One of the biggest mysteries in physics is why the universe is made entirely of matter, even though equal amounts of matter and antimatter should have been created during the big bang. All the matter and antimatter particles should have annihilated with each other since then, leaving only photons, but somehow one matter particle in a billion or so has survived to create the universe as we know it. Physicists at the BaBar experiment at Stanford in the US and the Belle experiment in Japan have now, for the first time, directly measured the amount of matter-antimatter asymmetry allowed by the Standard Model of particle physics.

“The result determines directly for the first time the magnitude of the fundamental matter-antimatter asymmetry in nature,” says Paul Harrison of Queen Mary College in London, who chairs the UK’s BaBar steering committee. However, as expected, the asymmetries measured by the experiments are not large enough to explain why matter dominates the universe. Physicists will therefore have to look beyond the Standard Model – which can predict the results of most particle-physics experiments to high precision – for an explanation.

What’s the difference

A process called charge-parity (CP) violation is responsible for the difference between matter and antimatter in the Standard Model. CP violation means that the laws of physics change slightly when a particle is replaced by its antiparticle and when all three directions in space are reversed. CP violation was first detected in kaons in 1964, and BaBar and Belle are the first experiments to detect it in another type of particle – the B meson. “The discovery of CP violation in the B system is an outstanding achievement,” says Matthias Neubert, a particle theorist at Cornell University in the US. “The particular significance of the result is that, for the first time, a large CP asymmetry predicted by the Standard Model has been observed.”

CP violation in the Standard Model can most easily be explained in terms of a triangle, with the amount of violation being proportional to the area of the triangle. The base of this “unitarity triangle” is one unit long, so physicists need to measure the values of two other lengths or angles to calculate its area. And the more values they measure, the better they can test the model.

Experimental results are commonly expressed as the sine of 2 ß, where ß is one of the angles. If there is no asymmetry, then sin2ß should be zero. On 6 July, the BaBar team report that sin 2 ß = 0.59, with error bars of 0.14 (arxiv.org/abs/hep-ex/0107013). There is only a 3 in 100 000 chance that the effect is due to statistical fluctuations. And on 23 July the Belle experiment at the KEK laboratory in Japan reported a value of sin 2 ß = 0.99 ± 0.14.

Both colliders have been built to operate as “B Factories” and produce large numbers of B mesons – particles that contain a bottom quark and an anti-down quark – and anti-B mesons. The two collaborations measured sin2ß by detecting the decay of the B particles into J/Psi particles and neutral kaons. CP violation means that, for this particular channel, the B mesons decay slightly slower than their antiparticles.

The Standard Model does not actually predict a value of sin2ß. Rather, like the charge and mass of the electron, it is one of 17 or so parameters that must first be measured in experiments before being included in the model “by hand”. However, both the BaBar and Belle results are consistent with the value of 0.72 suggested by other experiments and calculations based on the model.

CP violation can manifest itself in three different ways. In the indirect process first observed in neutral kaons in 1964, quantum mechanics allows particles to change into their antiparticles and back again in a process known as “mixing”. However, the two rates are different. In direct CP violation, which has also been observed in kaons, particles and their antiparticles actually decay in slightly different ways. However, physicists have so far not been able to perform the complicated calculations needed to convert these experimental results into a measure of matter-antimatter asymmetry.

The type of CP violation observed at BaBar and Belle results from the interference of decays with and without mixing. While it is extremely demanding to measure this form of violation – the B mesons only survive for about 10^-12 seconds – it is straightforward to relate the results to the fundamental matter-antimatter asymmetry.

Beyond the Standard Model

The next big challenge for both teams is to measure CP violation in the decay of the B meson into particles called pions. This would measure another angle, alpha, in the unitarity triangle to test the internal consistency of the Standard Model.

“If we get lucky,” says Harrison, “we might find a flaw in the Standard Model, since the dominance of matter in the universe strongly suggests that there are other forms of CP violation in nature that are not included in the theory.”

In 1967 the late Andrei Sakharov showed that, in addition to CP violation, two criteria must be met for matter to dominate the universe: the universe cannot be in thermal equilibrium, and there must exist certain processes that can change “baryon number”. However, reactions that change baryon number have never been observed, although they are allowed by certain extensions of the Standard Model.

“It is possible that the theory of how the matter-antimatter asymmetry in the universe built up may need modification,” adds Harrison. “But either way we win because there is something that we don’t fully understand about the universe and, therefore, there is something new to be found.”

Indeed, most extensions of the Standard Model introduce other parameters that violate CP symmetry. “It is a puzzle,” says Matthias Neubert, “that these effects are not seen in the vast data sets collected by the B factories at Cornell, Stanford, KEK and Fermilab.” Another mystery, he adds, is the fact that CP violation has not be observed in strong interactions, where the effect should be orders of magnitude larger than in weak decays.

“We are sure that the Standard Model must fail at some level and are stunned by the fact that no such failure is observed at the current level of experimental precision. Searches for new physics at the B factories will complement direct searches for new physics in experiments at the energy frontier.”