Mass hysteria

New Scientist vol 183 issue 2456 - 17 July 2004, page 38

 

Thousands of people are on the trail of the Higgs particle, which is believed to give things their mass. Are they on a hiding to nothing? Alexander Hellemans and Valerie Jamieson follow the hunt

 

FOR as long as Christopher Tully can remember, the dream has been there: understanding what gives everything its mass. Sure, the dial on your bathroom scales reflects how many atoms you contain. But when you go deep inside atoms, where mass comes from is a mystery. Why should anything weigh anything at all? If you can answer this long-standing puzzle, you open a window onto an ancient era when all the forces in the universe were bound into one, an era physicists are desperate to understand. Certainly the origin of mass is a prize worth having.

All it takes is some way to smash subatomic particles into each other at high enough energies for them to create a particle called the Higgs. This is the particle thought to give stuff its mass. Yet for almost 20 years, nature has played a game of peek-a-boo with particle physicists like Tully, fooling them into thinking they were nearing their goal. Each new particle accelerator has been built with high hopes that the Higgs will be found there, only for those dreams to be shattered.

Last month a team of researchers working on the DZero experiment at the Fermi National Accelerator Laboratory in Batavia, Illinois, indicated that the Higgs has given physicists the slip again. It is a situation that Tully, who works at Princeton University, found himself in four years ago when he and his colleagues thought they were on the verge of discovering the particle. Now hopes for finding it at the world's most powerful accelerator, the Tevatron at Fermilab, are fading and all eyes have turned to an even mightier machine, the Large Hadron Collider (LHC), being built at the CERN laboratory in Geneva, Switzerland. But some physicists are starting to wonder if even the LHC will catch this will-o'-the-wisp particle.

The Higgs is the only missing ingredient in the standard model of particle physics, a theory developed over the past three decades, and one that incorporates our best knowledge of the building blocks of matter and the forces that act among them. The theory says that all matter is made up of six types of quark - known as up, down, strange, charm, bottom and top - and other particles such as electrons and neutrinos.

According to the standard model, what we call mass is really an indication of how strongly particles interact with an invisible syrupy substance called the Higgs field. "It is really why things are the way they are," says Chris Quigg, a theoretical physicist at Fermilab.

Proposed in 1966 by theorist Peter Higgs of the University of Edinburgh, UK, and others, the Higgs field is similar to the fields that carry the forces of electromagnetism and gravity. Like these fields, the Higgs field fills the universe and seeps through every pore of empty space. Yet this field stands out because it discriminates between the particles that pass through it. The reason photons have no mass and zip through space at light speed is because the Higgs field ignores them. Meanwhile, the heaviest known particle, the top quark, has a mass as high as a gold nucleus because the Higgs field smothers it, hindering its passage through space.

Quantum mechanics says that the mass-giving field can also be thought of as a sea of electrically neutral Higgs particles that should be dislodged in collisions between subatomic particles with high enough energies. The big question is just how high should those energies be?

It isn't an easy question to answer. Though the standard model predicts the Higgs particle's existence, the theory alone cannot tell physicists what its mass should be. And without knowing its mass, researchers do not know where to look for it.

To try to make headway, physicists crash together particles accelerated to the speed of light. The energy released in the collisions translates into the mass of new particles. So if you want to make a colossal particle, you need to put more energy in.

In 1979 theorists thought that the Higgs might weigh as little as 10 times the mass of the proton - about 10 gigaelectronvolts (GeV).

Soon afterwards experiments looked for the Higgs and found nothing. Since then, improvements to the standard model have given us better clues about the Higgs's hiding place. Around 20 of the ingredients in the standard model, such as the masses of particles including the Higgs, cannot be predicted from scratch. Instead they have to be measured in experiments. Many theorists baulk at this ugly feature of the theory. The good news is that most of these ingredients, once measured, are linked by equations which give the theory some predictive powers. So to forecast the value of one ingredient, you need to know what other ingredients it depends on and then measure them as accurately as possible. The standard model says that the Higgs is most sensitive to the mass of the top quark and another heavyweight particle called the W, which mediates the weak nuclear force.

