Evidence for Top Quark Uncovers Last Fundamental Particle

Harvard physicists this past spring looked like knights from the Middle Ages who had just alighted upon the Holy Grail.

After nearly two decades of searching scientists presented nearly conclusive evidence of the existence of the top quark believed to be the last undiscovered building block of matter.

A total of 439 physicists from 35 institution and five countries collaborated on the project detailing their triumph in a 152-page paper submitted to the Physical Review journal. The experiment was conducted at the Fermi National Accelerator Laboratory in Illinois better known as Fermilab.

Though physicists were reluctant to term the results a "discovery," they said the new "strong evidence" for the top quark was the keystone they needed to validate the Standard Model, the prevailing theory explaining the constituents of matter.

Two Harvard contributors to the project--professor of Physics John E. Huth and Senior Research Fellow in Physics George W. Brandenburg--announced the results to a capacity crowd of 300 at a press conference in Jefferson Hall in April.

"Now the Standard Model is, in essence, complete. The one really big missing piece is found," Brandenburg said. "The goal of the last 20 years, namely understanding the Standard Model and filling it out, may have reached a final or at least semi-final' stage."

At least seven current Harvard graduate students and eight former Ph.D. candidate worked on the project.

The new findings have given physicists intellectual satisfaction, and may even help them explain how matter was first created said Brandenburg.

"Not only are we understanding matter and energy at a very microscopic level, but we're also under standing the origin of the universe," he said. "That's one of the really beautiful unifications that's happening between particle physics and astrophysics."

Over one year's worth of data was collected from Fermilab's Tevatron the highest energy particle accelerator in the world.

Protons and anti-protons were smashed against each other at high speeds in the four-mile -long underground circular tunnel of the accelerator, and top quarks were blasted out from the collisions.

Brandenburg, Huth and Professor of Physics Melissa Franklin helped design the three story high 5,000-ton detector which recorded the quarks emitted.

The data from the collisions corroborates the Standard Model theory which states that all matter consists of 12 basic particles: six leptons and six quarks. Leptons are a family of light particles the best known of which is the electron.

The quarks occur in pairs: up and down, charm and strange and top and bottom. A proton, for example, consists of two up quarks and one down quark.

The last quark conclusively identified was the bottom quark in 1977, sparking a race among scientists to find its partner the top.

In a seeming paradox, the mass of the top quark is nearly 200 times that of a proton. As a result scientists have likened this experiment to smashing together two tennis balls and finding a bowling ball in their place.

The increase in mass, however, is consistent with Einstein's theory that extra energy from high-speed particles can be translated into matter.

Margin of Error

Scientists were reluctant to call the results a conclusive discovery although there is only a.25 percent chance that the data was caused by non-top quark phenomena.

"A quarter of a percent is huge in the world of physics," said Franklin, who has been working on the project for 12 years.

The findings allow physicists to embrace a whole range of new initiatives including the calculation of the mass of another theoretical particle, the Higgs boson, said Franklin.

The mass of the boson is vitally important for physicists to develop a grand unifying theory linking together the four fundamental forces of nature: strong weak, gravitational and electromagnetic.

"Because the top [quark] is so massive, in a sense it's like a keystone in a bridge," Huth said. "When you find this keystone and put it in, the whole structure all of a sudden becomes apparent. That gives you a window to do the new interesting pieces of physics which you couldn't have really understood without the top."