What's the matter with antimatter?
- First, they select "events" in which they see one of the heavy mesons undergoing the desired decay. This is done by looking at all particle signatures in the detector and determining which combinations of particles may have been produced in the decay of interest, given the constraints imposed by Einstein's theory of relativity.
- Next, they analyze the decay products of the other meson to determine whether it was a B or a B-bar. This process is called "tagging", and it makes use of the fact that a B-bar meson decay tends to produce a certain particle, such as an electron, whereas the decay of a B usually produces the corresponding anti-particle, such as a positron.
- Third, by measuring the points of origin of the decay products of the two mesons, they can find the distance between them, which is typically about a quarter of a millimeter. They divide this distance by the velocity with which the mesons move, to obtain the difference between their decay times, known as dt, which is typically about a picosecond.
- Finally, they plot the number of events observed in different ranges of dt.
Introduction
Physicists at the Stanford
Linear Accelerator Center (SLAC) in California and the
High Energy Accelerator Research Organization (KEK) in Japan are colliding
particles and anti-particles at high energies to study minute
differences between the ways matter and antimatter interact. Their
goal is to contribute to our understanding of the workings of the
universe at its largest and smallest scales, from revealing the origin
of matter shortly after the Big Bang, to
uncovering the secrets of elementary particles and their interactions.
Anti-particles and antimatter
may sound like the stuff of science fiction, but for particle
physicists it's something they work with every day, just as a
carpenter works with nails. After decades of particle physics
experiments, we now know that every type of particle has a
corresponding antimatter particle, called an anti-particle. A particle
and its anti-particle are identical in almost every way - they have
the same mass, for example - but they have opposite charges. The
existence of the positron, the positively charged anti-particle of the
negative electron, was first hypothesized by Dirac in 1928. Its
existence was experimentally proven in 1933 by Anderson, who received
the 1936 Nobel
Prize for this achievement. Since then, physicists have discovered
the anti-particles of all the known elementary particles, and have
even been able to combine positrons with antiprotons to make antihydrogen "antiatoms".
From the physicists' point of
view, what is strange about antimatter is
that we don't see more of it. When we collide high-energy particles in
accelerators,
their energy is converted into equal amounts
of matter and antimatter particles (recall that according to
Einstein's famous formula, the energy (E) it takes to create matter
and antimatter of total mass (m) is E=mc^2). Since we see matter and
antimatter created in equal amounts in particle experiments, we expect
that shortly after the Big Bang, when the universe was extremely dense
and hot, equal amounts of matter and antimatter were created from the
available energy. The obvious question is, therefore, where did the
antimatter go?
Based on numerous
astronomical observations and the results of particle physics and
nuclear physics experiments, we deduce that all the matter in the
universe today is only about a billionth of the amount of matter that
existed during the very early universe. As the universe expanded and
cooled, almost every matter particle collided with an antimatter
particle, and the two turned into two photons - gamma ray particles -
in a process called annihilation, the
opposite of pair production. But roughly a billionth of the matter
particles survived, and it is those particles that now make the
galaxies, stars, planets, and all living things on Earth, including
our own bodies.
The survival of a small
fraction of the matter particles indicates that, unlike what we wrote
above, matter and antimatter are not exactly identical. There is a
small difference between the ways they interact. This difference
between matter and antimatter was first observed in particle
accelerators in 1964 in an experiment for which Cronin and Fitch were
awarded the 1980 Nobel Prize,
and its connection to the existence of matter in the universe was
realized in 1967 by Sakharov. The new generation of
experiments at SLAC and KEK, called BaBar and Belle, offer new tools with which to
probe the nature of CP violation, hopefully shedding light on what
happened a tiny fraction of a second after the Big Bang, and expanding
our understanding of elementary particles and their interactions.
These experiments work as follows: a particle
accelerator accelerates electrons and positrons to high energies.
They are then "stored" in bunches of about a hundred billion particles
each, running around in a circular accelerator called a storage ring
at about 0.99997 of the speed of light. Electrons are made to go one
way, and positrons go the other way, so that the bunches cross through
each other every time they go around the ring.
On some bunch crossings, a
positron and an electron come close enough to collide, and the high
energy that they have been given by the accelerator turns into a new
particle & anti-particle pair: a B meson and its anti-particle,
called a B-bar meson (mesons are particles composed of a quark
and an anti-quark). These mesons undergo radioactive decay within
about a picosecond (a trillionth of a second). Because they are quite
heavy - their mass is about five times that of the proton - they can
decay in numerous ways into different combinations of lighter
particles. Physicists detect differences
between matter and antimatter and determine the strength of CP
violation by measuring the ways the B and the B-bar decay. For
example, decays into particular sets of particles exhibit a peculiar
time structure which is different for B and B-bar decays. A plot appears to the left,
with events in which the other Meson was "tagged" as a B shown in red,
and those in which it was "tagged" as a B-bar shown in blue. You can
see that the plots are not the same: events with a B tag tend to have
a larger dt than events with a B-bar tag. This subtle difference is
exactly what we are looking for: this is CP violation, observed for
the first time in almost four decades!
