as research physicists. There are, however, nearly 6,500 ‘users’, who flit in and out from 500 universities and institutions around the world, most of them experimental physicists.
Tsesmelis is the head of a small group responsible for supervising the fitting-out of all the experimental chambers for these users, a job he describes as just as much fun as the 10-dimensional physics he once studied. “Reality sets in after you leave university.
You can’t exploit the physics if you haven’t built the machine.”
At the Large Hadron Collider, constructing the machine includes the millimetre-precise positioning of 1,232 superconducting dipole electromagnets that must be chilled to less than two degrees above absolute zero (or around -271°C). Torpedo-sleek and cobalt blue, these are the LHC’s brute force, which generate intensely powerful magnetic fields that compress beams of near-light-speed protons within two tubes with a diameter not much larger than toilet paper rolls.
The energy of these protons beams is measured in electronvolts, where one electronvolt is the energy an electron acquires when moving between the terminals of a one-volt battery. When the high-speed counter-circulating proton beams are deliberately collided at ATLAS and the other LHC Higgs boson detectors, however, they’ll each be carrying a total energy of about 7 trillion electronvolts (7,000,000,000,000) – a large number, which is why physicists prefer to refer to it as 7 TeV (with T standing for tera).
According to Chris Oram, the Canadian physicist who is the ATLAS coordinator, 1 TeV is like having a 1.5-volt AA battery for every star in the Milky Way galaxy.
The point of all this energy is to reproduce the conditions at the instant of the Big Bang, and thus create really massive elementary particles, which are also infinitesimally small and ephemeral in the extreme. Since E=mc2 means that energy and mass are essentially interchangeable, banging two 7 TeV beams together should produce offspring with the energy levels projected for the Higgs boson – which calculations suggest can’t be greater than 1.4 TeV and could be 10 times less.
Detecting the Higgs boson is shaping up as even more challenging than creating it, which is where the ultra-sophistication part of the package comes in. Collisions at these extreme energies rend the original protons, sending off a spray of other particles like muons, which themselves decay almost instantaneously. So even the fastest of the detectors inside ATLAS don’t register the Higgs itself, just the detritus from its decay.
There is a great deal of detritus. The LHC’s two proton beams are engineered to collide 40 million times a second inside ATLAS. The beams are like dense clouds, so many protons simply pass without hitting anything. On average, researchers expect about 23 proton-proton smashes each time the beams collide, which is something approaching a billion collisions per second.
Most of these collisions are boring and predictable. In the end, ATLAS’s layered detectors – four types nesting one within another, rather like those Russian matrioshka dolls – will capture 200 potentially important events a second. Over the course of an entire day of activity, theorists predict the Higgs boson will appear just once: a nanoneedle in a universe of haystacks.
And that’s just from the one detector experiment. There’s the equally complex Compact Muon Solenoid (CMS) under French soil on the opposite side of the LHC oval. It’s also looking for the Higgs and churning out the information equivalent of 10 Encyclopaedia Britannicas every second. As well, two other detector experiments known as ALICE and LHC-b, will respectively yield data about the quark-gluon plasma and B-mesons that may give clues to the nature of antimatter.
The astounding upshot is that when all LHC experiments are running full tilt, CERN will be giving birth to one per cent of all the information generated on the planet in any year. CERN realised it couldn’t afford to install enough computers to handle all this data or sufficient Internet capacity to provide access to all interested researchers. As a solution, it pioneered the next generation of the Net, called The Grid (see ‘A tsunami of data’, p61).
One information stream from the CMS experiment won’t be on The Grid; yet without it, there wouldn’t be any data. That data output comes from a beam condition monitor (BCM), developed by a team directed by Philip Butler, head of the physics and astronomy department at the University of Canterbury. The team’s goal is to use technology from high-energy physics to improve medical imaging. Their short-term aim was to get noticed, so the team shifted its attention from pixel detectors inside CMS to the vital BCM that sits beside the beam line.
“This raises our profile. We wanted to say: ‘Hey, New Zealand is doing something’,” says Richard Gray, a University of Auckland physicist who represents the team at CERN.
The beam monitor’s task is to sound the warning if the stream of high-energy protons starts to drift off course because of problems like a computer foul-up or electromagnet glitch. It’s a crucial warning device: those billions of near-light-speed protons carry
11 gigajoules – enough energy to melt 500 kg of copper. And while the CMS (weighing in at 12,500 tonnes) cannot be reduced to a molten puddle by a wayward beam, even the tiniest bump would fry its delicate detectors.
As well, the BCM has to blow the whistle superfast, so safety magnets can shove the beam aside through an ‘abort gap’ into a big metal block that absorbs the blast. Says Gray: “the proton bundles in the beam are 25 nanoseconds apart, and our monitor has the resolution to distinguish these.”
The New Zealand team pulled off this feat by using diamonds as the heart of their monitor. Not sparkle-on-your-finger gems, Gray points out, but the Chemical Vapour Deposition Diamonds grown in a 1 cm square that’s a mere half millimetre thick. Looking like a patch of cellophane, the diamonds are far more impervious to radiation damage than conventional silicon chips.
If a proton hits one of these diamonds, it triggers an internal electrical charge picked up by surface metal coatings. The charge is then converted to an optical signal that speeds along optical fibre cable to sound the alarm. ATLAS also incorporates a beam monitor of a different design, to avoid experimental bias.
Going on the experience of Fermilab’s Tevatron, there will be many such alarms. Nine months of fine-tuning were needed before the Tevatron beam ran for more than an hour uninterrupted, suggesting it could be well into 2008 before the LHC operates reliably at the initial target, one-tenth of its ultimate power.
Theorists calculate that even that reduced power level will produce some 10,000 Higgs bosons over 12 months. By all that’s right and just in particle physics, that ought to be more than enough to confirm the existence of the God Particle beyond all doubt.
In fact, so confident of success are some experimental leaders that there’s already debate about who gets listed as authors on the paper announcing the discovery. This is a major headache considering that the ATLAS and CMS teams are expected to have swollen to about 2,000 researchers each by next year.
A greater worry rumbles just below the surface: what if the Higgs is the LHC’s only discovery of any consequence? What if that’s all there is to show for the enormous expense? Especially when you consider that CERN was forced in 2005 to shut down most of its other experiments to compensate for cost overruns and missed deadlines of the LHC.
In particular, many physicists would be disappointed if LHC does not find evidence for supersymmetry. “Supersymmetry must exist if the universe is going to make sense,” insists the École Polytechnique’s Yves Sirois. “If there is nothing else new, we would have a party for detecting the Higgs. But the next day we would have a real headache,” he says.
And what if no Higgs particle is found – what then? The Higgs boson is such a crucial part of theoretical physics that if it were not found, many theoretical physicists would be lost. For years, countless papers have been written assuming the Higgs exists. But just because physics needs a Higgs, that does not necessarily mean it exists. In many ways, discovering that the Higgs does not exist would be a more interesting development.
In high-energy physics, however, such headaches have always proved temporary. The theorists find explanations as to why earlier theories didn’t work, and the experimentalists lobby for more powerful accelerators with more sophisticated detectors.
The God Particle could well turn out not to be the end of this quest to understand the ultimate nature of matter but instead the beginning of yet another phase.
Peter Calamai is a science writer for Canada’s Toronto Star.
