New form of matter-antimatter transformation observed for first time
Posted on Wednesday, March 14, 2007 @ 21:38:17 UTC by vlad
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 For the first time, scientists of the BaBar experiment at the
Department of Energy's Stanford Linear Accelerator Center (SLAC) have
observed the transition of one type of particle, the neutral D-meson,
into its antimatter particle. Mesons, of which there are about 140
types, are made up of fundamental particles called quarks, which can be
produced when particles collide at high energy.
The new observation will be used as a test of the Standard Model, the
current theory that best describes all the universe's luminous matter
and its associated forces.
The PEP-II accelerator complex at SLAC, also known as the B
Factory, allows the BaBar collaboration to study not only B-mesons but
also several other types of particles including the D-meson. A flurry
of particles in a variety of combinations is produced when electrons
and positrons smash together at high energy in the PEP-II collider
facility.
"Achieving the large number of collisions needed to observe this
D-meson transition is a testament to the tremendous capabilities of the
laboratory's accelerator team," said SLAC Director Jonathan Dorfan.
"The discovery of this long-sought-after process is yet another step
along the way to a better understanding of the Standard Model and the
physics beyond."
One of the most elusive results of this flurry is the
transformation of one particle into its anti-particle in a process
physicists call "mixing." Neutral K-mesons, observed more than 50 years
ago, were the first elementary particles to demonstrate this
phenomenon. About 20 years ago, scientists observed mixing with the
B-meson. Now, for the first time, the BaBar experimenters have seen the
D-meson transform into its anti-particle, and vice versa.
"This is a very exciting moment for us, having found the missing
puzzle piece for particle-antiparticle mixing," said BaBar spokesman
Hassan Jawahery, a physics professor at the University of Maryland.
D-meson mixing is remarkably rare. Of the BaBar experiment's several
billion recorded collisions, this study focuses on about a million
events containing a D-meson decay that are candidates for this effect.
The experimenters found about 500 events in which a D-meson had changed
into an anti-D-meson before decaying.
"SLAC's remarkable combination of a high-intensity accelerator and
a precision detector has had a tremendous impact on our ability to
probe very rare phenomena that are sensitive to the effects of new
physics," said SLAC Deputy Director Persis Drell, who also leads the
Particle and Particle Astrophysics Division.
By observing the rare process of D-meson mixing, BaBar
collaborators can test the intricacies of the Standard Model. To switch
from matter to antimatter, the D-meson must interact with "virtual
particles," which through quantum fluctuations pop into existence for a
brief moment before disappearing again. Their momentary existence is
enough to spark the D-meson's transformation into an anti-D-meson.
Although the BaBar detector cannot directly see these virtual
particles, researchers can identify their effect by measuring the
frequency of the D-meson to anti-D-meson transformation. Knowing that
quantity will help determine whether the Standard Model is sufficient
or whether it must be expanded to incorporate new physics processes.
"It's too soon to know if the Standard Model is capable of fully
accounting for this effect or if new physics is required to explain the
observation," said Jawahery. "But in the coming weeks and months we are
likely to see an abundance of new theoretical work to interpret what
we've observed."
Some 600 scientists and engineers from 77 institutions in Canada,
France, Germany, Italy, the Netherlands, Norway, Russia, Spain, the
United Kingdom and the United States work on BaBar. SLAC is funded by
the Department of Energy's Office of Science.
Source: Stanford University
Via: http://www.physorg.com/news93018835.html
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