How to catch a Higgs
>> Researchers are setting a trap for an as-yet-unseen elementary particle, a piece of subatomic matter that may lead to a whole new realm of physics.

It is as if a heavy gust of wind whipped through the world of elementary particle physics this past year, swirling around the Swiss-French border, blowing newspaper pages from Glasgow to Chicago, and lifting eyebrows to precarious heights before releasing them to drift earthward again. The source of the gale was none other than the Higgs boson, a piece of subatomic matter that a group of Chicago physicists and several thousand other scientists around the globe have been chasing for their entire careers.

The researchers believe the Higgs may have revealed itself at least once in the Large Electron-Positron (LEP) collider at CERN in Geneva last summer, and perhaps as many as four times. The glimpses-really no more than shadows scuttling along the wall of LEP's cavernous detector, if they were indeed even that-came just before the accelerator was scheduled to shut down, clearing construction space for another, much more powerful accelerator opening in five years. After heated international debate, copious data checking, and a two-month extension of LEP's run time, the $6 billion construction plan for the next particle collider could be put off no longer. As the lights went out at LEP in December, CERN's 3,000 physicists were left with a single, clean, yet perplexing image and the sense that something wonderful had whirled by.

Mark Oreglia, associate professor in physics, the Fermi Institute, and the College and a veteran Higgs hunter, was part of an eleventh-hour working group assembled at CERN to examine the odd data signals and attempt to determine whether they were hints of a Higgs. The working group resolved the question as a parent might curb the expectations of an overly excited child: yes, it is very exciting, but we'll have to wait and see.

The fundamental ideas of science, Albert Einstein insisted, are essentially simple and, therefore, comprehensible to everyone.

Quite simply, the Higgs boson is the subatomic particle that gives other particles their mass. Named for Peter Higgs, the retired Scottish theorist who proposed its existence in the early 1960s, the Higgs is a hot object of pursuit for experimental physicists because, after so much waiting, finally seeing it in the lab would be the first step toward understanding its nature and properties.

What does it mean for a particle to "give" another particle its mass? When the Higgs hubbub began late last year, many well-meaning journalists explained mass as the weight of something, and the Higgs as the mechanism that gives things weight. But mass is not the reason why a hunk of steel weighs a ton and a gooseberry weighs a few grams. That's a function of gravity. Rather, mass is why a thing has inertia, causing it naturally to resist a change of motion, which is why a gooseberry and a hunk of steel both stay right where they are until some intervening force (late-season rain, a forklift) moves them.

A Higgs boson is the particle that carries the inertia. The more tightly it gloms on to other particles, the less likely they are to be swayed by intervening forces. Melvyn J. Shochet, the Elaine M. & Samuel D. Kersten Jr. professor in physics, the Fermi Institute, and the College, describes the phenomenon created by the Higgs as, in essence, sticky muck: "It's like soldiers walking through mud. Interaction with the mud slows them down and makes it much harder to walk." In fact, particle physicists believe all of Nature is covered in a blanket of mud, and they would like very much to scoop up a glob from this Higgs field to study how, exactly, it gloms onto other particles.

Luckily, they won't have to wait another five years to do it. Just as LEP was shutting down in December, another particle accelerator 40 miles outside Chicago was preparing to fire up in March to resume the hunt for the Higgs. The Tevatron accelerator, located at the Fermi National Accelerator Laboratory in Batavia, Illinois, has undergone extensive equipment upgrades, increasing the intensity of its colliding beams and the machine's ability to tag data like that seen at LEP. Shochet and Henry J. Frisch, professor in physics, the Fermi Institute, and the College, are among the 2,000 scientists conducting experiments at Fermilab.

The Higgs, if it exists, will be the latest in a series of subatomic particles discovered during the last century. The fermions, named for the Chicago Nobel physicist, are the building blocks of atoms. They include three pairs of quarks (top and bottom, up and down, and the fancifully named charmed and strange) and three pairs of leptons (the electron, muon, and tau and their corresponding neutrinos).

