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.