Culminating nearly a decade of intense effort, two rival groups of physicists announced that they have found the elusive top quark - an ephemeral building block of matter that probably holds clues to some of the ultimate riddles of existence.
The announcements brought sustained applause and a barrage of questions from an overflow audience of physicists at the Fermi National Accelerator Laboratory, where the work was done. Fermilab has the world's most powerful particle accelerator.The two competing scientific teams, each with about 450 scientists and each using a separate detection system, reported that after a long chase in which there had been several false sightings of the top quark, this monstrously heavy but elusive particle has finally been cornered and measured. The results of the two groups' independent measurements differed somewhat, but when margins of error were taken into account, the scientists agreed that the results were consistent.
One of the teams, the CDF Collaboration (standing for Collider Detector at Fermilab) reported last April that it had found evidence of the quark's existence. But at the time, the group lacked enough statistical evidence to claim discovery, and the competing group, the D0 (for D-Zero) Collaboration, which had even less evidence of its own, branded the CDF announcement as premature.
The achievement claimed on Thursday by both teams leaves virtually no room for doubt, however, and the discovery was hailed as a landmark in science. Hazel O'Leary, who as secretary of energy heads the federal agency providing most of the money for research at Fermilab, called the discovery a "major contribution to human understanding of the fundamentals of the universe."
The finding confirms a prediction based on a theory known as the Standard Model that nature has provided the universe with six types of quarks; the other five, the up, down, strange, charm and bottom quarks had all been known or discovered by 1977.
Since the infancy of the universe shortly after the Big Bang - estimated at 10 billion to 20 billion years ago - only the up and down quarks have survived in nature, and the protons and neutrons that make up the nuclei of all atoms are built from combinations of these two quarks; the other quarks disappeared from the observed universe, but have been recreated by modern particle accelerators.
Dr. Leon M. Lederman, a winner Nobel Prize winner, said he doubted there could be any more quark types but that "we know there's a lot of dark matter out in the universe that we can't identify."