CP violation enters a new era

22 April 2002

New results show that CP violation in the decays of particles containing heavy quarks is becoming precision physics. These studies could soon lead to new insights into quark physics, in particular the mystery of how a universe apparently composed only of matter emerged from a Big Bang which initially produced matter and antimatter in equal amounts.

What physicists call CP violation ultimately distinguishes matter from antimatter. If physics is CP-symmetric, the behaviour of left-handed particles is the same as that of right-handed antiparticles, and vice versa. Since 1964 physicists have known that this convenient symmetry does not quite work. The difference between matter and antimatter is not simply a case of human convention.

Why CP violation happens is still a mystery, but one early outcome – when only three types of quark were known – was the realization that whatever the explanation of CP violation, it requires at least six different types of quark. In a world containing only three or even four kinds of quark, there would be no way of distinguishing matter from antimatter.

For some 35 years, CP violation experiments were confined to the study of neutral kaon particles (containing the strange quark). Only this physics arena provided the right conditions for seeing the small effect (a few parts per thousand). However, in principle CP violation could also be visible with any quark-based neutral particle which is difficult to distinguish from its antiparticle.

Accumulated results suggested that the best place to look would be the decays of the neutral B particles, containing the fifth or “b” quark. New high-energy B-factories – the electron-positron colliders PEP-II and KEKB – were constructed at the Stanford Linear Accelerator Center (SLAC) and the Japanese KEK laboratory respectively, to provide optimal conditions for this new physics. At these machines, major new experiments – Belle at KEKB and BaBar at PEP-II – provided their first tentative results in 2000, which were updated last (see How CP violation came to B).

Between them, these two experiments have gone on to accumulate some 100 million examples of B pairs. For B production, the machines are tuned to the energy region at and around the upsilon 4S resonance, with the best CP violation hunting ground being the decay of a neutral B particle into a J/psi and a neutral kaon.

The underlying quark transitions responsible for CP violation are described by a triangle in a special parameter space. The larger the area of this triangle, the greater the CP violation, and measurements aim to measure the angles and sides of the triangle.

The first angle to be measured, Β, is conventionally expressed as sin2Β. Now BaBar sees sin2Β as 0.75 ± 0.09 ± 0.04 (previously 0.34 ± 0.20 ± 0.05), and Belle sees 0.82 ± 0.12 ± 0.05 (previously 0.58 + 0.32 – 0.34 + 0.09 – 0.10). B particle CP violation definitely happens, and the effect is larger than the earlier suggestions.

In addition, Belle at KEKB has seen CP violation in the decay of neutral B mesons into two charged pions – the decay rate of the neutral B particle via this route is not the same as that of the neutral B antiparticle. This is analogous to the signal seen at Brookhaven in 1964 in the decays of neutral kaons which provided the first evidence for CP violation.

Vital to these new measurements is the performance of the electron-positron colliders at KEK and SLAC. This performance is usually expressed as luminosity, which is proportional to the rate of electron-positron collisions. KEKB’s luminosity has reached a remarkable 7.25 x 1033, not far from the ambitious 1034 design luminosity, but already the highest collider luminosity ever reached anywhere. PEP-II has exceeded 4.6 x 1033. Improving on these performances requires hard work and ingenuity, particularly as they are already not far from fundamental limitations (the “beam-beam limit”), but machine physicists remain optimistic when such creditable performances have been attained so soon after commissioning such complicated new machines.

The runs are continuing, and the results will probably be updated this summer.

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