Antimatter: What is It and Where Did It Go?
Lessons from the SLAC B Factory and This Year's Nobel Prize
SLAC physicist Aaron Roodman kindly agreed to provide this summary of
his October 28 public lecture.
All elementary particles, such as electrons, protons and neutrons, have
an antimatter version. For example, the anti-electron is exactly the
same as the electron, but it has the opposite electric charge; likewise
the anti-proton is just like the proton, again with opposite charge.
But the Universe is just made up of matter, with
virtually no anti-matter. If there were antimatter galaxies or stars there
would be regions of space where antimatter and matter are
annihilating each other, but this has not been observed. Why is there
no antimatter in the universe?
Theory predicts that just after the Big Bang there must have
been equal amounts of matter and antimatter. Early in the Universe
there must have been some interaction between elementary particles that
differed between matter and antimatter, and that produced a small
excess of matter over antimatter. Then all the antimatter annihilated
with most of the matter, leaving a small excess of matter and lots of
energy. That small excess of matter is all the galaxies, stars, planets
in the universe—it is us.
There are four forces in nature: the strong nuclear force,
electromagnetism, the weak force and gravity. But only the weak force differs between antimatter and matter.
It is response for some radioactive decays, but otherwise
has little impact in our daily lives. In 1964, a matter versus antimatter
difference, or asymmetry, was discovered in weak interaction decays of
a subatomic particle called the neutral K meson. In 1973, two Japanese physicists, Kobayashi and Maskawa, proposed that the weak force could cause a matter-antimatter
asymmetry. Their model called for the existence of at least six types of quarks,
though only three were known at the time. It
also predicted that large differences between matter and antimatter
would exist in decays of a specific particle, the B meson, the meson
containing a b quark. Over time, the
other three quarks have been discovered, but the model's predictions for the
B meson could not be observed.
The SLAC B Factory was built to search for matter-antimatter asymmetries
in the B meson. The B Factory's BaBar experiment detected matter-antimatter differences by measuring the decays of both
matter and antimatter B mesons and looking for interference patterns that
could signal asymmetries between the two. The B
meson is five times heavier than a proton, so it can decay literally
thousands of different ways. Because of the many decay possibilities,
it is possible to observe many different matter-antimatter asymmetries at
the B Factory.
The B Factory was designed to answer two key questions: 1) How
does the weak interaction cause a matter-antimatter asymmetry? and 2) Is
the matter-antimatter asymmetry in B mesons just from the weak force, or
is another new force also involved? Using many other weak force
measurements and the Cabibbo-Kobayashi-Maskawa model, BaBar scientists started with a prediction for
the size of matter-antimatter asymmetries in B meson decays, hoping to confirm the CKM model and answer question #1.
Ironically, due to the complicated processes that must be involved, the weak force alone
is not enough to produce the excess of matter
over antimatter in the early Universe. Another force is
needed, one that probably involves more massive and as yet undiscovered
particles. If physicists observe an unexpected matter-antimatter asymmetry,
this new source of asymmetry could be responsible for the disappearance of antimatter.
The SLAC B Factory produced B mesons for nine years, and almost a billion
mesons were observed by the BaBar experiment. The data showed a large matter-antimatter
asymmetry when the neutral B meson decays to the particle J/Psi and a
neutral Kaon. The size of this asymmetry is a very good match to that expected—strong evidence that
the theory of how the weak force
causes matter-antimatter asymmetries is correct. The 2008 Nobel Prize
in Physics was awarded to Kobayashi and Maskawa, who first proposed how the weak
force could explain such asymmetries. The evidence provided by BaBar, and another experiment
in Japan called BELLE, was cited by the Nobel committee as the key proof
of Kobayashi and Maskawa's model. However, since the
source of the preponderance of matter over antimatter in the Universe is still a
mystery. Babar is continuing to explore other matter-antimatter differences.
SLAC Today, December 11, 2008