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Steve Chaplin
University Communications

Last modified: Thursday, May 20, 2010

IU physicists, in DZero Collaboration, announce evidence of matter-antimatter imbalance

May 20, 2010

BLOOMINGTON, Ind. -- Physicists at Indiana University are joining fellow DZero Collaboration researchers from around the world in announcing evidence of a 1 percent deviation between the amounts of elementary matter and antimatter particles being produced from high-energy collisions at the U.S. Department of Energy's Fermi National Accelerator Laboratory.

CP Violation

Image by Fermilab

A strong magnetic field inside the DZero particle detector forces muons that emerge from proton-antiproton collisions to travel along a curved path. Two muons with opposite charge follow paths that curve in opposite direction (see graphic). Scientists first compared the muon distributions when the the magnetic field inside the DZero detector pointed in one direction (configuration 1) and then compared their distributions when the magnetic field had been reversed (configuration 2). If the matter-antimatter symmetry were perfect, the comparison of the muon distributions in the two configurations would yield the same result. Instead, the DZero experiment observed a one-percent deviation, evidence for a matter-antimatter asymmetry.

Print-Quality Photo

IU Department of Physics chair and DZero collaborator Rick Van Kooten, one of four professors and research scientists from IU Bloomington working on the experiment, called new evidence of a deviation of the Standard Model of particle physics "unexpected" and "a surprise." He described the 1 percent deviation as "huge, and definitely not seen in general particle collisions."

"We are very excited about this discovery as it is truly unexpected," he said. "This observation that particles can not only quantum mechanically change into their own anti-particles and vice versa, but also that the two processes are not equally likely at this level, is a surprise."

The Standard Model, in fact, does predict very tiny symmetry violations, "but nowhere at the level observed here," Van Kooten noted.

This new evidence of a 1 percent violation of matter-antimatter symmetry in the behavior of particles could help explain the dominance of matter over antimatter in the universe through understanding behavioral differences in particles and antiparticles.

Specifically, researchers found that after neutral B-mesons, including one oscillating into an anti-B meson, decayed following production from high-energy collisions, what remained was about a 1 percent difference in the resulting pairs of muons and antimuons. The difference could confirm contrasts in the properties of particles and antiparticles that physicists have long believed existed.

And that 1 percent difference could be the evidence that leads to recognizing new physics phenomena explaining the observed dominance of matter over antimatter, Van Kooten said. The new findings were made public May 17 by the DZero collaboration, which constitutes an international team of 500 physicists, and have been submitted for publication in the journal Physical Review D.

Also representing IU in the DZero Collaboration are professors Hal Evans and Sabine Lammers and research scientist Daria Zieminska. Zieminska was working on a similar analysis that was showing hints of the same anomalous violation when the announcement was made.

"For me it's doubly exciting," she said. "Our group is looking at the same particle, at its decay into a pair of lighter particles, instead of muons, and we see the very same trend."

Scientists said there is less than a 0.1 percent probability that the results can be explained by an already known effect.

The work was conducted using Fermi National Accelerator's Tevatron high-energy particle collider where protons are sent through a magnetic course to be smashed into one another at high energies. The result reported this week was based on data collected by the DZero experiment over the last eight years using over six inverse femtobarns in total integrated luminosity, which corresponds to hundreds of trillions of collisions between protons and antiprotons in the Tevatron collider. Prior to start-up of the Large Hadron Collider in Europe last winter the Tevatron was the world's most powerful particle collider.

To speak with Indiana University's representatives on the DZero Collaboration, please contact Steve Chaplin, University Communications, at 812-856-1896 or