The bosons, on the other hand, carry the forces that interact with the fermions, causing them to stick together, have inertia, and disintegrate under radioactive conditions. Four of the five predicted bosons have already been seen in the lab. The photon, or the particle of light, carries the electromagnetic force, which binds atoms together to create molecules and, eventually, berries and steel. The gluon carries the strong force, which holds atomic nuclei together by binding together quarks to form protons and neutrons. The Z and W bosons carry the weak force, which governs radioactive decay, the demise of all unstable particles. Last but not least, the Higgs boson carries the sticky muck that gives the fermions inertia, or mass.

Together, the roster of fermions and bosons and their interactions create the Standard Model, the summary of scientists' present understanding of Nature. Every physicist will tell you it's an imperfect model-or, more precisely, an inelegant model, because it requires parameters such as mass to be factored in, rather than accounting for them on its own-but it's what they have for now, and the Higgs is an important part of it.

"The most natural way for it all to work," Shochet explains, "would be for all the elementary particles to have no rest mass whatsoever-to have mass that only came from kinetic energy, the energy from motion, and potential energy, the energy associated with things interacting. But we know that's not the case."

Instead, the Standard Model has Nature doing a most unnatural thing: imparting masses to each of the particles with random abandon: allowing the photon to be massless and the neutrinos to be near-massless, while the most massive particle, the top quark, measures in at 175 billion electron volts and the second-most massive, the bottom quark, is a mere 5 billion electron volts. (Shochet explains an electron volt as "how much energy an electron gets when it goes through a one-volt battery"-a measurement system that draws on Einstein's theory of relativity and the notion that mass and energy are interchangeable. So the particles' masses are, in essence, how much energy they deposit when they go splat against the walls of the accelerator detectors.)

Solving the mass conundrum begins with finding the Higgs.

Energy, the poet William Blake wrote, is eternal delight. The search for the Higgs boson is a quest for energy: high enough levels of it, more accurate measurements of it, a better understanding of it. The small step between delight and light would not be lost on the visionary poet. Says Henry Frisch, a visiting scientist at Fermilab, "It's really intensity - it's luminosity - that we need. Which is to say we need more collisions."

Causing collisions is, in theory, simple work. Researchers begin with hydrogen, the simplest and lightest element, with one proton and one electron. LEP isolates the electron, while the Tevatron uses the proton. Huge numbers of the particles are cooled and stored, as are the particles' antimatter, which carries a reverse electric charge. The collider at LEP used antielectrons (or positrons), while Fermilab uses antiprotons.

The accelerators are built in circular tunnels-four miles in circumference at Fermilab, 17 at CERN-with magnets and high-voltage accelerating devices placed at regular intervals around the loop. The particles are released into the tunnel, and as they race around the loop, they pick up energy from the high-voltage field, fattening them for the slaughter ahead.

Meanwhile, the antiparticles are also fed into the tunnel but, because they have an opposite charge, they travel in the opposite direction. Like "threading a needle," Frisch says, magnets direct the flow of the looping particles and antiparticles into a vacuum pipe a few inches across. Heavily concentrated, their plump little bodies collide. At Tevatron, the collision occurs at an energy of 1.8 trillion electron volts, ten times the energy achieved at LEP. The particles and antiparticles annihilate each other. Splatters of energy go hurtling off in every direction.

The point at which the particle beams collide is in the epicenter of a huge computerized detector. The machine, an onion-like device with layers of specialized forensic materials, keeps track of the energy's direction, how much is splattered, and how much the splatters bend as they pass through a magnetic field.

The particles that hurtle out of the collision are not necessarily pieces of the protons and antiprotons. Rather, slamming together two beefed-up particles releases enough energy to create a flurry of new, highly unstable particles. These particles make it a few millionths of an inch out of the collision point before they can't resist their inherent weakness any longer, decaying into more stable particles such as photons and electrons.

