March 1, 1999
NEWS MEDIA CONTACT:
Luann O'Boyle 630-840-3351 (Fermilab)
luann@fnal.gov
For immediate release
FERMILAB PHYSICISTS FIND NEW MATTER-ANTIMATTER ASYMMETRY
Batavia, Ill. Scientists working at the Department of Energy's Fermi
National Accelerator Laboratory have announced a significant advance in
the understanding of the difference in the way matter and antimatter
behave. Moreover, the physicists, from Fermilab's KTeV experiment, said
they were ``shocked'' at the size of the long-sought result they reported
to a standing-room-only audience at a seminar at Fermilab on February
24. Indeed, there was an audible gasp from the audience of physicists
when University of Chicago graduate student Peter Shawhan gave the
group's observed value for a phenomenon called ``direct CP violation.''
``Our result,'' Shawhan said, ``is that epsilon prime over epsilon equals
28 plus or minus 4.1 times 10 to the minus 4.''
What did Shawhan mean, and why were physicists so surprised by what he
said?
Antimatter has a habit of surprising physicists, beginning with its
discovery in 1932, when physicist Carl Anderson first observed the
puzzling track of an anti-electron, or positron, in a cloud chamber.
Today, the prevailing theory of the fundamental structure of matter, the
Standard Model, holds that every particle of matter has a corresponding
antiparticle of antimatter. (Just as the antiparticle of the electron is
the positron, for example, every quark has an antiquark.) Early in the
evolution of our universe, matter and antimatter were equally abundant;
but today the universe appears to be made entirely of matter. Antimatter
shows up only in cosmic ray interactions and at particle accelerators
such as Fermilab's Tevatron, where antiparticles are produced in
high-energy particle collisions.
Among the collision-produced particles are mesons. Unlike long-lived
protons and neutrons, which are combinations of three quarks, mesons are
short-lived pairings of a quark and an antiquark. Certain mesons, known
as neutral kaons, are combinations of a strange quark or antiquark and a
down quark or antiquark. In 1964 scientists got another antimatter
surprise when a team led by physicists James Cronin and Val Fitch,
studying neutral kaons in experiments at DOE's Brookhaven Laboratory,
discovered a slight but definite asymmetry in the behavior of the
neutral kaon and its antiparticle an asymmetry called charge-parity,
or CP, violation. Until that discovery, physicists had believed that
particles and antiparticles behaved symmetrically, like mirror
reflections of each other.
``We were attempting to make a much better test of CP invariance,'' said
Fitch, who with Cronin was awarded the Nobel Prize for the discovery,
``and it turned out not to be invariant.''
This original CP-violating effect can be described as an asymmetry in
the mixing (or quantum-mechanical fluctuation) of the neutral kaon with
its antiparticle. Other manifestations of CP violation have been firmly
established at many laboratories around the world in the years since its
discovery, but they could all be traced to this original effect. Among
theories proposed to explain CP violation is the Superweak Theory, which
posits only mixing effects, with no CP violation in the decays of
neutral kaons into other particles.
Yet ever since 1964, physicists at laboratories around the world have
been attempting to observe an asymmetry in the DECAY, rather than the
mixing, of the neutral kaon. To do so, they have attempted to measure
the ratio e'/e, ``epsilon prime over epsilon,'' a ratio of different modes
of decay of neutral kaons into two pi mesons, or pions. If they found a
value different from zero, it would signal a new direct form of CP
violation.
``The Standard Model, if it correctly accommodates CP violation,
predicts a non-zero, but small, effect,'' said University of Chicago
physicist and KTeV cospokesman Bruce Winstein. ``But experiments up
until now had not firmly established such an effect. An experiment at
CERN, NA31, led by Heinrich Wahl, reported a significant effect 23 x
10-4, with a precision of 3.5 standard deviations, but that was not yet
enough to definitively say it was non-zero. Furthermore, a previous
Fermilab experiment saw an effect about three times smaller than the
CERN experiment, and not far enough away from zero to confirm the CERN
effect.''
The gasp at KTeV's new result came because it established the existence
of direct CP violation beyond reasonable doubt (almost 7 standard
deviations), and because it was much larger than anyone, including the
experimenters, had expected. The finding definitively rules out the
Superweak Theory as the sole source of CP violation. And, while the
Standard Model predicts a non-zero effect, the size of the KTeV result
raises questions about whether it can be accommodated within the
Standard Model.
Fitch, now professor of physics at Princeton University, summed up
reaction to the announcement: ``It is a most astonishing result. It is
quite unexpected, and very, very interesting.''
Secretary of Energy Bill Richardson shared the excitement of the
Fermilab scientists. ``Thirty-five years ago, scientists at a DOE
laboratory discovered a difference in the way matter and antimatter
behave,'' Secretary Richardson said. ``Now physicists at DOE's Fermilab
have taken a giant step in our understanding of this asymmetry between
matter and antimatter. I am proud of the role of the Department of
Energy in advancing the worldwide exploration of the way our universe
works at the most fundamental level.''
KTeV's Winstein pointed out that the new result, which is based on
analysis of only about 20 percent of the collaboration's total data from
a 1996-1997 physics run, is much more consistent with the earlier CERN
result than with previous results from Fermilab. ``We are excited to have
established direct CP violation,'' Winstein said, ``but we also want to
emphasize that CERN's NA31 deserves a share of the credit.''
To account for the difference from their previous results, the KTeV
analysis team has intensely scrutinized the earlier Fermilab
measurement, but, says Winstein, ``we have found nothing that could
account for the difference, other than an unlikely but still possible
fluctuation. We are eagerly awaiting the next results from our
colleagues at CERN in experiment NA48. That experiment has significant
strengths that complement KTeV's, and we expect them to report soon on
data they have already taken. And the physics community awaits the
results of a completely different approach taken by the KLOE experiment
at Frascati, in Italy.''
Cronin, co-discoverer of CP violation in 1964 and professor of physics
at the University of Chicago, confirmed the significance of the KTeV
announcement.
``It's been thirty-five years since CP violation was discovered,'' Cronin
said. ``This is the first time that we have finally learned something
new. It doubles our knowledge of CP violation; now there are two
parameters instead of only one. Until now, we could explain everything
in terms of slight kaon mixtures, but not any more. It's just
sensational!''
The KTeV experiment (for Kaons at the Tevatron) is an 85-member
collaboration of experimental groups from the University of Arizona, the
University of California at Los Angeles, the University of California at
San Diego, the University of Chicago, the University of Colorado,
Elmhurst College, Fermilab, Osaka University, Rice University, Rutgers
University, the University of Virginia, and the University of Wisconsin.
The new experiment began construction in 1992 and took its first data
1996. It used a beam of protons from Fermilab's Tevatron to create two
parallel beams of neutral kaons to search for CP violation. An
innovative particle detector constructed of cesium iodide crystals gave
experimenters unprecedented precision in making experimental
observations. Other technological innovations allowed the collaborators
to rule out background events and collect data at very high rates.
KTeV cospokesman Bob Hsiung, a Fermilab physicist, said the startling
KTeV result ``means that experiments in the near future should be able to
observe sizable CP-violating effects first in the mixing of B mesons and
then in their decays, as well as in rare kaon decays.''
Fermilab is operated by Universities Research Association, Inc., under
contract with the U.S. Department of Energy.