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May
14 , 2010 -Fermilab scientists find evidence for significant matter-antimatter
asymmetry
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When
matter and anti-matter particles collide in high-energy
collisions, they turn into energy and produce new particles
and antiparticles. At the Fermilab proton-antiproton collider,
scientists observe hundreds of millions every day. Similar
processes occurring at the beginning of the universe should
have left us with a universe with equal amounts of matter
and anti-matter. But the world around is made of matter
only and antiparticles can only be produced at colliders,
in nuclear reactions or cosmic rays.
“What
happened to the antimatter?” is one of the central questions
of 21st–century particle physics.
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Batavia,
Ill.—Scientists of the DZero collaboration at the Department of
Energy’s Fermi National Accelerator Laboratory announced Friday,
May 14, that they have found evidence for significant violation
of matter-antimatter symmetry in the behavior of particles containing
bottom quarks beyond what is expected in the current theory, the
Standard Model of particle physics. The new result, submitted
for publication in Physical Review D by the DZero collaboration,
an international team of 500 physicists, indicates a one percent
difference between the production of pairs of muons and pairs
of antimuons in the decay of B mesons produced in high-energy
collisions at Fermilab’s Tevatron particle collider.
The
dominance of matter that we observe in the universe is possible
only if there are differences in the behavior of particles and
antiparticles. Although physicists have observed such differences
(called "CP violation") in particle behavior for decades, these
known differences are much too small to explain the observed dominance
of matter over antimatter in the universe and are fully consistent
with the Standard Model. If confirmed by further observations
and analysis, the effect seen by DZero physicists could represent
another step towards understanding the observed matter dominance
by pointing to new physics phenomena beyond what we know today.
Using
unique features of their precision detector and newly developed
analysis methods, the DZero scientists have shown that the probability
that this measurement is consistent with any known effect is below
0.1 percent (3.2 standard deviations).
"This
exciting new result provides evidence of deviations from the present
theory in the decays of B mesons, in agreement with earlier hints,"
said Dmitri Denisov, co-spokesperson of the DZero experiment,
one of two collider experiments at the Tevatron collider. Last
year, physicists at both Tevatron experiments, DZero and CDF,
observed such hints in studying particles made of a bottom quark
and a strange quark.
When
matter and anti-matter particles collide in high-energy collisions,
they turn into energy and produce new particles and antiparticles.
At the Fermilab proton-antiproton collider, scientists observe
hundreds of millions every day. Similar processes occurring at
the beginning of the universe should have left us with a universe
with equal amounts of matter and anti-matter. But the world around
is made of matter only and antiparticles can only be produced
at colliders, in nuclear reactions or cosmic rays. “What happened
to the antimatter?” is one of the central questions of 21st–century
particle physics.
To
obtain the new result, the DZero physicists performed the data
analysis "blind," to avoid any bias based on what they observe.
Only after a long period of verification of the analysis tools,
did the DZero physicists look at the full data set. Experimenters
reversed the polarity of their detector’s magnetic field during
data collection to cancel instrumental effects.
“Many
of us felt goose bumps when we saw the result,” said Stefan Soldner-Rembold,
co-spokesperson of DZero. “We knew we were seeing something beyond
what we have seen before and beyond what current theories can
explain.”
The
precision of the DZero measurements is still limited by the number
of collisions recorded so far by the experiment. Both CDF and
DZero therefore continue to collect data and refine analyses to
address this and many other fundamental questions. “The Tevatron
collider is operating extremely well, providing Fermilab scientists
with unprecedented levels of data from high energy collisions
to probe nature’s deepest secrets. This interesting result underlines
the importance and scientific potential of the Tevatron program,”
said Dennis Kovar, Associate Director for High Energy Physics
in DOE’s Office of Science.
The
DZero result is based on data collected over the last eight years
by the DZero experiment: over 6 inverse femtobarns in total integrated
luminosity, corresponding to hundreds of trillions of collisions
between protons and antiprotons in the Tevatron collider.
“Tevatron
collider experiments study high energy collisions in every detail,
from searches for the Higgs boson, to precision measurement of
particle properties, to searches for new and yet unknown laws
of nature. I am delighted to see yet another exciting result from
the Tevatron,” said Fermilab Director Pier Oddone.
DZero
is an international experiment of about 500 physicists from 86
institutions in 19 countries. It is supported by the U.S. Department
of Energy, the National Science Foundation and a number of international
funding agencies. Fermilab is a national laboratory funded by
the Office of Science of the U.S. Department of Energy, operated
under contract by Fermi Research Alliance, LLC.
Media
Contact: Rhianna Wisniewski , Fermilab, +1-630-840-6733, rhianna@fnal.gov
Graphics
and photos are available at: http://www.fnal.gov/pub/presspass/press_releases/CP-violation-20100518-Images.html
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