nly rarely do scientists make a dis- covery requiring textbooks to be 0 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 parti- cle 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.

“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 mole- cules, 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.

To find this legendary particle, re- searchers must bash together a billion tril- lion of more familiar subatomic particles, such as protons, at energies higher than those ever achieved before in any labora- tory. 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

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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 Labe ratory (Fermilab) in Batavia, Il l . , physi- cists restarted on March 1 a rebuilt collid- er (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 Gene- va, Switzerland, constructing an entirely new accelerator, called the Large Hadron Collider (LHC), at the European Laborate ry 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 Fermilabs nose. An international team of researchers has designed the LHC

Central oval shows a highly magnified computer reconstruction of the impact point of the June 14, 2000, particle collision documented in the cover image. Off-center particle sprays (left and right ovals) may be breakdown products of a Higgs boson.

he standard model of physics describes with remarkable precision T all the known particles of the uni-

verse 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 identi- fies a dozen fundamental fermions, or mat- ter particles, which come in two families known as quarks and leptons. The model also specifies five forcecarrying particles, collectively known as bosons.

For all its successes, however, the the- ory omits a rather pivotal trait of parti- cles-their mass. “The major question in

it may not actually In this simulated Tevatron event, a short-lived Higgs boson exist. decays into two sprays (arrows) of particles.

SCIENCE NEWS, VOL. 159 MARCH 10,2001

particle physics is why any elemental par- ticle would have any mass at all. The most natural theory would have all the parti- cles 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 pass- ing through it. This cosmic molasses is called the Higgs field.

A sufficient jolt, like an extremely power- ful 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 de- void of spin, which is a quantum mechan- ical property analogous to the whirling of a top. Particles with spin have some in- trinsic magnetism, but not the Higgs.

over, if supersymmetry turns out to be the correct model of the universe, that light- weight Higgs would be only the first of five increasingly heavy forms of the particle.

Alternatively, could the standard mod- el-made yet more comprehensive with some still-to-be devised theory of gravi- ty-continue on as the best, most com- plete description of the particle universe? That would be possible if the Higgs boson’s mass is light or middleweight, theorists predict. If there’s no Higgs in that mass range, calculations within the stan- dard model lead to weird predictions, such as certain interactions among parti- cles taking place with probabilities greater than 100 percent.

Other Higgs masses lead to their own far-out consequences. For example, s u p pose the Higgs boson turns out t o be heavier than the standard model predicts and is not just one particle, but a pair. Then, the universe could be the stomping

f 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 door- way 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. “lt narrows the possibilities and points us in certain directions.”

Might the universe be filled with yet-un- seen 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). More

MARCH 10,2001

In 1964, Peter W Higgs (above) of Edinburgh University coined the idea of the mass-giving particle that now carries his name. Around the same time, a Belgian physics team, Robert Brout and FranGois Englert, independently achieved a similar insight.

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 di- mensions in the universe. However, theo- ries of extra dimensions remain too rudi- mentary to predict a Higgs mass, says Michael Dine of the University of Califor- nia. Santa Cruz.

he concepts of the Higgs field and the Higgs boson arose in the 1960s. T Physicists then were trying to under-

stand the relationship between the elec- tromagnetic force, which includes the attraction between electrically charged particles, and the weak force, which caus- es nuclear decay.

They knew that the carrier of the elec- tromagnetic force is the photon, the

SCIENCE NEWS, VOL. 159

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 dis- tances, 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 Scot- land 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-giv- ing 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 scien- tists keep hoping that one of those parti- cle will turn out to be a Higgs. The Super- conducting Super collider in Texas was to be dedicated to the Higgs quest, but Con- gress 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, r e searchers pushed the machine to its ener- gy limit and won a monthlong extension of life for their accelerator last fall. At that extreme, two of the LEP detectors record- ed 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 en- ergy units-was almost 115 billion elec- tron volts (GeV), or about the mass of an antimony atom.

Although the findings electrified the particlephysics community, confirming or disproving them would have required keeping the accelerator open yet another year. That would have delayed construc- tion 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.

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ermilab’s Tevatron pushes protons and antiprotons to the highest ener- F gies in the research world. Traveling

in opposite directions around a 6.5kile meter ring at nearly the speed of light, the particles collide and annihilate each other at two locations. There, detectors as large as threestory houses track the subatomic debris that spews from the submicroscopic fireballs.

What’s more, the newly upgraded Teva- tron is expected soon to generate colli- sions at a rate 20 times as high as it did before. Fermilab plans to make further im- provements in about 2 years that would boost the collision rate another seven- fold. To handle the tremendous jump in collisions and the increased amount of d e bris to be tracked, analyzed, and record- ed, 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 CeV. Within the still unexplored higher masses, physicists es- timate that a supersymmetric, light Hig- gs would fall below 130 GeV and a stan- dard-model Higgs below 170 CeV, and a Technicolor Higgs would weigh no less than 160 GeV.

If the Higgs mass actually is 115 CeV, 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 infor- mation 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 supersym- metry theory, is even more optimistic, pre dicting the announcement of a Higgs dis- covery 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 part- ners to known particles may show up at the Tevatron before the Higgs.

If the Higgs is heavier than 115 CeV, how- ever, then the chances for a Tevatron dis- covery 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 Teva-

tron’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 Teva- tron may have a better chance than scien- tists previously believed of discovering the Higgs boson-at any energy. That’s because, the Illinois and California r e searchers say, the collider is capable of creating the Higgs in the company of very massive fundamental particles, called top quarks- combination that no prior accel- erator had the energy to create. Physicists had previously ignored this production mode, however, because it yields only a few Higgs, says Fermilabs Joel Coldstein.

“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,” Coldstein 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 CeV, my convic- tion 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-hand- ed, Fermilab experimenters will have ac- quired 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 heav- ier Higgs.

Then there’s that other nagging possi- bility: 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-be- stowing entity called the Omega Smudge. In pursuit of it, researchers build an accel- erator 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 Hig- gs 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 exper- ience, and it is almost certainly more exciting.” 0

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154 SCIENCE NEWS, VOL. 159 MARCH 10,2001