The inevitability of malfunction aside, the process is continual: 24 hours a day, every day of the year, for years and decades at a time. The result at Tevatron will be something like 5 million collisions per second. The odds of finding a Higgs in those millions and billions and trillions of collisions is slim: maybe three in a year if the physicists are lucky. That's why increasing the number of collisions - that delightful concept called luminosity - is so critical.

But it also presents an incredible challenge: how, for Pete Higgs's sake, do you keep track of and sort through all those collisions? Luckily, physicists have been doing this long enough to know what to expect from the constant spray of colliding particles. They know the properties of all the particles that can possibly come hurtling out (except the Higgs), which means they know each particle's preferred direction of splatter, how much energy (or mass) it deposits in the detector, and how much each particle bends as it passes through a magnetic field.

The physicists also generally know what kinds of particles to expect, and how often those particles will show themselves. Since Tevatron collides protons (which are made up of quarks), they know to expect huge numbers of insignificant quarks to come spraying out. They call this quark-spray "background," the chaff from which they must separate the wheat. The wheat, that single golden grain, they call a "signal." The image left behind by LEP when it closed down was a clean, beautiful signal with all the properties of a Higgs.

By their fruits you shall know them," the apostle Matthew wrote almost 2,000 years ago-an apt description of how physicists hope to know a Higgs when they see one.

Physicists can't actually see into the beam pipe, and even if they could, too many collisions are occurring and the golden grains decay too quickly to be glimpsed by a human or mechanized eye. So researchers use clues from a detector to retrace the particles' steps back to the collision point. Each step backward in time tells a little more until, eventually, they can identify and measure a particle with enormous precision.

"There are hand-waving descriptions of particle physics, and these carry you a certain distance," explains James E. Pilcher, professor in physics, the Fermi Institute, and the College. "But we feel much happier and more confident in our progress when we can really make precise predictions. If given enough time and energy after we do the calculations and make a prediction, then we can do some magnificent measurements to compare observation with our predictions."

Observation begins in the detector's outer layer, at the 10-meter-wide "calorimeter." As its Latin root implies, the calorimeter measures the "heat"-that is, the energy-deposited by splattering particles. Chicago is one of 150 institutions contributing pieces to the calorimeter to be constructed at CERN. Anyone who takes a wrong turn in the Fermi Institute on campus may come upon such a piece-shaped like an oversized slice of fruit tart, four feet in length, eight inches deep, slightly more than a foot at its widest end and tapering to half that at the vertex. Threaded with fiber optic cable and assembled, the pieces eventually will form a hulking 192-piece crust to wrap around the beam pipe at CERN's Large Hadron Collider (LHC).

"The signals begin in the calorimeter as light," Pilcher explains. Scintillations are converted by the fiber-optic lines into electrical pulses, and the detector records the signals. The next step backward is to measure the bend of the particles as they pass through a magnetic field before entering the calorimeter. A particle's willingness to bend tells physicists its charge. One step deeper into the detector, and physicists arrive at layers of silicon wrapped around the vacuum beam pipe. As particles hurtle out of the vacuum, they pass through the silicon, leaving behind a series of dot-like signals. Physicists have simply to connect the dots and see the "track" of a single particle.

In the vast majority of cases, the dots don't connect or they lead directly back to the collision point, both of which indicate background-those superfluous quarks that fly out of a parent proton or antiproton during annihilation. But very, very rarely the dots lead to a point other than the collision. And that is one of those interesting signals indicating enough energy was achieved to kick out some other highly unstable particle which decayed too quickly to be seen. Taken together, the deposits in the calorimeter, the trajectory in the magnetic field, and the tracks left in the silicon tell exactly what was kicked out.

Except that it isn't quite that simple. Given the luminosity of the machines-the million collisions occurring each second-there's just too much data to be stored for study. Explains Frisch, "These events are on the order of a quarter of a megabyte each. We make a million a second. So in principle, the detector is capturing 250 gigabytes per second"-compared to, say, an Apple G4 with its much-lauded four gigabytes of memory-"but we have no way to store that much data. It sits on the detector for a little while, and then we have to make the decision. Imagine you're taking pictures with a digital camera at the rate of 250 gigabytes per second. And you have to look at every one of those pictures and say, yes, I want that one but not that one. That's what we've done here at Chicago: picture recognition."

