WHEN Alice stepped through the looking-glass in Lewis Carrolls classic childrens story, she found a world populated by talking chess pieces and bread-and-butterflies. The standard model of particle physics is hardly less strange. It says that there is indeed a looking-glass world, which is a near-perfect mirror image of this one. This week, physicists at the Stanford Linear Accelerator Centre (SLAC) in America and KEK, a high-energy physics laboratory in Japan, have unveiled the first results from experiments meant to test this theory. And, in doing so, they have come a step closer to finding out why the universe of people, planets and stars, rather than shadowy reflections of such things, exists at all.
The standard model explains how the fundamental building-blocks of the universe fit together. Among other things, it says that, for every sort of particle that matter is made of, there is an antiparticle, which is like a mirror image of its this-world counterpart, with the same mass but opposite in all other respects (such as electric charge). In other words, the looking-glass world is related to this one by two fundamental symmetries of nature: charge and parity (handedness). Occasionally, however, the standard model allows these symmetries to break down, in a phenomenon known as charge-parity (CP) violation. This is a good thing, as otherwise the universe would not be here at all.
CP violation helps to explain a troubling astrophysical conundrum. If the looking-glass world were a pure one, matter and antimatter would always be perfect opposites of one another. In that case there would have been equal amounts of them at the beginning of the universe and they would have annihilated each other completely in a burst of light shortly afterwards. That did not happen. Some matter was left over. So there must have been an asymmetry that made more matter than antimatter in the first place. Physicists believe that it was the symmetry-breaking inherent in CP violation that allowed matter to beat antimatter in the primordial race for survival. By the time the universe was a billionth of a second old, it was all over, and matter had won.
The standard model is not that clever, however. The amount of symmetry-breaking that it allows underestimates the amount of matter in the universeby a factor of 100 billion. The first evidence that such an asymmetry does indeed occur came in 1964, when physicists explored the decay of electrically neutral particles called kaons. Since then, they have been searching for additional sources of CP violation that could beef up the numbers. It is for this purpose that the SLAC and KEK experiments were built.
SLAC and KEK are what are known in the business as B-factories. These are particle accelerators that smash together electrons and their antiparticles (called positrons) at very high speeds, to create a shower of new neutral particles called B-mesons, or B0s for short. A B0 is made up of two quarks (fundamental bits of matter) or, more precisely, a heavy, whimsically named bottom antiquark and a quark that can be one of several other types. A B0 also has a looking-glass-world antiparticle, called an anti-B0. Neither of them tends to hang around for very long. After a mere billionth of a second or so, B0s and anti-B0s disintegrate into other sorts of bizarre species that stock the cosmic zoo.
The ones that particularly interest the physicists at SLAC and KEK, however, pop up through a rare route known as the golden channel, which happens around once in every 50,000 decays. The golden channel results in two particles called a charmonium (or j/psi) and a K-short. The charmonium and the K-short have the interesting property of being their own mirror-images, and it is this that provides a test of the symmetry-breaking anomaly of CP violation.
If the world of matter and the looking-glass world of antimatter were exactly equal and opposite, then the decay of a B0 into a charmonium and a K-short should perfectly mirror the decay of an anti-B0 to a charmonium and a K-short. This should show up in equal rates for the two decays. It is rather like rolling a ball down a hill: if two sides of the hill are symmetrical, the ball should take the same amount of time to roll to the bottom on either. A difference in decay time provides a measure of the amount of symmetry-breaking. It also provides a test of whether the standard model is right. Since they would like to fix the small problem of the amount of matter in the universe, the physicists at SLAC and KEK have been hoping that it is not.
This weeks results hint at that. They suggest that B-meson decays may require another source of CP violationand so the standard model is by no means the end of the story. This is exciting. But the physicists cannot heave a sigh of relief, pack up and go home just yet, as the numbers are far from unequivocal. The errors at this stage are so large that the results could still be in line with the standard model, or even be consistent with no symmetry-breaking at all. There is only one way to find out: continue churning out B0s, and wait for the smoke to clear.