Top quark `had to exist' -- It did -- It's found

By Ann Morris

Special to the Florida State Times

The hunt lasted over a decade, cost millions of dollars, and involved the combined efforts of hundreds of the brightest minds from all over the world. What could be the object of such an intensive search? How about an infinitesimally small subatomic particle proposed to be one of the fundamental units of matter?

On March 2, physicists at the Fermi National Accelerator Laboratory in Batavia, Ill., announced that they had finally discovered a particle called the top quark. Their announcement marks a colossal milestone in the history of physics and of humankind's search for the structure of matter.

The top quark is a key component of what is known as the Standard Model, the prevailing theory, first proposed by Murray Gell-Mann and George Zweig of Caltech in the 1960s, that explains how matter and energy fit together at the subatomic level. The Standard Model predicts the existence of six different quarks, all of which have now been discovered.

In the United States., the six quarks are called "up, down, top, bottom, charm and strange." The names are based partly on their charges -- positive or negative -- and partly on the whims of the scientists picking the names.

Confirmation of the top quark's existence was critical -- without it, the Standard Model would have fallen apart -- along with an abiding faith among physicists that what they've figured out so far is, in fact, the way nature works.

"Because of the symmetry of nature, the top quark has to exist," says FSU high- energy physicist Vasken Hagopian. "If we didn't discover it 100 percent, then it would be a disaster; we would say `Hey, we don't know what we're doing.'"

Hagopian is among 10 Florida State physicists, including four from the campus' Supercomputer Computations Research Institute (SCRI), and several post- doctoral and graduate students who belong to one of two international research teams that were engaged in a friendly competition to find the top quark for more than a decade.

But neither team claimed victory -- or both did -- they announced the discovery together.

The hunt was based on the grounds of the most powerful particle accelerator in the world -- the Tevatron accelerator at Fermilab, just outside Chicago. The two groups, called CDF and DZero (the FSU physicists work with the DZero group), each contain more than 400 researchers representing about 40 universities and research institutions worldwide.

How did these groups find the tiny particle? Why, by smashing to bits lots of bigger particles, of course. The scientists fed streams of protons and antiprotons (particles just like protons but with opposite charge) into a gigantic underground ring several miles across. The protons and antiprotons, which travel in opposite directions, were accelerated around the ring until eventually they reached nearly the speed of light. At this point the protons and antiprotons were smashed into each other, producing a shower of particles that flew off in every direction. The scientists then searched the "debris" of particles for traces of the top quark.

Like prospectors from the Old West, the physicists sifted through the subatomic silt created during a proton-antiproton collision, hoping the top quark would emerge. And, more often than not, what actually turned up was merely fool's gold -- particles that looked like the top quark but weren't, or ordinary particles that were of no interest at all.

The problem is that the top quark only sticks around for about a billion-trillionth of a second after it is created. After that, it breaks down into other particles -- like electrons, for example. So the physicists had to search instead for the decay products the top quark left behind, sort of like detectives picking through a suspect's garbage for clues.

To make matters worse, there are many processes, called background noise, which mimic what physicists believe is the top quark's decay pattern. This means that the physicists had to collect enough evidence to rule out the possibility of background noise before they could make any kind of claim.

Because of the confounding complexity of the search, it took physicists from both groups more than 10 years to accumulate enough evidence to become convinced that they had actually discovered the top quark. "What was particularly important," says FSU physicist Harrison Prosper, "was that we wanted to be certain that... the background calculation was correct, because... these events are not clear-cut. It's not as if you see this reaction in your machine... and say, `A-ha!' clearly, obviously this is due to the top quark."

The physicists, however, were finally able to claim victory. And with their triumph, the last puzzle piece of the structure of matter was put into place. Does this mean that the physicists' task is done? Hardly. "Finding the top quark is really exciting," says Prosper, "because it gives you the feeling that you're on the right track. But we have only just begun to answer the really deep questions."

One of those questions may well take physicists beyond the frontiers of the Standard Model to an even greater quest -- the search for the origin of mass. Scientists believe that the mass of the top quark is about equal to that of an atom of gold. If so, this would make the top quark by far the heaviest of all the fundamental particles. Some physicists believe that because of its improbably large mass, the top quark must be connected in some way to the mechanism that causes mass. Investigating that mechanism, say high- energy physicists could open the door to a whole new era of physics.

Or not. It could be that scientists will find they've got it all wrong, says Prosper, and scientific paradigms like the Standard Model will be replaced by completely new theories, just as quantum mechanics replaced Newtonian physics at the beginning of the 20th century.

High energy physics at FSU

The High-Energy Physics program (HEP) at FSU was started by Dr. Joseph Lannutti (Ph.D., Berkeley) in 1957. Fresh out of graduate school when he arrived at FSU, Lannutti was hired to create a program that would bring diversity to the FSU physics department, which at the time was focused primarily on nuclear physics.

"It was a very difficult thing in the physics department to build high- energy physics," said Lannutti, who is now an associate vice president for research. "When I came here there were more nuclear physicists (than high-energy physicists) and they were afraid I was going to take over the department. Now, we have about as many (high energy physicists) in number as nuclear physicists."

In the past 37 years Florida State's HEP program has grown to include some 36 physicists and has become one of the strongest groups of its kind in the country. It has participated in large scale collaborative efforts at other national and international laboratories around the world.

Roughly half the high-energy physicists in the FSU group come from the Supercomputer Computations Research Institute (SCRI), which was actually created to supplement the needs of high-energy physics research. "High- energy physics has always been the biggest computational user on campus," says Lannutti. "HEP needed more computational power, so that was a big push for SCRI."

Despite HEP's successes Lannutti has had to struggle to obtain adequate financing for the ever-expanding program, a difficult thing to do given the enormous scope of high energy physics research. "It's a constant battle to stay alive," says Lannutti. "It may look easy, but much of my life has been spent in sleepless nights worrying about what's going to happen."

Funding obstacles notwithstanding, the FSU high-energy physics program continues to increase in stature and expertise, maintaining a strong commitment to basic research and international collaboration. Such large scale collaborative research, says Lannutti, provides an additional benefit -- it fosters a spirit of cooperation and community among scientists around the world.

"We have a large mixture of citizenship," says Lannutti. "Since you're talking about a lot of people, it becomes a large social experiment, too." -- Ann Morris