Scientists Get to the Bottom of the Top Quark Puzzle : Physics: Team believes it has found evidence of the elusive subatomic particle that is part of all matter.
Scientists at the Fermi National Accelerator Laboratory announced Tuesday that they have detected strong evidence of an elusive particle at the heart of all matter in the universe, providing the best proof yet of the hidden structure of the material world.
“We aren’t looking at the face of God, but we are deciphering his handwriting,” said Thomas Muller, a UCLA physicist who was a member of the team that detected the presence of the top quark, as the subatomic particle is known.
Ending years of rumor and speculation, the announcement culminated a 17-year search that cost more than $100 million and involved 440 researchers from 49 universities and five national governments--the United States, Japan, Canada, Taiwan and Italy.
Discovery of the top quark, if confirmed, completes one set of subatomic building blocks whose existence is predicted by the prevailing theory, called the Standard Model, of the particles and forces that determine the fundamental nature of matter and energy. In the whimsical lexicon of modern physics, the elementary particles are called quarks, leptons and bosons.
Murray Gell-Mann, the Caltech physicist who won a 1969 Nobel Prize for pioneering the theory of the Standard Model, said the news was “splendid.”
“Not finding it would have been shocking,” Gell-Mann said. “We theorists would have to fall on our fountain pens. It would be an inconceivable disaster for theorists if they could not find it.”
Ron Ruth, head of the accelerator theory department at the Stanford Linear Accelerator Center, said, “It is the final piece of the puzzle. This puts to rest the doubts.”
The discovery may be the last hurrah for American high-energy physics, under whose banner three generations of scientists have dominated the search for the fundamental building blocks of matter. Only one other particle--called the Higgs boson--has yet to be tracked down, and, with the cancellation of the Superconducting Supercollider project last year, no U.S. atom smasher will be powerful enough to detect it.
The finding of the top quark has an impact similar to filling in the last blank in the periodic table of the elements. Many of the elements have little or no direct impact on everyday life, but their existence affirms scientists’ picture of the composition of matter.
All matter is composed of atoms, which are made up of electrons, neutrons and protons. Those in turn contain even more elementary pieces, such as quarks. Gell-Mann named them after a word he found in a cryptic reference in the novel “Finnegan’s Wake” by James Joyce.
There are six kinds of quarks in all, linked to each other in pairs. The top quark, the most massive and last to be detected, has puzzled physicists for so long that they hope finally understanding it will lead to new insights into the origin of matter. The top quark by itself is as heavy as an entire atom of gold.
The so-called bottom quark, which is linked to the top quark, was detected by Fermi researchers 17 years ago. But the top quark was remarkably hard to find because it decays so quickly, existing for less than a trillionth of a trillionth of a second before breaking down into lighter particles.
No one has laid eyes on a top quark or captured its direct image on film. Instead, physicists at Fermi collected evidence of its brief existence by tracking the distinctive trail of byproducts it leaves behind. The search is particularly difficult because a top quark can decay in several different ways, producing a number of possible electronic signatures.
The Tevatron particle accelerator at Fermi outside Chicago is the most powerful in use today, spinning protons and antiprotons at close to the speed of light in a giant underground ring four miles in circumference.
When the particles collide, they create a tiny fireball of energy as intense as the Big Bang itself when the universe was barely a trillionth of a second old--its energy safely contained by Tevatron’s massive magnetic field. As they smash, particles fly off in all directions. For that hellish instant, the Tevatron contains the hottest spot in the solar system--still just barely enough energy to create the conditions for the top quark to appear.
By repeating such collisions millions of times, researchers believe they caught a dozen glimpses of the top quark on the Tevatron’s 5,000-ton detector.
Data from test runs in August, 1992, and June, 1993, provided the most persuasive evidence of the top quark, said Melvyn J. Shochet, a physicist at the University of Chicago who is a spokesman for the Fermi project. The particles detected, Shochet said, were actually the ones into which a top quark would eventually break down, “the grandsons and granddaughters,” he said.
The top quark could decay in several different ways, and two of those patterns were spotted. “Our confidence grew when we discovered the second interaction,” Muller said.
In one of the apparent paradoxes that abound in particle physics, researchers had predicted that the top quark would be heavier than the proton from which it came. From their data, Fermi scientists measured the top quark’s mass at about 200 times that of a proton, well within the range predicted by the theory.
At a Fermi conference late last year, participating physicists agreed that they should write a journal article analyzing their results, but wrangled over what their videotapes showed. Although they were certain they had highly significant results, they could not agree on whether they had gathered enough data to be absolutely sure they had actually created the missing particle.
At a news conference Tuesday to announce the discovery, William C. Carithers Jr., a senior staff physicist at Lawrence Berkeley Laboratory who is a visiting researcher at Fermi, said, “The evidence is too large to ignore, but too small to cry ‘Eureka!’ ”
The researchers were anxious to stake their claim to the discovery as soon as practical but equally anxious to avoid the embarrassment of a public error. After all, a European lab had announced the discovery of the top quark previously and then retracted its claim because of mistakes in interpreting the background of vast numbers of particles.
Last week, the team submitted its 150-page manuscript to a peer-reviewed science journal called Physical Review D. A separate team of 420 researchers at Fermi is attempting to independently confirm the findings.
Fermi officials said they decided not to wait for the usual outside scientific review and make their announcement at the news conference. Word of the Fermi data was leaking out, and the group felt confident enough to declare it had found evidence of the top quark.
Some misgivings still linger.
John Peoples Jr., director of the Fermi Lab, said Tuesday, “I am persuaded that the evidence is very strong.” But, he said, “there are some inconsistencies.”
Carithers said that there is only a “1-in-400 chance” the scientists are wrong.
Shochet said he would like to see “two or three times” as much data “fingerprinting” of the top quark before declaring a definitive discovery, but that “this is the first time you have a serious hint of something.”
