July 24, 2002 -- At
the International Conference on High Energy Physics in Amsterdam
on July 25, the BaBar collaboration working at the the Department
of Energy's Stanford Linear Accelerator Center (SLAC) will announce
their new measurement of a parameter known as sin 2b (sine two beta)*,
that expresses the degree of asymmetry between matter and antimatter.
This result is the product of three years of intense research and
is a perfect example of how understanding of our universe advances
through high energy physics research.
"This research at
SLAC is a wonderful demonstration of the scientific method."
said Raymond Orbach, director of the Department of Energy's Office
of Science. "What was a theoretical prediction until recently,
has been measured with exquisite accuracy by the BaBar experiment,
providing a solid platform of understanding to allow physicists
to formulate the next question. This is how science advances."
Something is missing in physicists' understanding of how our universe
evolved into its current state. At the Big Bang, equal quantities
of matter and antimatter should have been created, and subsequently
annihilated each other leaving nothing but energy. However the matter
universe is here as undeniable proof of the victory of matter over
antimatter in this initial cosmic encounter. To establish experimentally
the effect that allowed matter to dominate has been a central theme
in high-energy physics research. The BaBar experiment at SLAC and
the Belle collaboration at the KEK laboratory in Japan have been
working relentlessly to tie down an exact measurement of this effect
called Charge Parity (CP) violation.
Both experiments follow
the infinitesimally short lives - a trillionth of a second - of
particles called B mesons and those of their antimatter counterparts,
called anti-B mesons or "B-bars." Any difference in behaviour
of these otherwise exact opposites indicates a difference between
matter and antimatter and confirms the existence of CP violation.
The first results, announced in the summer of 2001(SLAC Press Release
July 26, 2001, BaBar Physicists Find a Striking Difference Between
Matter and Antimatter) gave clear evidence for CP violation in B
"This was a major
discovery but much more data was necessary to turn sin 2b into a
fundamental constant of particle physics, " said BaBar science
team leader Stewart Smith of Princeton University. "The new
result from BaBar for sin2b is 0.74 ± 0.07. This result comes
after three years of intense research and analysis of 88 million
events. It is the fruit of tremendous effort from the 500 collaborators
on the BaBar experiment and the excellent professional abilities
of the SLAC accelerator team. The accelerator performed remarkably
allowing us to do outstanding physics, and the conditions at SLAC
were ideal for conducting world-class science."
The new measurement from
BaBar fits the theoretical expectations based on the "Standard
Model" that explains subatomic particles and their interactions.
on BaBar has been tremendously exciting," said Hassan
Jawahery of the University of Maryland, the
experiment's physics coordinator. "Only three years ago CP
violation in B mesons was just a plausible scenario, but now the
precision of our results has anchored CP violation as one of the
foundations of the standard model. This is the beauty of doing science.
We have moved from obscurity to clarity in a very short time and
have created knowledge that will remain forever."
The millions of B and
anti-B meson events that the physicists needed to hone the accuracy
of their measurement were produced in collisions between beams of
electrons and their antimatter counterparts, called positrons, in
storage rings called PEP-II.
"PEP-II and BaBar
completed the longest known run of data collection at a colliding
beam facility ever-18 months of exceptional performance," SLAC
director Jonathan Dorfan said. "PEP-II delivered its aggressive
100 million events goal, set when the accelerator was first switched
on in July 1999. This is a remarkable achievement for such a 'young'
machine. BaBar logged over 95% of the PEP-II data - an unprecedented
achievement by a detector of that complexity. Sustaining that level
of performance for 18 months was heroic."
The precision of BaBar's
new result derives from the huge amount of data flooding out of
the detector. The experiment uses the largest database in the world
(SLAC Press Release April 12, 2002, World's Largest Database Reaches
"At SLAC, a Department
of Energy Laboratory, we have created a truly international scientific
collaboration," commented Smith. "The success of BaBar
is based on the power and efficiency of collaborative science. More
than 500 scientists and engineers from 75 institutions in Canada,
China, France, Germany, Italy, Norway, Russia, the Unied Kingdom,
and the United States are working on BaBar. Great advances recently
in our ability to distribute data around the collaboration have
had enormous impact. Our data is now analyzed not only in the United
States, but at linked major computer centers in France, Italy and
the United Kingdom as well."
The accuracy of the CP
violation measurements coming from BaBar and Belle has established
the magnitude of the effect beyond doubt. However, this knowledge
shows that the degree of CP violation now confirmed is not enough
on its own to account for the matter-antimatter imbalance in the
happened in addition to CP violation to create the excess of matter
that became stars, planets, and living creatures," said Jawahery.
"In the future, the BaBar experiment allows us to examine rarer
processes and more subtle effects that will give us an even clearer
understanding and may point us towards the processes which caused
our universe to evolve into its current state. It is a very exciting
For more about SLAC, the BaBar Collaboration, and the BaBar database:
http://www.slac.stanford.edu/ and http://www-public.stanford.edu/babar/
Photographs of the B
Factory and BaBar detector can be found at: http://www.slac.stanford.edu/slac/media-info/babarphotos.html
All photo credits should be "Photo: SLAC.