Physicists Present Evidence for Top Quark

Last of 12 Building Blocks Undiscovered

Physicists presented the world yesterday with strong evidence for the existence of the subatomic top quark--which scientists consider to be one of the 12 fundamental building blocks of matter.

A total of 439 physicists from 35 institutions and five countries submitted a 152-page to the Physical Review journal yesterday describing the breakthrough.

The announcement represents the most significant development in two decades of searching for the top quark at the Fermi National Accelerator Laboratory in Illinois.

Two Harvard participants in the research--George W. Brandenburg, senior research fellow in physics, and Professor of Physics John E. Huth--described their findings to a capacity crowd of about 300 students and professors yesterday afternoon in Jefferson Hall 250.

Brandenburg said evidence for the existence of the top quark could eventually help scientists gain a greater understanding of the origin of the universe.

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

In addition to Brandenburg and Huth, at least seven current graduate students, eight former Ph.D candidates and Professor of Physics Melissa Franklin have worked on the project.

The new findings lend credence to the Standard Model theory, which posits that all matter in the universe is composed of 12 fundamental particles: a set of six leptons, or light particles which include the electron, and six quarks.

The Standard Model predicts three pairs of quarks: up and down, strange and charm, and top and bottom. In 1977, the bottom quark became the last of these particles to be conclusively identified--a development which immediately sparked a search for its partner, the top quark.

"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, semi-final, stage."

Top quarks were culled from highspeed collisions between protons and antiprotons in the Tevatron particle accelerator at Fermilab--a circular underground track four miles long which rams particles together at speeds approaching that of light.

Fermilab's collider detector, a complicated machine over three sto- ries high and weighing more than 5,000 tons,recorded emissions of top quarks from thecollisions, Brandenburg. Huth and Franklin allhelped to design the detector.

The new finding have only a .25 percent chanceof resulting from non-top quark phenomena. Butphysicists still refused to call the new data aconclusive discovery.

"It's statistically not significant," saidFranklin, who has worked on the project for 12years. But "a quarter of a percent is huge in theworld of physics."

The finding could also be helpful incalculating the mass of another theoreticalparticle, called the Higgs boson. Understandingthe Higgs boson, Franklin said, is critical indeveloping a grand unifying theory linkingtogether the four fundamental forces of nature:strong, weak, electromagnetic and gravitational.

The paper released yesterday is based on ayear's worth of data collected by running theFermilab's accelerator at an energy of 17 billionelectron-volts. Top quark sightings during theyear, or "events" in physics jargon, generatederuptions of excitement among scientists, saidHuth, a self-dubbed "top godfather" in dataanalysis.

"There was a point in the analysis when mostpeople were sort of skeptical of top because therewasn't any evidence, but then there was a certainpoint when evidence started to pile up, when allof a sudden people started to think maybe there'ssomething here," Huth said. "That was ratherexciting because all of a sudden there was animmediate rush to put the pieces together in someway."

Brandenburg said there was no single moment ofdiscovery because the data analysis accumulatedevidence for the top quark bit by bit.

Huth said there to four times more data muststill be collected before the top quark'sexistence can be officially confirmed. Only whenthe particle's existence is completely assured canphysicists begin further research on the nature ofmatter in the universe, Huth said.

"Because the top [quark] is so massive, in asense it's like a keystone in a bridge," Huthsaid. "When you find this keystone and put it in,the whole structure all of a sudden becomesapparent. That gives you a window to do the newinteresting pieces of physics which you couldn'thave really understood without the top.