By Sally Pobojewski
News and Information Services
U-M physicists helped make history on April 26 when scientists working at Fermi National Accelerator Laboratory announced evidence for the elusive top quark, which researchers around the world have been searching for since 1977.
Even though the announcement presented evidence, not a conclusive discovery, it was enough to put the top quark on the front page of such newspapers as The New York Times, The Chicago Tribune and USA Today, prompting millions of Americans to turn to one another and ask: “What’s a quark?”
Quarks are one of the basic building blocks of everything in the universe—including protons, neutrons and other subatomic particles that make up the nuclei of atoms. During the 1960s and 1970s, physicists developed a theory called the Standard Model, based on the existence of six types of quarks, to explain the origin and structure of matter. Over the next two decades, physicists found five quarks, but no one was able to detect the sixth, or top, quark until now.
If it is confirmed, credit for the discovery will go to 440 physicists who are members of the Collider Detector Facility (CDF) experiment at Fermilab. Five faculty members, five research associates and 12 students from the Department of Physics are part of the CDF collaboration.
“The key role played by U-M physicists in the possible discovery of the top quark is very exciting and a source of great pride for our university,” says Homer A. Neal, vice president for research and professor of physics. “Even though it required hundreds of physicists to conduct this experiment, the role of our faculty was absolutely critical to its success.”
“This is our first glimpse of the top quark,” notes Myron K. Campbell, associate professor of physics and a member of the CDF collaboration. “It’s a significant discovery, because it validates the Standard Model, and gives us important clues to the next big question—how do particles acquire mass?”
According to physics Prof. Dante E. Amidei, who is part of the CDF team, the top quark’s huge mass is what enabled it to elude physicists for so long. “The top quark has the mass of an entire gold atom. It is 35 times more massive than any similar particle,” Amidei explains. “The bigger the mass, the more energy you need to create the particle. Fermilab has the only particle accelerator in the world capable of producing the energy it takes to create a top quark.”
To produce a particle as massive as the top quark, CDF physicists had to reproduce conditions that existed shortly after the “big bang,” which created the original top quarks and everything else in the universe 18 billion years ago.
To do this, they used Fermilab’s particle accelerator to collide beams of protons and antiprotons moving nearly at the speed of light. When beams collide, some of the energy in each collision turns into subatomic particles. Out of more than a trillion collisions, however, CDF physicists found only 12 that they believe produced a top quark.
“Top quarks decay into other particles almost as soon as they’re produced,” Campbell explains. “We had to develop extremely sensitive detectors to filter out the background noise and record their faint, transitory signals.”
Campbell led a group of CDF physicists who developed an electronic trigger that sorted through 300,000 collisions per second to identify one or two potential top quarks. “In addition to members of the physics department, students and computer-aided design resources in the College of Engineering were crucial to the successful construction of the trigger,” Campbell says.
Amidei coordinated construction of a silicon vertex detector so precise it could record the position of particles immediately after a collision occurred. “Our research group’s analysis of signals from this detector proved essential to identifying top quark events,” Amidei says.
“Both pieces of technology were vital to the success of the CDF collaboration,” Neal says. “If the trigger wasn’t reliable or the silicon tracker didn’t work, the entire experiment would have been a failure.”
Neal is in a position to know. He is a member of the DZero collaboration—another group of physicists who also have been searching for the top quark at Fermilab. The DZero experiment began several years after the start of the CDF experiment. “Together, the two experiments should provide a comprehensive picture of the top quark, its decay properties and its interaction with other particles,” Neal says.
As with most scientific research, it’s impossible to foresee where research on the top quark may lead. “It would be foolish to say this will lead to any immediate benefit,” Campbell says, “but who knows? As each property of matter was discovered, it was soon harnessed for people’s benefit. The discovery of electromagnetism led to electricity. Our understanding of the atom led to radiation therapy for cancer. If we discontinue basic research, we could be shortchanging our children’s future.”
Physics Profs. J. Wehrley Chapman and Rudolf P. Thun, and Dolly Yu-Ting Wu, assistant professor of physics, are the other U-M faculty members of the CDF collaboration.
