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Physicists get first look at single top quarks
Elusive Particles Key to Understanding Mysteries of the Universe
CONTACT: Susan Blessing
(850) 644-1032; sblessing@fsu.edu
March 2009
TALLAHASSEE, Fla. -- Scientists at the U.S. Department of Energy’s Fermi
National Accelerator Laboratory (Fermilab), including six from The
Florida State University, have made the first observation of the
production of single top quarks, one of the elementary particles of
matter. Researchers believe that top quarks provide clues to solving
longstanding mysteries of the universe.
The observation of single top quarks resulted from the painstaking
analysis of billions of proton-antiproton collisions recorded by the DŲ
(DZero) detector in Fermilab’s Tevatron, the world’s highest-energy
particle collider. Physics researchers from Florida State who
participate in the DŲ collaboration are Associate Professor
Todd Adams,
Professor
Susan Blessing, staff physicist
Sharon Hagopian, Professor
Harrison Prosper, postdoctoral research associate Jedranka Sekaric and
Professor
Horst Wahl, along with their graduate students.
Previously, top quarks had only been observed when produced by the
strong interaction between elementary particles. That process leads to
the production of pairs of top quarks. The production of single top
quarks, which involves the weak nuclear force, is much harder to
identify experimentally. But almost 14 years to the day after the top
quark discovery in 1995, the production of single top quarks has now
been observed.
“I am simply elated,” said Prosper, the Kirby W. Kemper Professor of
Physics at Florida State. “Fourteen years ago, the search for single top
quarks seemed an almost impossible task. Yet several Ph.D.s later --
including two from Florida State -- my colleagues and I finally
succeeded.”
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These two 'Feynman diagrams' represent the processes that lead to the production of single quark events of the kind seen by DZero.
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Searching for single-top production is an extremely difficult business
because only one in every 20 billion proton-antiproton collisions
produces a single top quark. Moreover, the signal of these rare
occurrences, called “events,” is easily mimicked by other processes,
referred to as “background,” that occur at much higher rates.
At 200 times the mass of a proton, which is roughly the mass of a gold
atom, the top quark is by far the heaviest elementary particle -- yet it
has no discernable size. Understood as an ingredient of the particle
soup created just after the Big Bang, today top quark pairs exist only
fleetingly within atoms, according to the laws of quantum theory.
Therefore, in order to study the top quark in detail it must be created
experimentally in a high-energy particle accelerator, such as the
Tevatron, that can recreate the conditions of the very early universe.
“This was a difficult analysis, carried out by a very dedicated and
persistent group,” said Blessing, a professor of high-energy physics and
director of Florida State’s Women in Math, Science and Engineering
program. “Their search began 15 years ago and required that we
understand our detector and the data extremely well. This bodes well for
future searches.”
To make the single top quark discovery, the researchers spent two years
combing through the results of proton-antiproton collisions recorded by
the DŲ experiment. The DŲ collaboration is an international team of
nearly 500 scientists studying high-energy particle collisions.
The Fermilab collaborators identified several thousand events that
looked the way single top events are expected to appear. Using
sophisticated statistical analysis and detailed modeling of background
processes, the team showed that a few hundred collision events produced
the real thing.
The researchers submitted their results to Physical Review Letters on
March 4.
“This discovery, in which Florida State University scientists played a
pivotal role, is a spectacular example of the truism that, even in a
collaboration of some 500 scientists, individuals can make significant
contributions,” Prosper said.
High-energy physics is about what makes up the world and what holds it
together. Its Standard Model is the most comprehensive theory ever
created and explains in detail the interactions between all elementary
particles.
Having more precise information about the top quark gives scientists
clues in their search for another missing puzzle piece, the Higgs boson,
which many physicists believe will solve longstanding mysteries about
the universe, such as why particles like electrons have mass. The work
done by FSU scientists is also helping to prepare the ground for what
many believe will be a new era of discovery at the Large Hadron Collider
(LHC), the world’s highest-energy particle accelerator. The $10 billion
LHC is located at CERN, the European Laboratory for Particle Physics, in
Geneva, Switzerland.
Major funding for the study was provided by the U.S. Department of
Energy.
Fermilab, located in Batavia, Ill., is the United States’ top facility
for research in the field of high-energy physics. Read more about it at
www.fnal.gov.
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