Such a strategy relies on precision measurements made by thousands of physicists working on a variety of experiments all over the world, most importantly at the Large Electron Positron (LEP) collider at CERN and at Fermilab's Tevatron. Since 1995 these experiments have collected sightings of the W particle and top quark and accurately measured their masses. By feeding these details into the standard model equations, researchers figured the most likely value for the Higgs mass hovered around 95 GeV, and that it should certainly be less that 220 GeV.

With a clearer idea of the hunting ground, recent experiments at LEP and the Tevatron looked for signs of the Higgs among the debris produced in particle collisions. Particle physicists expect the Higgs to exist only for a fleeting moment before decaying into other particles, which are caught in the detectors. By piecing together evidence from particle tracks and the energy they dump in their detectors the physicists hope to reconstruct the Higgs's final moments and pin down its mass. What makes the Higgs so slippery is that it is expected to decay in different ways, depending on its mass. So physicists are not even certain what telltale signs to look for.

So far, direct searches for the Higgs have yielded nothing definite. The closest physicists have come to spotting it was in September 2000 when teams at LEP's four experiments reported finding a handful of particles that stood out against the background of other particles. Tantalisingly, they had a mass around 115 GeV, close to the prediction from the standard model. Although there were too few Higgs candidates to claim an outright discovery, particle physicists were ecstatic.

But the euphoria was short lived. Within weeks the evidence started to crumble, and in the month before the LEP accelerator closed down in November 2000 to make way for the LHC, there were no further sightings of the wannabe Higgs particles. The frustrated LEP teams could claim only that should the Higgs exist, it must be heavier than 114 GeV. "We were so close to finding it," says Tully who helped mastermind the search at LEP. Most LEP physicists believed the Higgs had given them the slip. Reluctantly, they passed the baton to researchers at the Tevatron, almost certain that the collider's two experiments, CDF and DZero, would bag the ultimate prize.

But the Higgs particle isn't yielding its secrets so easily. The Tevatron smashes together protons and antiprotons with a collision energy close to 2000 GeV, which should be enough to produce Higgs particles. The trick to finding them lies in how well you can spot any rare appearances that they make against the background of ordinary particles that mimic its vital signs. To boost their chances of finding the Higgs, CDF and DZero have to measure as many collisions as possible. "In principle we can detect a Higgs between 100 GeV and 200 GeV," says John Conway of Rutgers University in New Jersey and member of the CDF collaboration, "but it is all a matter of how much data we get."

And that's the trouble. For the past three years, the Tevatron has been dogged by technical problems that have limited the amount of particles packed into its beams. As a result, the collision rate has fallen far short of what is needed for a speedy discovery. "Repairs and upgrades have brought us back on track," says Stephen Mrenna of Fermilab.

But with only five years left to collect the readings, many physicists think they have already lost too much time. "It is very unlikely that we will have adequate sensitivity to make any statement about the Higgs," says Stephen Reucroft, a physicist based at Northeastern University in Boston, who works on CDF.

Last month, that view was reinforced by a new result that pushes the Higgs further out of the Tevatron's reach. The prediction comes from the DZero experiment, which collects sightings of the top quark and measures its mass. Instead of taking a simple average of the different top-quark masses, the experiment gives more emphasis to top quarks with a well-measured mass.

The new analysis slashes the uncertainty on the measurement and nudges up the top quark's mass (Nature,vol 429, p 638). Feeding this new value into the standard model alters the expected mass of the Higgs, pushing the most likely estimate up from 96 GeV to 117 GeV. "The fact that the Higgs mass shifted up as far as it did was a surprise to me," Tully admits.

Crucially, it pushes the upper limit of the Higgs mass from 219 GeV and to 251 GeV, dealing a blow to the Tevatron's chances of finding it before the CDF and DZero detectors switch off in 2009. "It's probably beyond the sensitivity of current experiments," says Ron Madras at the Lawrence Berkeley National Laboratory in California.

