Αρχική > επιστήμη > Scientists at Cern’s Large Hadron Collider near end of the search for the Higgs boson

Scientists at Cern’s Large Hadron Collider near end of the search for the Higgs boson

The £5bn particle accelerator has operated for a year, generating billions of pieces of data in the hunt for the Higgs boson, nature’s building block. But it hasn’t been found and scientists are running out of places to look

Cern Large Hadron Collider

About 10,000 scientists work at Cern’s laboratories near Geneva, under which lies the Large Hadron Collider in a 17-mile circle. Photograph: Julia Gavin/Alamy

For almost 20 years, Bill Murray has been hunting the Higgs boson, the elusive subatomic particle that is thought to give mass to the basic building blocks of nature. In those two decades, the 45-year-old Edinburgh-born researcher has watched the search for the holy grail of physics narrow to a tighter and tighter group of targets – a process of elimination that has peaked over the past 12 months with the start-up of the Large Hadron Collider at Cern, the European particle physics laboratory. An avalanche of nuclear collisions – created by beams of high-energy protons smashing together – have been generated. But no trace of the Higgs has been found in the resulting nuclear debris.

Only a very narrow range of Higgs targets are now left – and some scientists are beginning to get twitchy, including Murray. "In 1993, I got a job by telling people that I wanted to find the Higgs," he says. "It is only in the last month that I have started to think that it might not exist after all."

Murray is not alone. "In the last year, we have eliminated most energy ranges that could contain the Higgs," says Sergio Bertolucci, Cern‘s director of research. "It is like pumping water out of a pond. We have virtually emptied the pond and have only a couple of muddy puddles left in which to find the Higgs. If it is not there, we will have to admit it does not exist."

Physics has clearly reached a dramatic juncture. In the next few months, the Higgs boson – the last major particle in the standard model, the current theory of subatomic physics, that has still to be observed – will either be found or its existence disproved. Theoreticians will be shown to be right and will earn Nobel prizes. Or their work will have been demonstrated as being wrong and accepted science will be overturned, forcing scientists to devise some new and exciting theory to explain why objects – and humans – have mass.

Not surprisingly, these prospects are having a profound effect on the 10,000 scientists who work at Cern, a jumble of office blocks, laboratories and meeting halls outside Geneva. Data is pouring from the collider beneath their feet. But which group will be first to make sense of that information is unclear. Hence the buzz around the centre, an effect that is particularly noticeable in its large, ramshackle, neon-lit cafeteria which, day and night, is filled with shifts of technicians, researchers, and theoreticians. Perched on plywood seats at long Formica tables, earnest young researchers swig beer and argue over the minutiae of particle physics. Some fiddle with their laptops, others scribble on notepads. Many gaze at the cafeteria’s huge plasma screens that display details of the behaviour of the collider’s particle beams on which their livelihoods depend. "The tensions that are now being created inside the experiments are now very serious," admits Murray, who acts as Higgs convenor for the Atlas experiment group. "I haven’t seen people coming to blows but there are certainly plenty of intense verbal discussions going on."

The Higgs particle was postulated by a group of physicists – who included Peter Higgs of Edinburgh University – in 1964 to explain how other subatomic particles have mass. The theory hypothesises a sort of lattice, referred to as the Higgs field, that fills the universe. A particle moving through this field creates distortion, in the form of a boson, and that lends mass to the particle.

The idea was accepted, grudgingly, by scientists – despite a lack of evidence to support it. The problem was the absence of any means to create energetic enough collisions to create a heavy particle like the Higgs. Finding the Higgs was one reason – but certainly not the only one – for spending around £5bn to build the Large Hadron Collider, an instrument built on the scale of London’s Circle line but constructed to an accuracy of a billionth of a metre.

It is a staggeringly ambitious, superbly engineered device and, although delayed by a series of technological hiccups, it has now been running perfectly for the past year, generating five times more data than expected. When its beams of protons smash together, billions of collisions are produced every second – spraying particle debris through dense layers of detectors. "It is the job of our computers to retain details of the interesting collisions – and only the interesting ones," says Pierluigi Campana, spokesman for the LHCb, one of the collider’s main experiments. "It is a very difficult balance to achieve. You don’t want to throw away promising results but you don’t want to be swamped with useless ones. Getting that balance right, in programming our computers carefully, has been one of the great intellectual achievements of this project."

Details of about 3,000 events a second are retained. It may only be a small fraction of the device’s output but it represents an immense repository of information. If stored on DVDs, Cern’s total output of data would have created a stash taller than Mont Blanc. And somewhere in this great inventory of digital signals – which are distributed weekly to computing centres round the world – lies the secret of the Higgs boson.

"Every week, we collect data about 75 million potentially interesting collisions," says Guido Tonelli, spokesman for CMS, another of the collider’s main experiments. "First we validate that data – in other words, we check its quality. Then we release it to physicists so that they can try to spot signs of a Higgs or another interesting particle. Occasionally, they find an interesting anomaly, so we call all the groups together – and we try to kill it. In other words, we try to show that any anomalous result is a product of a software error or a glitch. Only when such a result survives this scrutiny do we accept that we are on to something. To date, no sign of a Higgs boson has survived that scrutiny."

The Higgs hunt therefore rests on the business of disproof. Thousands of scientists, working with one of the world’s most sophisticated devices, have laboured like this for the past year.

"Matter and energy are interchangeable and we measure subatomic particles in terms of their energy," says Fabiola Gianotti, head of Cern’s Atlas experiment. "We have now shown the Higgs particle cannot be lighter than 114 billion electron volts [GeV] or heavier than 145 GeV. All that is left is the range between these two energies. It might seem tight but the standard model favours a Higgs boson that exists in this region. The trouble is that this range is also filled with particles produced by many other reactions. That makes it very difficult to distinguish the sign of a Higgs from the signals produced by the other particles. Once we study more and more observations, the subtle behaviour of the Higgs will eventually be revealed, I believe. So no, I am not the slightest bit worried because we have not yet found the Higgs. We will succeed – and soon."

In general, most Cern scientists agree. The Higgs may be hiding but it appears to be concealing itself in an expected place. Certainly, Murray remains confident. "My wife has put a bottle of champagne aside for the day we find the Higgs. I still believe that by this time next year I will have opened that bottle."


Two sets of scientists at Cern are competing to find the Higgs. One is involved with the Atlas experiment, the other runs the CMS project. Both labs are based in underground halls along a 17-mile circular tunnel built below the French-Swiss border. This is the Large Hadron Collider. Subatomic particles called protons are accelerated in both directions, and at the Atlas and CMS sites these beams cross over and smash into each other. The resulting explosive interactions generate new types of particles, including – possibly – the Higgs. These pop fleetingly into existence before disintegrating.

The crucial point is that the greater the energy generated by a collider, the bigger are the particles it creates. Matter and energy are interchangeable. And most predictions suggest the Higgs is relatively big, hence scientists’ past failure to produce them. At present, the LHC’s beams are running at half power. In 2012, the machine will run at full power and produce collisions that should answer a swath of questions beyond proving the existence of the Higgs.

For example, the particles which make up stars and planets account for only a tiny fraction of the universe’s mass. Something else is out there, an invisible form of matter – called "dark matter"; subatomic entities called supersymmetric particles are a favourite candidate to account for this. The LHC should be able to produce these, if they exist.

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