High-energy physics

The matter with antimatter

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The Universe is made of matter, not antimatter, and 'CP violation' in particle decays could be the reason. Results from experiments measuring this effect at last confirm the predictions of a 30-year-old theory.

Every elementary particle has an anti- particle, a counterpart with precisely the same mass and the opposite electric charge. It would seem natural that all of the interactions of antiparticles are just the opposite of those of particles. But in 1964, in a Nobel-prize-winning experiment, Cronin and Fitch showed that this is not so1. Their measurement showed up a tiny difference, one part in a million of the strength of the weak interaction. New measurements from the BaBar experiment2 at Stanford, USA, and from the Belle experiment3 at Tsukuba, Japan, seem finally to have established a definite origin for this small difference in the behaviour of particles and antiparticles. The new data are a triumph for a theory proposed 30 years ago. But they are also a disappointment. The results so far do not help us understand the most obvious matter–antimatter asymmetry in nature — the fact that we see matter, but no antimatter, in the Universe at large.

The asymmetry in question is a basic one. It is well established that the laws of physics are the same irrespective of the position, time or orientation of the observer. From this statement, it would seem obvious that the laws of physics would also be the same when reflected in a mirror. But in 1956, following the suggestion of Lee and Yang, it was found that the weak interactions that mediate radioactive decay are completely mirror-asymmetric: nuclear β-decay preferentially produces particles that spin in the left-handed sense and antiparticles that spin in the right-handed sense. The interaction, however, seemed to respect the combined operation of mirror reflection (which turns a left-handed spin into a right-handed one) and interchange of particle and antiparticle. Physicists refer to these two operations as P ('parity') and C ('charge conjugation'), so the combined operation is called CP.

The Cronin–Fitch experiment, however, showed that the CP symmetry could be violated in specific particle decays. But it was not clear how to develop new equations to include this effect. CP violation arises only when particle masses are generated, and the origin of mass in particle physics is even now a mysterious issue, one that the elusive Higgs boson might solve.

Conversely, CP violation, not C or P violation alone, is required if an unstable heavy particle decays asymmetrically into matter and antimatter. Just after the Cronin–Fitch result, Andrei Sakharov suggested that CP violation could make particle decays in the early Universe asymmetric and thus could create an excess of matter from a hot Universe containing equal amounts of matter and antimatter4. The idea that the present Universe contains only matter as the result of a CP asymmetry in the fundamental laws is very attractive, but to evaluate it we need to know the origin of CP violation. CP violation arises in expressions for particle masses. In quantum mechanics, a particle with mass is a quantum state, which may be a mixture of some more basic states. CP violation results from that mixing, when the mixing amplitudes are complex numbers with different phases. But which specific particles give rise to the CP violation that we observe?

In 1972, when the model of weak interactions was still speculation and even quarks (the elementary particles that are the constituents of particles such as the proton and the neutron) were viewed with scepticism, Makoto Kobayashi and Toshihide Maskawa put forward the idea5 that CP violation originates in the mass mixing of heavy quarks. Quarks were expected to have mass mixing; Cabibbo and others had shown this theoretically6,7. At that time, though, only three species of quarks were known (called up, down and strange), and all three had small masses. Kobayashi and Maskawa required more quarks, six in all, to build a theory with intrinsic phase differences in the mixing terms. It was a wild idea. But, wild or not, these extra, heavy quarks (charm, bottom and top) have since turned up in experiments — the sixth, top, quark only in 1995.

If heavy quarks are needed for CP violation, it is logical that the decays of heavy quarks would show much larger CP asymmetries. Bigi, Carter and Sanda8,9 proposed looking in the decay of a B0 meson (a particle that contains a heavy bottom, or b, quark together with a light quark) and specifically in its decay to two other mesons, called Ψ and K0. This decay can occur by two different paths (Fig. 1): either the B0 can convert directly to ΨK0, or it can convert to its antiparticle B̄0, which then turns into ΨK0. In quantum mechanics, when a reaction can occur by two different paths, interference results. In this well-chosen reaction, the relative phase of the interfering terms is precisely the CP-violating phase of the Cabibbo– Kobayashi–Maskawa (CKM) model. If CP violation is fundamentally a large effect, the phase should be large.

Figure 1: Decay of a B meson.
figure1

The B0 meson (made of bottom and down quarks) can decay into two other mesons, known as Ψ and K0 (made of charm quarks and strange and down quarks respectively). The decay can occur directly, or the B0 can convert to its antiparticle, the B̄0 meson, through a weak interaction between its constituent quarks (left); the antiparticle can also decay to ΨK0 (right) through another weak interaction involving the W boson. Because there are two pathways to the final state, quantum interference arises between them. The BaBar2 and Belle3 experiments exploit this as a means of measuring CP violation.

Experimentally, measuring this phase is challenging. This particular decay can be observed in only 1 in 100,000 B decays, requiring huge numbers of high-energy collisions and unprecedented accelerator performance. New accelerators were built in Japan and the United States to meet these needs and have been operating for three years. They now hold nearly all the performance records for colliding-beam particle accelerators, and their corresponding detectors, Belle and BaBar, have collected a total of 300 million B decays.

At this summer's International Conference of High Energy Physics in Amsterdam, the BaBar and Belle experimental groups presented the results on the CKM phase that they have derived from these data2,3. The two results are precise and consistent. Remarkably, the value of the CKM phase they measure is exactly that needed to explain the magnitude of CP violation seen in the Cronin–Fitch experiment nearly 40 years ago. In the next few years, these measurements of B decay will be sharpened and new measurements from BaBar and Belle, and from the experiments at the Fermilab Tevatron, will come into play.

There is much at stake, for, despite many attempts, no one has been able to use the CKM model of CP violation to create the matter excess of the Universe through Sakharov's mechanism. Only a small asymmetry is needed in the early Universe, as today we have only one leftover proton for 109 photons. But simple calculations in the CKM model give a prediction of one proton to 1018 photons, and no cosmological model has been found that improves this by more than a few orders of magnitude. The problem is that CP violation in the CKM model requires not only the heavy-quark but also the light-quark masses, and these latter terms are unimportant at the high temperatures of the early Universe10. Models of CP violation that involve heavy objects only — for example, models with additional CP-violating phases in the mass mixing of Higgs bosons or other undiscovered particles — can readily account for the observed excess of protons.

Unless the new particles are extremely heavy, their CP effects should also show up in B decays. If these particles are present, the precise B-decay experiments of the next few years should show anomalies: perhaps qualitative differences from the CKM predictions, perhaps 10–20% discrepancies that the CKM model cannot account for. At present, no one knows what the result will be. Later in the decade, millions of top quarks and other exotic particles will be produced at CERN's Large Hadron Collider in Geneva, Switzerland, offering an opportunity to search directly for CP violation associated with these heavy states.

For the moment, Kobayashi and Maskawa appear triumphant. Their 1972 model of heavy quarks seems to explain all of the CP violation seen today in particle-physics experiments. If it cannot also explain the Universe, that is a task for stranger particles still waiting to be discovered.

References

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Correspondence to Michael Peskin.

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Peskin, M. The matter with antimatter. Nature 419, 25–27 (2002) doi:10.1038/419024a

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