Now physicists are pinning their hopes on the $2.6 billion LHC which is due to start up in 2007. Currently under construction, this behemoth will smash protons together to produce 14,000 GeV of collision energy.

So switch on LHC and watch the Higgs particles roll out, then? Well, not so fast. The LHC and its gigantic detectors ATLAS and CMS are on a scale unlike anything ever attempted before. Both instruments are the size of an office building and contain myriad detectors arranged in layers like a Russian doll. Even if everything goes according to plan and LHC starts colliding protons in 2007, accelerator physicists will need time to optimise the machine. That means LHC's beams will start off with far fewer protons than the design calls for. As accelerator physicists get to grips with the machine, they will gradually squeeze more and more protons into the beams. If experience at previous proton accelerators is anything to go by, it could take months, if not years, to reach the design values. Then the physicists working on the ATLAS and CMS detectors will need months to interpret the signals coming out of their apparatus. "It might take two years of well-studied data before we can say anything about the Higgs," Tully says.

Even with lots of perfect measurements, finding the shadowy Higgs will prove tricky, as physicists working on the ATLAS detector discovered last year. To test their skill at finding unknown particles, Ian Hinchliffe from the Berkeley lab ran a computer simulation of a week's worth of proton collisions in the ATLAS detector. Among the 6 million collisions generated, Hinchliffe buried several new particles and made the data available to the collaboration to sift through. "It wasn't easy," says Peter Jenni, spokesman for the ATLAS collaboration, "but people did find new particles." What they failed to find, however, was the Higgs particle Hinchliffe had hidden in the data.

But what if particle physicists are completely wrong about the Higgs? "The Higgs mechanism is such a nice idea that most people are sold on it," says Michael Albrow at Fermilab. "But maybe something else is there." In May, theorist Alan White at Argonne National Lab in Illinois suggested that a new type of quark, rather than the Higgs, might be responsible for giving particles their mass. Though the idea is speculative, if White is right the new "sextet quarks" should show up at the LHC.

Some argue that the failure of experiments to pin down the Higgs particle is a sign that it does not exist. "Even if the Higgs mechanism is the way that nature gives particles their masses, there could still be no Higgs particle to detect," Reucroft says.

CERN theorist Alvaro de Rujula believes the best outcome would be for experiments not to find the Higgs, forcing physicists to revise some details of the standard model. "Then we would learn something revolutionary," he says.

But we don't need a null result to tell us the standard model needs work. Even though it has been widely fêted, the standard model has serious shortcomings as a theory. Because it ignores gravity, it cannot explain what happened shortly after the big bang when the four forces of nature were thought to be rolled into one. And although it does a good job of linking the ingredients of particle physics together, it does not explain where these ingredients come from in the first place or why they are the way they are. "We know the standard model isn't the end of the story," Albrow says.

Physicists think the answers could come from a theory called supersymmetry, which predicts that every particle we know has a heavy doppelgänger just waiting to be discovered in high-energy collisions at LHC. Yet the theory offers a crucial lifeline to researchers at the Tevatron, one that could see them pull off the most important discovery yet in particle physics. Instead of one Higgs particle, supersymmetry predicts five, all with different masses. The theory says the lightest Higgs is likely to weigh in at around 115 GeV and is certainly lighter than 140 GeV. That means there is a good chance that the supersymmetric Higgs is lighter than the standard model version and places it well within the Tevatron's grasp. What's more, supersymmetry says that its lightest Higgs is most likely to decay into bottom quarks, which the CDF and DZero teams are adept at spotting. So instead of finding the last ingredient of the standard model, they might unlock the door to the theory that replaces it.

Either way, it seems we will have to wait a bit longer before our dream of understanding mass is fulfilled. But Quigg is in no doubt that it will happen: "I bet my professorship that the Higgs particle, or whatever stands in its place, will be found."


Alexander Hellemans

Valerie Jamieson
Alexander Hellemans is a writer in Naples, Italy
 
 

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