For 50 years, physicists have been hunting for the missing piece in the puzzle of the universe. At last, says Graham Farmelo, a sighting may not be too far away .
How detecting the Higgs boson in the LHC would look. Finding the elusive last piece of the puzzle may not be far away, says Graham Farmelo.
When will it show its face? Since the existence of the Higgs particle was first predicted almost half a century ago, thousands of physicists have spent many millions of pounds in an attempt to pin it down, as yet to no avail. Last week, the web was humming with rumours that experimenters at Fermilab, near Chicago, had observed the particle using their Tevatron atom-smasher. But the lab’s authorities moved quickly to quash the gossip, using its Twitter feed to dismiss the “rumours spread by one fame-seeking blogger”. If nature really has chosen to involve the Higgs in its grand scheme, it is doing an excellent job of keeping it secret.
At first glance, it seems odd that an obscure subatomic particle has attracted so much attention. It’s not just that it would be much too small for any human being to see – theorists predict that it will weigh billions of times less than a typical dust particle, and will have only the briefest of lives. After each one is born, death should follow about a hundred trillionths of a trillionth of a second later as it falls apart to produce other particles.
Yet physicists care deeply about the Higgs, because its putative existence follows from an elegant theoretical idea that helps explain why almost all of the most basic particles have mass. The Higgs theory, named after its co-author – a distinguished, now-retired theoretician at the University of Edinburgh – does a lot to explain why you and every material thing around you are not as insubstantial as light.
At a deeper level, what makes the Higgs particle so important is that it represents the one unconfirmed part of perhaps the greatest triumph of modern science – the theory of the fundamental particles and the main forces between them. Known by the stupefyingly dull name of the Standard Model, it is based on quantum theory and relativity, and gives an excellent account of virtually all subatomic experiments ever performed. If the existence of the Higgs particle (or quite possibly several of them) could be demonstrated, theoreticians would be much more confident that this part of the Model is correct, and not a convenient fiction.
These experts have used their equations to make plenty of specific predictions about how the Higgs should behave. The problem is that they have not been able to say precisely how heavy it is, making life difficult for the teams of experimenters searching for it. By the year 2000, it was possible to specify only that it was pretty heavy by subatomic standards, weighing at least 120 times as much as a proton.
The only machines with sufficient power to generate a particle this heavy are the Tevatron and the Large Hadron Collider (LHC) in Switzerland. As a result, their host laboratories – Fermilab and Cern – are the only places in the entire world that stand a chance of claiming the prize of being the first to see the Higgs. In public, the two labs say that it doesn’t matter who wins, but in private the competition is becoming more intense by the month.
If it is the LHC that finally locates it, it would be a triumph for the largest and most complex machine ever built. Sometimes called “the fastest racetrack on Earth”, its aim is to look in unprecedented detail at the subatomic world by smashing together protons that have been accelerated to within a whisker of the speed of light. About 600 million collisions take place every second inside several gigantic detectors, each of them a masterpiece of precision engineering. It is then up to scientists, aided by their global network of computers, to examine the shower of debris from each collision and shed light on the innermost workings of the universe.
Technology like this does not come cheap. The total cost of setting up the LHC was about £6 billion, with Britain paying the annual equivalent of a fancy cappuccino for every adult in the UK. For this, we get to be one of the leading players in this multinational project, and to benefit from its spin-offs.
After serious helium leaks forced it to be switched off only days after the extraordinary festival of publicity that heralded its completion in September 2008, the LHC is now in exquisite shape. Meanwhile, the Tevatron in Chicago was collecting data and is currently in the best position to catch first sight of the Higgs.
“I personally am very confident that if the Higgs particle is there, it will turn out to be around 140 times as heavy as the proton,” says Stefan Söldner-Rembold of the University of Manchester, a physicist working at the Tevatron. That would put it just about within range for the Tevatron to produce and observe.
At the moment, the LHC is running at half its projected maximum energy, with two of its colossal particle detectors focused on hunting for the Higgs. However, it is highly unlikely that the experimenters will be able to pin the particle down before the machine begins to run at full power, from around 2013. “It could well be that the Tevatron will see the first hints of the Higgs particle,” says British theoretician John Ellis at Cern, “but it will take the LHC to pin down it down with complete confidence.” So perhaps the prize for getting the Higgs to show its face clearly will be shared.
If experimenters can definitively prove the Higgs’s existence, the discovery will be a triumph for the whole enterprise of theoretical physics, and its effort, in Stephen Hawking’s enticing phrase, to “know the mind of God”. But what if it doesn’t exist? Even though the LHC was designed to investigate many other things, including antimatter, and could eventually discover the existence of dozens of new particles, to find out that its creators have been on a decades-long wild goose chase might be regarded as something of an embarrassment.
Certainly, the public will be surprised after all the media hoopla about the Higgs, and its allegedly divine importance (the Nobel-winning experimenter Leon Lederman once called it “the God particle”, but has never successfully explained why, beyond the need for a catchy title to sell a book).
Physicists are nothing if not inventive, and some already have explanations up their sleeves if the Higgs does not exist – some of the hottest new theories involve the existence of additional dimensions, beyond the usual ones of space and time.
But most believe in their bones that the Higgs is about to shed its shyness and show us its face. The leading theoretician Nima Arkani-Hamed, at the Institute for Advanced Study in Princeton, is so convinced that he has bet a year’s salary on the Higgs being found, either at Fermilab or the Large Hadron Collider.
My own guess is that his money is safe, and that we’ll see the Higgs within the next five years as part of another, long-awaited golden age in particle physics.
No comments:
Post a Comment