Fermilab Scientists Discover Rare Single Top Quark
Written by Nancy
collision, recorded by the DZero collaboration, is among the
single top quark candidate events. The top quark decayed and
produced a bottom quark jet (b jet), a muon and a neutrino.
Credit: DZero collaboration.
Scientists at Fermilab have observed particle collisions
that produce single top quarks, a 1 in 20 billion find. This
discovery confirms important parameters of particle physics,
including the total number of quarks. Previously, top quarks
had only been observed when produced by the strong nuclear
force. That interaction leads to the production of pairs of
top quarks. The production of single top quarks involves the
weak nuclear force and is harder to identify experimentally.
This observation occurred almost 14 years to the day of the
top quark discovery in 1995.
Fermilab’s Tevatron, located near Chicago, Illinois is
currently the world’s most powerful operating particle
accelerator, and the discovery was made by scientists
working on together on collaborations. Scientists say
finding single top quarks has significance for the ongoing
search for the Higgs particle.
The Fermilab accelerator
complex. Credit: Fermilab
"Observation of the single top
quark production is an important milestone for the Tevatron
program," said Dr. Dennis Kovar, Associate Director of the
Office of Science for High Energy Physics at the U.S.
Department of Energy. "Furthermore, the highly sensitive and
successful analysis is an important step in the search for
Searching for single-top
production makes finding a needle in a haystack look easy.
Only one in every 20 billion proton-antiproton collisions
produces a single top quark. Even worse, the signal of these
rare occurrences is easily mimicked by other “background”
processes that occur at much higher rates.
Discovering the single top quark production presents
challenges similar to the Higgs boson search in the need to
extract an extremely small signal from a very large
background. Advanced analysis techniques pioneered for the
single top discovery are now in use for the Higgs boson
search. In addition, the single top and the Higgs signals
have backgrounds in common, and the single top is itself a
background for the Higgs particle.
To make the single-top
discovery, physicists of the CDF and DZero collaborations
spent years combing independently through the results of
proton-antiproton collisions recorded by their experiments,
CDF is an international
experiment of 635 physicists from 63 institutions in 15
countries. DZero is an international experiment conducted by
600 physicists from 90 institutions in 18 countries.
The CDF detector, about the
size of a 3-story house, weighs about 6,000 tons. Credit:
Each team identified several
thousand collision events that looked the way experimenters
expect single top events to appear. Sophisticated
statistical analysis and detailed background modeling showed
that a few hundred collision events produced the real thing.
On March 4, the two teams submitted their independent
results to Physical Review Letters.
The two collaborations earlier
had reported preliminary results on the search for the
single top. Since then, experimenters have more than doubled
the amount of data analyzed and sharpened selection and
analysis techniques, making the discovery possible. For each
experiment, the probability that background events have
faked the signal is now only one in nearly four million,
allowing both collaborations to claim a bona fide discovery
that paves the way to more discoveries.
“I am thrilled that CDF and
DZero achieved this goal,” said Fermilab Director Pier
Oddone. “The two collaborations have been searching for this
rare process for the last fifteen years, starting before the
discovery of the top quark in 1995. Investigating these
subatomic processes in more detail may open a window onto
physics phenomena beyond the Standard Model.”