Physicists find a new, striking difference between matter and antimatter

By Michael Riordan

An international collaboration of physicists conducting experiments at
the Department of Energy’s Stanford Linear Accelerator Center (SLAC) has
discovered a second fundamental difference between the behavior of matter
and that of antimatter. They observed this intriguing phenomenon — known as
charge-parity (CP) violation and first seen decades ago in experiments with
another particle — in disintegrations of heavy, short-lived subatomic
particles called B mesons. The collaboration reported its result in a paper
submitted July 5 for publication in Physical Review Letters, a leading
scientific journal.

“After 37 years of searching for further examples of CP violation, physicists
now know that there are at least two kinds of subatomic particles that
exhibit this puzzling phenomenon, thought to be responsible for the great
preponderance of matter in the Universe,” said Princeton University physicist
Stewart Smith, spokesman of the collaboration. “We are poised for further
discoveries that should open up new directions for particle physics.”

The international collaboration includes more than 600 scientists and
engineers from 73 institutions in Canada, China, France, Germany, Great
Britain, Italy, Norway, Russia and the United States. They built and have
been operating the sophisticated 1,200-ton detector, named BABAR, which was
used to make the discovery.

The detector records subtle distinctions between decays of B mesons and
those of their antimatter counterparts, called anti-B mesons. Both are more
than five times heavier than protons and survive just over a trillionth
of a second. Physicists employed the detector to observe an unmistakable
difference, or asymmetry, between the rates at which B and anti-B mesons
decay into a special set of specific final states.

From these measurements, they calculated a parameter called sin 2b (sine two
beta), which expresses the degree of asymmetry between matter and antimatter.
A non-zero value of this parameter is clear evidence for CP violation among
B mesons.

In the paper just submitted, the BABAR collaboration reported measuring a
value of sin 2b = 0.59 +/- 0.14, which is substantially different from zero.
There are now fewer than 3 chances in 100,000 that the actual, physical
asymmetry could be consistent with zero.

This BABAR result is easily the most precise measurement of sin 2b reported
to date. Earlier measurements made at Fermi National Accelerator Laboratory,
the Japanese National Laboratory for High-Energy Accelerator Research (KEK),
and at SLAC by the BABAR collaboration are consistent with the present result
but not as accurate. The value just reported agrees with expectations based
on the Standard Model, today’s dominant theory of particle physics.

The precision of the BABAR result was made possible by the outstanding
performance of the PEP-II B Factory at SLAC. Built in collaboration with
the Energy Department’s Lawrence Berkeley and Lawrence Livermore National
Laboratories, this pair of 2.2-kilometer storage rings collides unequal-
energy beams of electrons and their antimatter counterparts, called
positrons. Piermaria Oddone, now deputy director of the Berkeley lab, first
proposed this innovative experimental approach, which greatly enhances the
accuracy of many B meson measurements.

“The B Factory has performed beyond expectations, permitting the BABAR
collaboration to make the world-class measurements on B mesons,” said
SLAC Director Jonathan Dorfan, who played a pivotal role in designing and
building this particle collider. Since it began operating in June 1999, the
B Factory has produced more than 32 million pairs of B mesons, from which
data the present BABAR result was extracted.

The mysterious phenomenon of CP violation was first discovered in a 1964
experiment led by James Cronin and Val Fitch at the Brookhaven National
Laboratory. Their group observed this behavior in decays of subatomic
particles called K mesons, which are about one tenth as heavy as B mesons
and live much longer; the two physicists shared a Nobel Prize for the
discovery.

Several observations of CP violation have since occurred in experiments with
K mesons. But until the recent BABAR discovery, no other subatomic particles
had clearly exhibited this exceedingly rare phenomenon. Having this second
striking example of CP violation should aid theorists trying to understand
what causes it.

Scientists are interested in this puzzling behavior because it can help
explain the abundance of matter in the Universe. In 1967, Russian theorist
Andrei Sakharov used CP violation to suggest how the present matter-
dominated Universe could have emerged from one that contained exactly
equal amounts of matter and antimatter during the earliest moments of the
Big Bang.

The Stanford Linear Accelerator Center is a national laboratory for high-
energy physics and synchrotron-radiation research operated by Stanford
University on behalf of the U.S. Department of Energy. The Department’s
Office of Science funded the construction of the B Factory at $177 million
and contributed about 60 percent of the cost of the BABAR detector, with
the remainder coming from foreign sources. “The foreign contributions
to this experiment, both monetary and scientific, have been absolutely
crucial to its success,” noted former BABAR spokesman David Hitlin of the
California Institute of Technology.

Contact:

Michael Riordan, SLAC

(650) 926-2620, michael@slac.stanford.edu

Comment:

Stewart Smith, SLAC

(650) 926-4775, ajsmith@slac.stanford.edu

Jonathan Dorfan, SLAC

(650) 703-9484, jonathan@slac.Stanford.edu