World's Most Powerful Particle Collider Finds Evidence Of Rare Single Top Quark

"I am delighted by the DZero results and what they portend for the future," said Fermilab Director Pier Oddone.

Starting from a million billion proton-antiproton collisions produced by Fermilab's Tevatron, the world's most powerful particle collider, the DZero collaboration used modern sophisticated analysis techniques to search for about 60 collisions, each containing a single top quark. Up to now, scientists had observed the top quark only in subatomic processes involving the strong nuclear force, which produces pairs of top and antitop quarks. Those observations, made by both the DZero and CDF experiments at Fermilab, led to the discovery of the top quark in 1995.

"Observing a few single top quarks in a sea of billions of particle collisions represents an extraordinary technical tour de force," said Dr. Robin Staffin, Associate Director for High Energy Physics in DOE's Office of Science. "The power and sophistication of experimental analysis techniques like those developed by the DZero experimenters augur well for exciting discoveries to come."

Separating a handful of single-top events from two billion events recorded since 2002 allowed the DZero collaboration to determine one of the most important unmeasured parameters of the Standard Model of particles and forces, known as Vtb (pronounced "VTB"). This parameter determines the rate of single-top production: If there are only six quarks, as scientists have observed so far, theory predicts the magnitude of Vtb to be close to one. Departures from this magnitude would be a sign of new physics, indicating the existence of additional quarks or other fundamental particles. The DZero collaboration determined the magnitude of Vtb to lie within the range of .68 to 1.0 with a 95 percent probability, consistent with the Standard Model. Collecting more collision data and further refining the analysis will yield tighter constraints on the value in the upcoming months.

The signature of single-top events is easily mimicked by other subatomic processes that occur at much higher rates. To stand a chance of seeing the single-top signal, physicists at the DZero experiment had to develop sophisticated selection procedures. The first stage selected approximately 1,400 candidate events. Of these candidates, only about 60 single-top events were expected, and experimenters exploited every bit of information to unambiguously establish their presence. DZero scientists used three different techniques to combine some 50 discriminating variables to represent the results. These distributions allowed physicists to recognize the presence of single-top events-much like a mother's uncanny ability to distinguish between identical twins. The results of the analysis will be submitted this week for publication in Physical Review Letters.

"This analysis is an important milestone in our continuing search for the Higgs boson, the missing keystone in the Standard Model," said DZero cospokesperson Terry Wyatt, of the University of Manchester, UK. "The discovery of the Higgs boson would help explain why particles have mass. Observing the Higgs requires us to see very low rate signals in the presence of substantial backgrounds. The sophisticated analysis techniques we are honing in our current analyses will be directly applicable to our Higgs searches."

DZero cospokesperson Dmitri Denisov, of Fermilab congratulated his fellow collaborators.

"This exciting result would not be possible without the tireless efforts of over 600 DZero scientists and the wonderful accelerator that produced all these collisions," Denisov said. "Bravo to the Tevatron and the people who make the machine work. The search to understand the fundamental forces of nature is mankind's timeless quest, and it has led us to the discovery of powerful physics laws, new technologies and benefits for society."