Dec. 4, 1998
Contact: Fariss Samarrai
(804) 924-3778


LOOKING FOR TIME REVERSAL ASYMMETRY

	Searching for asymmetry between matter and antimatter explores
the very frontiers of the universe as well as its origin.  After
trawling the oceans of data produced at the Fermi National Accelerator
Laboratory (Fermilab) in 1996-97, a team of University of Virginia
physicists have found new evidence of a violation of the symmetry of
interactions under reversal of the direction of time.  
	A general theorem of physics known as the CPT theorem posits
that in any
reaction the simultaneous operation of changing particles to
antiparticles (C), mirroring the coordinate system (P) results in a
modified reaction that reverses the direction of time (T), which should
run at the same rate as the original reaction.  If the application of
any two of the three operations does not result in a reaction that runs
at the same rate, then the third operation must compensate in just the
right amount so that overall the CPT theorem is preserved.  Previous
experiments observed CP (charge-parity) violation and had to infer that
there was also a time reversal violation.
	Essentially they generate the temperature levels present shortly
after the Big
Bang in order to create a huge number of kaons.  The physicists study
the kaons, and search for time reversal violation in their decays. This
phenomenon was first hypothesized to be a necessary condition to explain
the existence of our universe which is dominated by matter with very
little antimatter present.  It was first developed by Soviet dissident
Andrei Sakharov, known as the father of the hydrogen bomb. In 1980
Cronin and Fitch won the Nobel prize for Physics for their observation
of CP violation.  Only four such observations have been made so far,
including the latest work by the U.Va. group.
	The physicists use the Fermilab facilities outside Chicago to
create an intense kaon beam by protons hitting a target. Kaons produced
in these collisions travel through the experimental apparatus and decay,
and then they are analyzed for decays with asymmetric distribution,
indicating a violation of time reversal symmetry.
	It can be shown that time reversal violation implies that
particles and
antiparticles behave slightly differently, according to Brad Cox,
principal investigator at the U.Va. High Energy Physics Laboratory and a
key investigator for the Fermilab experiments.  "We think that in the
beginning of the universe there was just enough difference in behavior
of particles and antiparticles so that a small excess of particles
survived that made it possible for our present universe to exist.  This
small excess is what makes up the matter of the universe, the stars and
galaxies, and everything else that we observe around us."
	Through the KTeV (kaons at the Tevatron) experiment conducted at
Fermilab, the first direct observations of time-reversal asymmetry were
reported in October 1998.  The time reversal phenomenon is a rare event
even at the sub-atomic level, and contradicts our every day experience.
If you were to film events involving the effects of gravity,
electro-magnetism or other strong forces, you would see that these run
at the same speed whether you viewed them running
forward or if you reversed the film. This is because gravity,
electromagnetism and the strong nuclear force are time-symmetric.
Viewing the results of the experiment shows that this weak force of
nature apparently allows for an exception. So far, a time reversal
effect of thirteen percent has been observed in the weak decays of
kaons.
	Cox and his colleagues are working in collaboration with
physicists from
the University of Chicago, Colorado, UCLA, Rutgers, Rice, Arizona,
Wisconsin
and from Osaka (Japan), and with physicists from Fermilab.  The research
is funded by the U.S. Department of Energy.  The latest findings have
been able to produce this evidence for time reversal violation because
their experiment is one thousand times more sensitive than previous
experiments.   ###