Only rarely do scientists make a discovery requiring textbooks to be rewritten. Yet physicists say they now may be on the verge of a “Eureka!” of that magnitude.
Within just a few years, clear signs of a never-before-seen subatomic particle known as the Higgs boson are expected to show up in the world’s most powerful accelerators, where the energy of particle collisions can form new particles. Although physicists have found many other exotic fundamental particles since the 1930s — some so important that their discovery earned Nobel prizes — finding this particle would be different.
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“All the discoveries in the last century, in a sense, were finding more of things like those already found — until this. The Higgs is a completely new kind of object never known to exist before,” says Gordon L. Kane of the University of Michigan in Ann Arbor. Indeed, if it weren’t for the Higgs boson, all matter would be on the left side of Albert Einstein’s famous formula, E = mc2. Without the Higgs, nothing — not molecules, this magazine, you, Earth, the sun, or anything else — would exist as matter. Everything would always be in the form of energy dashing along at the speed of light.
The Higgs plays such a crucial role in shaping the universe as we know it that it was dubbed the God particle by Leon M. Lederman, who won the 1988 Nobel Prize in Physics for codiscovering the muon. Lederman is now at the Illinois Math and Science Academy in Aurora.
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To find this legendary particle, researchers must bash together a billion trillion of more familiar subatomic particles, such as protons, at energies higher than those ever achieved before in any laboratory. Only then, theoretically, will a Higgs boson occasionally pop out of a dramatic fireball. The price tag for the undertaking, not to mention the technical challenge, is enormous. A single accelerator being built for this work will cost $4 billion.
The scientific stakes are also colossal. Prestige, fame, and, probably, a Nobel prize are in store for those researchers who find the Higgs first. Consequently, the top high-energy physics laboratories of the United States and Europe are pitted against each other in a race to the goal.
At Fermi National Accelerator Laboratory (Fermilab) in Batavia, Ill., physicists restarted on March 1 a rebuilt collider (SN: 6/19/99, p. 399), the Tevatron, with the search for the Higgs as its top priority. “We’d do anything possible to be able to find the Higgs,” says Fermilab Director Michael S. Witherell. “Everyone agrees it’s the discovery our field needs to move to its next level of understanding.”
As Tevatron research gets underway, heavy equipment will be busy near Geneva, Switzerland, constructing an entirely new accelerator, called the Large Hadron Collider (LHC), at the European Laboratory for Particle Physics (CERN).
If the Tevatron hasn’t nailed the Higgs by some time between 2006 and 2008, the then-completed LHC may grab the prize from under Fermilab’s nose. An international team of researchers has designed the LHC to generate collisions seven times as energetic as the Tevatron’s collisions and to create a more plentiful stream of particles to analyze. At Fermilab, “everyone here is nervous and excited. . . . Either we find the Higgs before the LHC is finished, or forget it,” says Joseph Lykken, a Fermilab theorist.
Despite all the sound and fury, physicists concede that this race may ultimately have no winner. Although well-examined and widely accepted theory declares that the Higgs is out there, it may not actually exist.
The standard model
The standard model of physics describes with remarkable precision all the known particles of the universe and the interactions that occur between them (SN: 7/1/95, p. 10; 7/29/00, p. 68). Only gravity has yet to be integrated into the model. The standard model identifies a dozen fundamental fermions, or matter particles, which come in two families known as quarks and leptons. The model also specifies five force-carrying particles, collectively known as bosons.
For all its successes, however, the theory omits a rather pivotal trait of particles — their mass. “The major question in particle physics is why any elemental particle would have any mass at all. The most natural theory would have all the particles with no mass, just like the photon,” says Melvin J. Shochet of the University of Chicago and Fermilab. But there’s plenty of mass in the universe, making such a theory obviously wrong.
To patch these flaws in the standard model, theorists proposed the existence of some sort of influence that permeates all of space, weighing down particles passing through it. This cosmic molasses is called the Higgs field.
A sufficient jolt, like an extremely powerful particle collision, can set the molasses quivering. Such a vibration amounts to a particlelike manifestation of the field — a Higgs boson. Theorists predict that there’s also a second, even thicker molasses that only affects quarks — the constituents of protons and neutrons — and gives them much more of their mass than the Higgs field does. But the Higgs is the only source of mass shared by all particles that have mass.
The Higgs boson, however, is not a form of matter. And, although it’s called a boson, it doesn’t carry force, as do the other bosons. For example, photons and gluons provide the forces that hold atoms together.
Unlike other standard-model particles, the Higgs boson interacts with another particle in proportion to the particle’s rest mass — its mass when standing still. Yet at least part of that mass only exists because of the interaction between the particle and the Higgs boson.
