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OCTOBER 2003
Volume 96, Issue 1
 

GRAPHIC:  ResearchInvestigations
If not the Higgs, then what?

The headlines held an air of defeat: “Smallest Particles Are the Biggest Challenge,” lamented the Chicago Sun-Times; “No Sign of the Higgs Boson,” cried New Scientist; “Below-par Performance Hampers Fermilab Quest for Higgs Boson,” sighed Nature. Although Fermilab began its quest for the Higgs—the subatomic particle that, in theory, gives other particles mass—in 2001, recent estimates show that the world’s most powerful accelerator won’t be able to squeeze out enough collisions to find or disprove the boson’s existence. At least six years’ worth of collisions were needed, at a rate of 5 million collisions per second, running 24 hours a day, every day of the year—factoring in downtime for repairs. In July Fermilab told the Department of Energy, which owns the lab, that the 20-year-old Tevatron accelerator is showing its age, and downtimes are stretching longer than previously expected.

IMAGE:  A recent Fermilab collision produced a W boson and two photons, one with very strong sideways momentum—a one-in-a-million event.
Image by Henry Frisch

A recent Fermilab collision produced a W boson and two photons, one with very strong sideways momentum—a one-in-a-million event.

Yet physics professor Henry Frisch, one of many visiting scientists from Chicago at Fermilab, is unshaken by the news. “The Higgs is an answer to one key question: what gives particles mass? But that’s only one of a list of important questions,” Frisch shrugs, “and in my mind it’s not even at the top of the list.”

Dressed in sandals, khaki trousers, and a well-worn, white button-down shirt, the silver-haired Frisch, who was on the team of Fermilab physicists that sighted evidence of the top quark in 1994, refuses to talk specifics about alternate lines of research that might capture the imagination the way the Higgs once did.

“The Higgs was a very approachable story [see “How to Catch a Higgs,” April/01]. You could ask a physicist, ‘What is it that you do?’ And he could say, ‘I’m looking for the Higgs, it’s this particle that gives things mass.’ And that made it very easy for everyone.” But it also made everyone—including some physicists, Frisch adds—a bit myopic. So Frisch insists on widening the view before talking specifics. At this moment in particle physics, he begins, “there are clues, there are mysteries, and there are questions.”

The clues include the patterns observed among the subatomic particles and the forces that interact with them. For example, physicists know of three "generations" within the fermions, the building blocks of atoms. There are three pairs of quarks (top and bottom, up and down, and charmed and strange) and three pairs of leptons (the electron, muon, and tau and their corresponding neutrinos). Frisch compares the generation pattern to the periodic table of elements, which explains how electrons arrange themselves.

A second clue, Frisch says, exists in the forces carried by particles called bosons, causing fermions to stick together, to disintegrate under radioactive conditions, and to have mass. "Although their strength is different from each other at the temperature at which we now live, at a very high energy—at the Big Bang, say—they all have the same strength," Frisch says. "We know this because as we build bigger and bigger accelerators, the forces get closer and closer in strength."

The last big clue is mass. “The particles we’re built of are very light,” he notes. Yet far heavier fermions appear among the unstable particles created by high-energy particle collisions. The bosons, too, have varying masses. “Everything we see, we see with photons, which are massless. Yet the W and Z, cousins of the photon, are very heavy. It’s a clue,” Frisch says. “There’s a pattern there.”

The “No. 1 mystery,” meanwhile, “is that gravity Is so much weaker than these other forces.” Gravity differs in that physicists can’t put it in the same mathematical formula as the other forces.

For Frisch the clues and the mystery prompt three why-and-how questions. First, what determines how the periodic table of fundamental particles is arranged? And is it related to the three space and one time dimensions in which we live? Perhaps other dimensions are required to explain the three-generation, fermion/boson split.

Second, where does mass come from? “Well, maybe it’s the Higgs mechanism,” Frisch concedes, “but maybe it’s something else.”

The third big question circles back to the mystery: Why is gravity so much weaker than the other forces? Some physicists suggest that gravity is the same strength as the other forces but exists in more dimensions. “Maybe the dimensions are curled up in themselves in a small radius,” Frisch says, “and the only part of gravity we see is the part that sticks out. Maybe it isn’t that gravity is weaker, it’s spacetime that’s different than what we think it is.” How, he continues, do the strengths of the forces relate to the fundamental structure of spacetime?

The problem with searching for the Higgs, Frisch says, has been the nature of the search itself. “If I were to say, ‘Go discover an animal you’ve never seen before,’ and you decided you were going to find a two-headed rhinoceros with a purple tail, it’s very likely that you’ve made the wrong choice.” Similarly Fermilab—taking up its search for the Higgs on the heels of a possible sighting by the Large Electron-Positron (LEP) collider at CERN in Switzerland—set out to see what LEP had seen.

“A better approach,” says Frisch, “is to make a catalog of known animals, and every time you see an animal, check it against your catalog.” To that end, Frisch and a team of physicists spent five years creating a computerized catalog of possible “signatures” that the Tevatron collider might spit out. So far he’s seen some rare ones indeed: he cites a signature with two photons, an electron, and a neutrino. Creating three fundamental force particles during a collision is a sign of “very heavy things,” Frisch says, perhaps evidence of “a whole heavier layer than we see now.”

The Standard Model makes highly precise predictions for what will be seen at Fermilab. What physicists need to do, says Frisch, is to test the predictions as broadly and precisely as we can. "We have the world’s highest-energy accelerator and more powerful detectors than ever before. Let’s push the theory to within an inch of its life and see if it holds up," he says. "The history of science is that whenever we explore completely new territory we discover that Mother Nature is more creative and wilder than we had dreamed."—Sharla A. Stewart

 


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