What's the matter with antimatter?


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.

Is antimatter real science or science fiction?

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".

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.

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).

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?

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?

One survivor for every billion.

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 universe and the particles.

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.

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

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.

Making quarks

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 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

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.

To expose this behavior, the physicists conduct the following analysis:

  • 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.

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.

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!

What does this mean for me?

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.

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?

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.

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.

Abner Soffer
Colorado State University