THE BIGGEST high-energy physics experiment in history is set to start its first test run at Fermilab in September, and scientists are in an unabashed state of excitement over what they might find.
For physicists it will be a rare opportunity to open the door to a new world of subatomic particles never before seen but which may provide fundamental information for a better understanding of matter, energy, space and time.
”How excited are high-energy physicists around the world? They are blue,” said Leon Lederman, director of the Fermi National Accelerator Laboratory near Batavia. ”Anytime a machine opens up a totally new domain, it`s an exciting time.”
After six years of construction, Fermilab`s new Tevatron atom smasher is champing at the bit to begin banging protons and antiprotons together at forces never before achieved, thereby regaining the crown as the world`s most powerful particle accelerator.
But the supercold giant buried under the prairie 30 miles west of Chicago had to wait for the completion of a $64 million ”eye” to see the incredibly small and fantastically short-lived particles created in the tremendous collisions.
The ”eye” is the world`s largest particle detector, and one of its first jobs will be to look for the elusive Higgs particle. The Higgs is thought to live for only one billionth of a billionth of a second, but it holds the key to breaking the logjam now impeding a clearer view of how the universe works.
Atom smashers and their detectors are to physicists what telescopes are to astronomers and microscopes are to biologists. These instruments allow scientists to explore worlds that would otherwise be forever invisible to the naked eye.
Physics is the mother lode of all science. Everything in the universe is made up of elementary particles that play out their roles in stars, plants and humans through four forces. The knowledge so far gained from studying these particles and their forces has laid the groundwork for all the other sciences and all technology.
That`s why scientists around the world are racing to build ever bigger atom smashers. The more power a smasher has, the deeper it can probe into Nature`s fundamental building blocks.
Probing these mysterious depths is like trying to discover the contents of a sack full of golf balls, baseballs and bowling balls. Earlier atom smashers were like throwing golf balls at the sack: They were able to knock golf balls out, but they didn`t tell scientists anything about the baseballs and bowling balls inside.
When the original Fermilab 4-mile-round acclerator was completed in 1971, it was the world`s most powerful atom smasher. But its power was equivalent to throwing golf balls.
To get baseballs out, scientists needed to hit the sack with objects as massive as baseballs. A consortium of European countries did that in 1983 when their Super Proton Synchrotron (SPS) at CERN, Geneva, collided protons and antiprotons together to produce the world`s highest energies.
Out of the sack came the baseballs, the W and Z particles. The dramatic discovery of these particles solidified the Standard Model theory of the universe and unified two of the four forces, electromagnetic and weak, into the electroweak force.
The Standard Model has been remarkably successful in explaining all of the known facts that have come out of high-energy physics experiments. A cornerstone of the theory is the prediction that three of the four forces
–electromagnetism, which governs electricity and light; the weak force, which regulates radioactive decay; and the strong force, which binds the nuclei of atoms together–are different manifestations of the same force. The fourth force, gravity, is the odd man out, but scientists hope eventually to unify it with the other three.
The electromagnetic force is carried by the photon, which has no mass and travels at the speed of light. The weak force is carried by the W and Z particles, which are very massive, about 100 times as massive as a proton. They exist for extremely brief periods and do not travel much farther than the diameter of a nucleus.
But, at higher energies, approaching those that existed at the beginning of the universe in the Big Bang some 15 billion years ago, the electromagnetic and weak forces were thought to be the same. They froze out into their present forms as the early universe cooled.
The key to understanding how these forces broke apart is the Higgs particle. Somehow the Higgs, which exists for such an extremely short period, converted some of the original Superforce into electromagnetism, the weak and strong forces and gravity, according to theory.
By reversing this sequence and heating up matter close to what it was like after the Big Bang, scientists were able to unify the electromagnetic and weak forces. The CERN atom smasher blew away the Higgs field and revealed that the W and Z particles and the photon were one and the same particles at high energy.
Finding the Higgs would go a long way toward unifying the electroweak and strong forces. That would be an electrifying breakthrough that would leave only gravity unaccounted for. Understanding the Higgs also may help explain one of the profoundest questions of physics, why some particles have mass while others don`t.
The Higgs particle is the bowling ball. It is thought to be at least 200 times more massive than the proton. Higgs has eluded detection because no previous atom smasher was powerful enough to generate the energies needed to create it.
With Fermilab`s Tevatron, scientists hope to knock the Higgs out of the sack by banging it with an energy that is three times greater than the CERN machine.