For five years Frisch, Shochet, and a team of physicists have created the new computerized trigger at Fermilab that will recognize the golden grains of wheat among superfluous piles of chaff.

So what, after all this, will a Higgs look like when one is finally caught? It will look a lot like the signal left by LEP: four jets of spray going in very specific directions, with very specific trajectories, leaving behind very specific quantities of energy, all of which will indicate the final particles into which two unstable particles decayed before they could reach the silicon and leave tracks of their own.

More than 40 years of crunching the numbers of the Standard Model with Mr. Higgs's theoretical particle as a function of the ever-increasing levels of energy reached at these machines tells physicists exactly how the four jets will add up. Two jets will be the sprays of fermions that result when a W or a Z boson decays. Physicists have seen enough to know a W or a Z by the fruits of their decay. Two jets will spray in the opposite direction and come from something that's never been seen before, something whose decay products are two bottom quarks. It will be a W or a Z plus whatever decays into two bottom quarks because the mathematics say so: add up the end products, and the total equals the energy achieved during annihilation.

What physicists do know, Mark Oreglia explains, is that the Higgs "likes mass." Of course it does: its nature is to glom onto things, and the heavier the particle available to glom onto, the happier (and more easily detected) the Higgs is. After 15 years LEP was only able to give a possible glimpse of a Higgs at the very end of its life. The physicists pushed the machine as hard as it could be pushed and then finally agreed to close it down and move on. The working group recognized how incredibly interesting that single, perplexing image was, but the odds of producing another golden-grain signal with a Z and two bottom quarks were too slim and the cost of putting off construction of the LHC was too high.

Will the delightfully luminous Fermilab accelerator, with ten times the energy of LEP and a smart new trigger able to recognize golden grains in ten millionths of a second, see a Higgs? The answer is still only maybe, though the odds are better. Chicago theoretical physicist Carlos Wagner devotes his work to predicting how long it will be before the Tevatron might trap a Higgs. He estimates six years at least, by which point the LHC in Geneva will be starting up with nearly eight times more energy than Tevatron and 100 times more luminosity (effectively moving the "energy frontier" from U.S. to European ground-a fact lamented repeatedly by the Chicago physicists). Still, Oreglia says, another signal like LEP's is not enough. Physicists need about ten clean signals to say with certainty they've seen a Higgs. Then they'll spend many more years creating Higgs events and studying them with greater and greater precision to understand its properties.

But even then, says Oreglia, the Higgs will not be the final, glorious answer many well-meaning journalists have proclaimed, often calling it "the God particle," after the title of a 1993 book on the Higgs by physics Nobel laureate and Chicago professor emeritus Leon M. Lederman-a metaphor that causes Oreglia to visibly cringe. Seeing a Higgs is merely one step toward understanding that sticky notion of mass. "Right now without the Higgs boson," Oreglia says, "a mass is nothing more than a parameter in a theory"-that is, a set of numbers with no rhyme or reason that must be plugged into the Standard Model rather than being apparent on their own. How much mass is generated by the Higgs field-how exactly the Higgs gloms onto other particles to cause their all-over-the-map masses-is what physicists call its "coupling" habits.

As physicists begin to quantify many years from now how a Higgs couples to each particle, they hope finally to see a pattern in the numbers. And then, says Oreglia, a new theory may take the place of the Standard Model-still an incomplete summary, but more complete than we have now.

The ultimate goal in all of this theoretical brainstorming, number crunching, calorimeter assembling, silicon triggering, and continual running of experiments is a single - Einstein would say simple - unified understanding of Nature, where all the rest masses of particles are zero, where all the forces end up as simply manifestations of the same force, and where astrophysicists' theories of outer space are explained by what happens in subatomic space and vice versa.

"It's not just about the Higgs. The stakes are much higher," says Frisch. His meaning is clear: the possibilities are endless, we have only to fire up the machines and see what we will see.

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