May 24, 2010 Why is there matter in the universe and not antimatter, its opposite?
Physicists at Fermi National Accelerator Laboratory, including John Ellison, a professor of physics at UC Riverside, have announced that they have found evidence for a significant violation of matter-antimatter symmetry in decays of B-mesons, which are exotic particles produced in high energy particle collisions.
To arrive at their result, the research team, known as the DZero collaboration, analyzed billions of proton-antiproton collisions at Fermilab's Tevatron particle collider, and found a 1 percent excess of pairs of muons over pairs of antimuons produced in the decays of B-mesons. Muons, which occur naturally in cosmic rays, are fundamental particles similar to electrons but 200 times heavier.
Ellison said this result is exciting and surprising since it is not predicted in the Standard Model, the comprehensive theory that explains the interactions between all fundamental elementary particles.
He explained that the dominance of matter we observe in the universe is possible only if there are differences, called "CP violation," in the behavior of particles and antiparticles.
"The reason this is important is that CP violation -- the fact that physics does not look the same when particles and antiparticles are interchanged and all processes are mirror-reflected -- is one of the three ingredients identified by Andrei Sakharov, the famous Soviet physicist and dissident, needed to explain the matter-antimatter asymmetry observed in our universe," Ellison said. "That the universe is completely dominated by matter is a mystery because the Big Bang theory predicts that there should have been equal amounts of matter and antimatter."
According to Ellison and his DZero peers, the explanation for the dominance of matter in the present day universe is that the CP violation treated matter and antimatter differently and allowed the early universe to evolve into a situation with matter dominating completely over antimatter.
"CP violation as predicted in the Standard Model has been observed before but at a level many orders of magnitude too small to explain the asymmetry," Ellison said. "This is the first evidence for anomalous CP violation. If confirmed by further measurements, this points to new physics phenomena in particle interactions that give rise to the matter-antimatter asymmetry, and may be another step forward in our understanding of why matter dominates over antimatter in the universe."
The DZero result is based on data collected over the last eight years by the DZero experiment at Fermilab. Besides Ellison, the UC Riverside co-authors of the paper, submitted for publication in Physical Review D, are Ann Heinson, Liang Li, Mark Padilla, and Stephen Wimpenny.
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.
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