Finally, of all fundamental particles in the theory, the Higgs is the only one devoid of spin, which is a quantum mechanical property analogous to the whirling of a top. Particles with spin have some intrinsic magnetism, but not the Higgs.
The Higgs boson
If physicists do find the Higgs boson, they’ll want to study the particle in detail to help them understand the mass-giving mechanism. But that’s not the only tantalizing secret of physics they’ll want to pursue.
The Higgs boson has become a doorway to the future of physics. Whatever new, more comprehensive picture of the universe lies ahead depends in large measure on what the mass of the Higgs turns out to be.
“The standard model shows us very well how the universe works. The Higgs will be the first discovery that tells us why the universe works the way it does,” Kane says. “It narrows the possibilities and points us in certain directions.”
Might the universe be filled with yet-unseen particles that are partners to all the ones already known? If the Higgs boson’s own mass turns out to be relatively small, it would bolster so-called supersymmetry theories that include such a mirror world of particles (SN: 4/13/96, p. 231). Moreover, if supersymmetry turns out to be the correct model of the universe, that lightweight Higgs would be only the first of five increasingly heavy forms of the particle.
Alternatively, could the standard model — made yet more comprehensive with some still-to-be devised theory of gravity — continue on as the best, most complete description of the particle universe? That would be possible if the Higgs boson’s mass is light or middle-weight, theorists predict. If there’s no Higgs in that mass range, calculations within the standard model lead to weird predictions, such as certain interactions among particles taking place with probabilities greater than 100 percent.
Other Higgs masses lead to their own far-out consequences. For example, suppose the Higgs boson turns out to be heavier than the standard model predicts and is not just one particle, but a pair. Then, the universe could be the stomping ground for a host of very heavy particles proposed by a set of alternative models of particle physics, including one called Technicolor.
A very heavy Higgs, even beyond what Technicolor would require, may even have implications for the number of dimensions in the universe. However, theories of extra dimensions remain too rudimentary to predict a Higgs mass, says Michael Dine of the University of California, Santa Cruz.
The concepts of the Higgs field and the Higgs boson arose in the 1960s. Physicists then were trying to understand the relationship between the electromagnetic force, which includes the attraction between electrically charged particles, and the weak force, which causes nuclear decay.
They knew that the carrier of the electromagnetic force is the photon, the most familiar massless boson. Theorists then postulated that one or more other bosons mediate the weak force. Because the weak force acts only over short distances, scientists inferred that those bosons had to have lots of mass. But no one could explain what would make them so heavy, while the photon has no heft at all.
In 1964, theorists in Belgium and Scotland concluded independently that there must be a pervasive field in the universe that is responsible for the mass in these weak-force bosons. Further work showed that this field could bestow mass on all fundamental particles that have mass. Researchers have dubbed the mass-giving field the Higgs field, after the Scottish physicist Peter W. Higgs of the University of Edinburgh.
While theorists have had a heyday with Higgs physics ever since, experimentalists have run into brick walls for nearly 30 years. Since no one knows how much the Higgs weighs, experimenters have been colliding particles at greater and greater energies, which, in turn, produce heavier and heavier clouds of particles. The scientists keep hoping that one of those particle will turn out to be a Higgs. The Superconducting Super Collider in Texas was to be dedicated to the Higgs quest, but Congress considered it too expensive to see to completion (SN: 10/30/93, p. 276).
Just last fall, however, experimenters at CERN thought they may have caught the very first signs of the Higgs boson, in debris from particle smashups at the Large Electron Positron (LEP) collider.
After a distinguished 11-year career, LEP was scheduled for dismantling last September to make way for the Large Hadron Collider. In a last-ditch effort to urge the Higgs boson out of the LEP, researchers pushed the machine to its energy limit and won a monthlong extension of life for their accelerator last fall. At that extreme, two of the LEP detectors recorded five collisions harboring tantalizing hints that the coveted Higgs had formed and then instantly disappeared (SN: 12/9/00, p. 381). There were also a dozen other less convincing events. The mass of these fleeting particles — expressed in energy units — was almost 115 billion electron volts (GeV), or about the mass of an antimony atom.
Although the findings electrified the particle-physics community, confirming or disproving them would have required keeping the accelerator open yet another year. That would have delayed construction of the more powerful LHC, while adding millions to its cost. Although the decision rankled and disappointed many LEP scientists, CERN’s director-general opted on Nov. 8 to shut down LEP and to push forward with the LHC as quickly as possible.
With that decision, CERN passed the baton to Fermilab, at least until 2006, when the LHC might begin taking data.
