7/28/00

CONTACT: Michael Riordan, SLAC (650) 926-3990

Background: B Factory Physics

Most people have learned about antimatter from science-fiction television programs such as “Star Trek,” where it is used to power warp drives. Many believe antimatter is as fictional as the warp speeds these drives are supposed to generate, but it is in fact quite real. Antimatter is commonly produced in cosmic rays and by particle accelerators. Its existence was predicted in 1928 by the British physicist Paul Dirac and confirmed by the subsequent discovery of many different antiparticles.

Antiparticles are virtually identical to the ordinary, garden-variety particles of everyday life, but their electrical charge and a few other, more obscure properties are reversed. The electron has a negative electrical charge, for example, while its antiparticle, the positron, has exactly the same mass but is positively charged. Similarly, protons and antiprotons weigh the same but have opposite charges. Most neutral particles, like the neutron, also have antiparticles that are distinctively different even though their electrical charge is the same – zero.

The relationship between a particle and its antiparticle is much like that between a hole and the pile of dirt you get from digging it. In a certain sense, they are opposites, but you make them both at the same time. You cannot have one without the other. Whenever matter and antimatter are created out of pure energy, according to Einstein's famous E = mc2, they appear as particle-antiparticle pairs. Today, antimatter is easily made in the laboratory, at particle accelerators such as the Stanford Linear Accelerator. Although antiparticles are being created continually by cosmic rays, they don't remain around for long because when an antiparticle encounters its opposite number, the two annihilate each other, and their mass reverts back into energy – at least temporarily. This annihilation process provides the greatest amount of energy per unit of mass theoretically possible. If antimatter could be stored in sufficient quantities and its annihilation with matter controlled (two extremely difficult tasks), it might eventually be used as rocket fuel. This is what makes antimatter so interesting to science fiction writers.

Matter, Antimatter and the Big Bang

Since every known particle has an antiparticle, one can conceive of an entire galaxy, solar system or planet made entirely of antimatter. Astrophysicists have in fact searched for antimatter galaxies and even larger structures, by looking for the violent activity that would occur at their boundaries, where annihilations with matter particles would give rise to copious photons. But no such structures have been identified so far, up to extremely large scales billions of light years in extent. We seem to live in a Universe dominated by matter.

This fact leads us to a big mystery. If matter and antimatter can be made or destroyed only in matching amounts, and the laws of physics are exactly the same for both, then how can it be that the Universe contains so much matter but so little antimatter? There has to be some difference in the laws of physics between matter and antimatter.

According to the widely accepted Big Bang theory, matter and antimatter existed in equal, copious amounts when the Universe was a fraction of a nanosecond old. But almost all these particles and antiparticles quickly disappeared in a blaze of mutual annihilation. Astrophysicists estimate that only about one in a billion or so particles survived, evolving into the matter-dominated Universe we know today. The rest ended up as photons in the cosmic background radiation – a uniform bath of microwaves that is the dull afterglow of this cosmic holocaust.

If matter and antimatter were perfect mirror images of one another, physicists argue, they should have annihilated each other completely. Since this is manifestly not the case, experimenters have been searching for intrinsic differences between matter and antimatter that can help explain why some matter managed to survive.

CP Violation

So far, physicists have found only a few subtle differences between matter and antimatter. In the mid-1960s, physicists were studying a subatomic particle called the K meson created by an accelerator at the Brookhaven National Laboratory in New York. They observed a small discrepancy – just a few parts per thousand – in the ways that K mesons and their antiparticles decay. Physicists cite this discrepancy as evidence for a more general phenomenon known as “CP violation,” or charge-parity violation, corresponding to a small but fundamental difference between matter and antimatter. Despite over three decades of searching, however, no other particles had shown similar instances of this phenomenon by the end of the century.

In 1967 the Russian physicist Andrei Sakharov conceived a mechanism by which a tiny excess of matter over antimatter could arise in the early Universe. His idea required, among other things, that CP violation had to occur in the high-energy physics processes going on during the first split nanosecond of existence. Once it was established, that matter excess would eventually go on to form all the galaxies, stars, planets, and people visible today.

The small degree of CP violation observed in K meson decays can be readily accommodated by the Standard Model, today's dominant theory of the elementary particles and their interactions. But it is much too tiny to account for all the matter in the Universe. When theoretical physicists make their best calculations of how and when the matter excess arose, based on the Standard Model, they get far too small a value. Something must therefore be wrong with this theory. In order to test it more stringently, more and better examples of CP violation are required.

Asymmetric B Factories

What experiments, physicists asked, can test whether or not the Standard Model explanation for CP violation is actually correct? The asymmetric B factories now yielding results at SLAC and in Japan were explicitly designed to address this question. They produce copious quantities of short-lived subatomic particles called B mesons, heavy cousins of the K meson, which contain a bottom quark instead of a strange quark. In detailed experiments on these electron-positron colliders, physicists look for differences in the ways that B mesons and their antiparticles decay, which most expect will give additional examples of CP violation.

The two asymmetric B factories each have twin storage rings, one carrying energetic electrons and the other positrons circulating in opposite directions. One distinctive feature is that the energies of these particles are not equal; the electron energy is roughly triple that of the positrons. By clever manipulation using magnets, the circulating bunches of electrons and positrons are brought together inside a large particle detector – known as BaBar at Stanford and Belle in Japan. Here electrons meet positrons and annihilate with them, often producing pairs of B mesons and their antiparticles (called “B-bars”). The detector records the decays of these B and B-bar particles into other, lighter particles.

Because of the energy difference between the two clashing beams, the B and B-bar travel in the direction of the electron beam with known speeds until they each disintegrate about a trillionth of a second after their birth. The distance (and hence the time) between their two individual decays is determined by a precision tracking device at the heart of the detector that is accurate to about 50 microns, or a few times the width of a human hair. Knowing this time difference is essential for physicists who wish to measure any asymmetries in B and and B-bar decays. If found, such asymmetries will give additional evidence for CP violation.

Because such asymmetries are expected to arise only in relatively rare decay modes, physicists need to create many millions of pairs of B mesons in order to make convincing measurements. Certain special “golden events” (in which a neutral B or B-bar decays into a J/psi particle and a short-lived neutral K meson), for example, occur only about once in every 100,000 decays. If Nature were completely symmetric, neutral B and B-bar particles would both transform into this state at equal rates; any difference between the rates of these two processes would be solid evidence for CP violation. Earlier measurements at Fermilab have suggested that such an asymmetry exists and that it may fall within the range of values expected in the Standard Model, but the experimental errors are large. If the measurements at the B factories fall outside this range, there will be great excitement, as further measurements continue and theorists attempt to account for this result.

Whatever the outcome, the study of CP violation and other B meson physics has only just begun at the B factories. A long program of other measurements is scheduled, aimed at examining still rarer and more difficult-to-observe modes of B meson decay for additional evidence of CP violation. Only after detailed study of all these modes and comparison with Standard Model predictions will physicists have the answers they seek. These answers will certainly improve our knowledge of the fundamental laws of Nature. They also may stimulate some completely new ideas about how matter came to dominate over antimatter in the Universe.

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By Michael Riordan and David Salisbury