”I don`t know if we`ll see the Higgs when we start running in September but you never know,” Lederman said. ”Higgs is a mystery because it stands for everything we don`t understand about particles today.
”It will be a lot more interesting than the W and Z because they were predicted with a great deal of confidence,” he said. ”They are the finising touches of the Standard Model. Now we will be poking beyond the Standard Model into the real jungle of ignorance.”
Besides Higgs, the jungle includes the search for supersymmetry particles that may be able to bring gravity into the unification fold, and the exploration of quarks, which are now thought to be the fundamental building blocks of matter, to see if they are actually made up of even smaller particles.
”This is the first time we`re going to be able to make sharp, definitive explorations of these questions,” Lederman said.
The immense Tevatron that will do the explorations is made up of more than 1,000 superconducting magnets built in the same tunnel as Fermilab`s original main ring. The main ring fired beams of protons at stationary targets to break their nuclei apart, producing a maximum center-of-mass energy of 20 to 30 billion electron volts.
The Tevatron is about 100 times more powerful. By colliding beams of protons against beams of antiprotons, with each beam traveling at near the speed of light (186,000 miles a second), the colossal force of collision generates 2,000 billion electron volts.
CERN has upped the ante in the atomic demolition derby sweepstakes by starting construction on a 20-mile circular atom smasher called the Large Electron Positron collider (LEP) to recapture the high-energy crown. It is expected to be finished in 1989.
Not to be outdone, the U.S. physics community is pushing for the construction of the Superconducting Super Collider (SSC). This machine, about 80 miles in circumference, would have an impact energy 20 times greater than the Tevatron. It is hoped to be in operation by 1994.
But, until LEP and SSC are ready, the Tevatron will remain king of the atom smashers. After its initial start-up in September, the Tevatron will be shut down for about 10 months to allow for construction of a second detector. The first experiment with the Tevatron, the search for the Higgs particle, may take several years to conduct. It ranks as the biggest high-energy physics experiment ever performed because it requires the construction of the world`s largest detector and a cast of 200 physicists.
The experiment that led to the discovery of the W and Z particles at CERN had been the biggest to date. That team, led by Carlo Rubbia of Columbia University, had about 140 physicists. Ten years ago the biggest physics experiment involved 30 physicists.
”We are not sure what will be found with the detector–that is the excitement of high-energy physics–but we do know that it will make significant contributions to our knowledge of the subnuclear world where current understanding is incomplete,” said Roy Schwitters, the Harvard physicist heading the detector group.
The 5,000-ton detector is a massive undertaking involving unparalled precision. It is the product of three national laboratories–Fermilab , Argonne and Lawrence Berkeley–10 U.S. universities and two foreign countries, Italy and Japan.
”A major high-energy physics enterprise such as this one is really a form of high adventure, much like the Spanish voyages of discovery or a Himalaya climbing expedition,” Schwitters said.
As the high-powered Tevatron opens up the fantastically small world of subnuclear particles, the detector must be able to record the interactions that are so fleeting that they never become part of the visible world.
A typical atom is about a tenth of a trillionth of an inch in diameter. Protons and neutrons, which make up the nuclei of atoms, are about 100,000 times smaller than an atom. Quarks, which make up protons and neutrons, are a thousand times smaller than a proton.
Fortunately, Nature has provided scientists with a neat trick to explore these elementary particles. Albert Einstein`s theory of Relativity showed that energy can be converted to matter and vice versa.
The energy produced in a collision of protons and antiprotons, for instance, will create a wide variety of particles that are normally never seen because they exist so briefly. Antiprotons are antimatter. They are reverse mirror images of protons. While protons are positively charged, antiprotons are negative.
”In high-energy physics you don`t have a concept of a permanent particle,” Schwitters said. ”A particle can transmute its energy into other particles. It`s a very transient world for these quarks. They come and they go.”
In the Tevatron, beams of protons will collide with similar sized bunches of antiprotons. A proton is a bag of three quarks. As the two beams approach each other, some of the quarks in the proton bags will collide with quarks in the antiproton bags.
The detector will be able to watch these collisions at the rate of 50,000 a second. But only one out of every 100,000 of these collisions are hard enough to produce interesting particles worth studying, Schwitters said.
These collisions are like two billiard balls hitting head on. The quarks shatter and their energy produces jets of other particles and more quarks. It is hoped that among this debris a Higgs particle will be created.
”You don`t see them (Higgs particles) in ordinary life, but they play a hidden role in influencing all the world around us,” Schwitters said.
