Chicago scientists find evidence for top quark

Twelve Chicago physicists were part of a 439-member collaboration that announced this week it had finally found strong evidence for the top quark, the sixth and final fundamental particle that will complete the Standard Model of matter and energy. Scientists have been searching for the "top" for nearly 20 years.

Melvyn Shochet, Professor in Physics and co-spokesperson for the collaboration, told reporters Tuesday at a press briefing at the Fermi National Accelerator Laboratory (Fermilab) that he and his colleagues have won the high-stakes race to find evidence for the existence of the top quark.

"The top quark has been for a long time the missing link in the Standard Model," said Shochet. "If our picture was correct, then it had to be there, and this is the first evidence that it exists."

He added, "The top quark is incredibly massive. Because it's so heavy, it may be that as we study its properties, it will enable us to better understand the process by which all objects acquire mass."

The two-year experiment was carried out at Fermilab's 6.3-kilometer Tevatron accelerator using the Collider Detector at Fermilab (CDF).

Scientists search for the building blocks of matter by accelerating protons and antiprotons to nearly the speed of light by sending them whizzing around Tevatron's superconducting accelerator ring. When the protons and antiprotons collide, they produce a wide variety of particles that constitute the fundamental building blocks of nature.

The top quark is extremely elusive. It is created in only about one collision in every hundred billion, and it decays almost immediately into other particles, meaning that only the signature of its passage can be detected, never the particle itself. To make matters worse, its signature of decaying subatomic particles can be mimicked by more ordinary subnuclear processes.

By measuring the debris from nearly a trillion subatomic particle collisions over the past several years, Shochet and his collaborators have seen the top's signature in about a dozen collisions. They determined that the top has a mass of about 174 billion electron volts, which makes it by far the most massive particle known -- 30 times more massive than the next heaviest quark, the "bottom."

Although the scientists say their experiments show a probability of 99.75 percent that they have seen the top quark, they are more comfortable using the word "evidence" rather than "discovery."

Henry Frisch, Professor in Physics and a member of the CDF team since its inception in 1977, explained, "The standards of discovery in science are very high, so even with 99.75 percent likelihood, it's still appropriate to say 'evidence.' It's the natural conservatism of scientists." Frisch added that with expected improvements to the Fermilab accelerator, the scientists should soon be able to obtain many times more data, making even the most conservative among them comfortable with claiming "discovery."

Because the top quark is the last of the six quarks that physicists believe make up all the universe's matter, its existence is a confirmation of the best current theories of physics. But one question many scientists are now asking is, "Why is it so heavy?"

The top quark's mass is nearly the same as that of an entire atom of gold -- many times heavier than the proton and antiproton that collided to produce it. It's as if two baseballs hit each other and a bowling ball popped out. Because of the connection Einstein found between matter and energy, E=mc2, the energy of the collision is converted into mass. To produce a particle as massive as the top quark, the energy of the collision has to be very high, near the limit of the Tevatron's current capability.

Scientists believe the top quark was produced in the first nanoseconds after the Big Bang. Finding it locks in the Standard Model, but the search won't end there.

"We regard this as the beginning of an exploration in a totally new region, rather than the closing of a door," said Frisch. "The top mass may provide a clue to the origin of mass in the universe, but we need to continue the exploration to go beyond just the clue. Improvements to the accelerator will allow us to produce many more top quarks so that these questions can be explored."

Gathering more data will confirm the team's finding, and it will also fill in important details about how top quarks are produced.

The scientists submitted a paper on their finding on Friday to the journal Physical Review, and they described the work at a scientific seminar at Fermilab on Tuesday.