Protons and antiprotons
Fermilab’s Tevatron pushes protons and antiprotons to the highest energies in the research world. Traveling in opposite directions around a 6.5-kilometer ring at nearly the speed of light, the particles collide and annihilate each other at two locations. There, detectors as large as three-story houses track the subatomic debris that spews from the submicroscopic fireballs.
What’s more, the newly upgraded Tevatron is expected soon to generate collisions at a rate 20 times as high as it did before. Fermilab plans to make further improvements in about 2 years that would boost the collision rate another sevenfold. To handle the tremendous jump in collisions and the increased amount of debris to be tracked, analyzed, and recorded, experimenters have rebuilt both of the Tevatron’s huge detectors.
Although LEP researchers may not have found the Higgs, they did show that the particle, if it exists, can’t have a mass smaller than 113 GeV. Within the still unexplored higher masses, physicists estimate that a supersymmetric, light Higgs would fall below 130 GeV and a standard-model Higgs below 170 GeV, and a Technicolor Higgs would weigh no less than 160 GeV.
If the Higgs mass actually is 115 GeV, as the LEP results suggest, the Tevatron will require 2 to 3 years of operation to pile up evidence as convincing as that from the defunct European collider. And it will take 5 years or more to accrue enough data to make a truly convincing case that Fermilab scientists have discovered the particle, according to some researchers.
Others are betting a discovery will be in hand sooner.
Hendrik J. Weerts, a physicist from Michigan State University in East Lansing and a Tevatron researcher, for instance, expects the hot prospect of discovery to speed up the pace of research. The information acquired during LEP’s last days is like a newly discovered dinosaur bone, Weerts says. “Once you have a bone, the excitement is much higher than if you’re just looking.” If the Higgs really exists at 115 GeV, it will be in hand by 2004, he predicts.
Gordon Kane, an architect of supersymmetry theory, is even more optimistic, predicting the announcement of a Higgs discovery within 2 years. He says if signs of a 115-GeV Higgs start to pile up early at the Tevatron, the physics community will quickly consider it a confirmation that the LEP data were actually due to Higgs bosons. Kane also notes that the lightest particles that are supersymmetric partners to known particles may show up at the Tevatron before the Higgs.
If the Higgs is heavier than 115 GeV, however, then the chances for a Tevatron discovery go down. Physicists associated with Fermilab maintain, however, that they can convincingly snag the Higgs even if its mass is up to 130 GeV as long as the Tevatron’s collision rate meets expectations.
Adding to the fervor, a new analysis by scientists at Fermilab and the University of California, Davis suggests that the Tevatron may have a better chance than scientists previously believed of discovering the Higgs boson — at any energy. That’s because, the Illinois and California researchers say, the collider is capable of creating the Higgs in the company of very massive fundamental particles, called top quarks — a combination that no prior accelerator had the energy to create. Physicists had previously ignored this production mode, however, because it yields only a few Higgs, says Fermilab’s Joel Goldstein.
“Our argument is that these events are so spectacular that even if you have only a handful, they should really stick out of your data,” Goldstein says. He and his colleagues present their findings in the Feb. 26 Physical Review Letters.
Others outside of Fermilab aren’t so optimistic.
“If it’s heavier than 115 GeV, my conviction is that the Tevatron is out of the game,” says CERN’s Patrick Janot, who was physics coordinator for LEP’s Higgs search.
It’s easier to rule out particular masses for a Higgs boson than it is to establish the particle’s reality. Even if the Tevatron’s Higgs search does come up empty-handed, Fermilab experimenters will have acquired valuable information. They’ll have shown that the particle doesn’t exist at masses up to about 190 GeV. It would then be left to CERN and the LHC to find a heavier Higgs.
Then there’s that other nagging possibility: that there is no Higgs. That’s the premise of a recent science fiction book called White Mars (2000, St. Martin’s Press) by veteran sci-fi author Brian W. Aldiss and Oxford University mathematician and theoretical physicist Roger Penrose.
In their tale, the LHC becomes 100 times as powerful as currently expected but finds no trace of the Higgs. Instead, by 2009, hints show up of another mass-bestowing entity called the Omega Smudge. In pursuit of it, researchers build an accelerator circling the moon, but it, too, fails. The book ends in the 22nd century, with scientists out beyond the solar system still looking for the source of mass. By then, they’re working on a detector as big around as the rings of Saturn.
Physicists on the front lines of the Higgs search today acknowledge that their coveted particle may not exist. But if it isn’t a Higgs that’s the basis of mass in the universe, then something else — call it a smudge, if you like — has to be the answer.
“There’s no boring way out of this,” Witherell insists. If something other than the Higgs is there, he says, then “that even goes beyond our past experience, and it is almost certainly more exciting.”