Using their results, BaBar
and Belle have measured with high precision a parameter called sin(2
beta), which describes part of the mechanism thought to be responsible
for CP violation. According to our understanding of particle physics,
if sin(2 beta) had been equal to zero, there would have been no CP
violation, and matter and antimatter would have been identical.
Recalling that the difference between matter and antimatter is
necessary for the existence of matter in the universe today, a zero
value for sin(2 beta) would have meant that the universe would have
been a totally different place, with no stars or planets, not even
people to ponder the mysteries of the universe and the underlying laws
of physics. Having measured sin(2 beta),
BaBar and Belle are now collecting more data about B and B-bar decays
and measuring more CP violation parameters, to improve our
understanding of the difference between matter and antimatter.
Is antimatter real science or science fiction?
Matter and antimatter are created together.
Fig 1. Particles
and antiparticles (such as the pair highlighted in pink) are created
in pairs from the energy released by the collision of fast-moving
particles with atoms in a bubble chamber. Since particles and
antiparticles have opposite electrical charges, they curl in opposite
directions in the magnetic field applied to the chamber.
For example, you
can see matter and antimatter particles created in the bubble chamber
photo on the left. The photo shows many bubble tracks generated by
charged particles passing through superheated liquid. Due to the
magnetic field applied to the chamber, positive particles curl to the
right and negative particles curl to the left. The two curled tracks
highlighted in pink show an electron-positron pair created by the
collision of a gamma ray photon
(a highly energetic particle of light) with an atom in the chamber, in
a process called pair production.
(Can you find additional electron-positron pairs in this photo?
Learn
more about bubble chambers)
Where did all the antimatter go?
One survivor for every billion.
The universe and the particles.
Physicists call this difference
CP violation. It sounds like fancy jargon,
but it just means that if you are conducting a particular experiment
on particles, from which you deduce a certain theory of the laws of
physics, then conducting the same experiment on anti-particles would
lead you to deduce different laws. The only way to end up with a
consistent set of physical laws is to incorporate the
matter-antimatter difference into your theory. Because this difference
is small, conducting any old experiment would not reveal it. For
example, if your experiment involves gravity, you would find that
apples are attracted by massive bodies like the earth, and that
anti-apples are also attracted by massive bodies. So gravity effects
matter and antimatter identically, and this experiment would not
reveal CP violation. A much more sophisticated experiment is required.
Sophisticated experiment
Making quarks
Physicists have built a living-room size detector
(see
pictures) around the collision point in order to detect the
lighter particles which are produced in the decay of the two mesons.
These detectors allow them to identify the types of particles
produced, measure their momenta and energies, and trace them to their
points of origin to within less than a 10th of a millimeter.
The huge amounts of data collected by the detectors is stored
in large databases and analyzed by computer "farms" with many hundreds
of computers. Together, BaBar and Belle have produced almost 300
million B meson & B-bar meson pairs, and physicists around the
world are hard at work analyzing the mountains of data and publishing
their results. 300 million is a large number, but when it comes to
some CP violation measurements, it can be barely enough.
Measuring CP violation
To
expose this behavior, the physicists conduct the following analysis:
How to see it
Fig 2. The difference between
the red and the blue lines shows the difference in how a
particle and its antiparticle behave. This is CP violation, and
indicates that matter and anti-matter are not exactly opposites.
What does this mean for me?
Particle physicists are motivated to study CP
violation both because it's an interesting phenomenon in its own
right, and because it is intimately related to the universe as a whole
and to our very existence within it.
What next?
More data is coming from other experiments as well. Physicists
at the CDF and D0 experiments in Fermilab are also studying the decays
of B mesons produced in collisions of protons with anti-protons.
Additional experiments using the Large
Hadron Collider at CERN, which will produce
B mesons copiously by colliding protons with protons at even higher
energies, are scheduled to begin operation in a few years.
There are many open questions that these experiments seek to
address. Some of the most intriguing questions are prompted by the
fact that the matter-antimatter difference we see in the laboratory
appears too small to be solely responsible for all the matter in the
universe today.
This suggests that there may be additional
differences between matter and antimatter, additional sources of CP
violation that we have not been able to detect yet, but which could
have played an important role during the very early universe, when
most matter and antimatter annihilated and a small fraction of the
matter survived.
Physicists are searching for these unknown CP
violation effects. We never know what exactly this quest will yield,
but as has always been the case in the history of particle physics, we
expect to learn a great deal about nature in the process.
Colorado State University
abi@slac.stanford.edu