From the September 1964 issue

Fundamental law doubted at Dubna

One of the most discussed topics at the 12th International conference on high-energy physics [at the Joint Institute for Nuclear Research, Dubna, 5–15 August] was the result of an experiment at the 33-GeV alternating gradient synchrotron in Brookhaven, US, by four physicists from Princeton University (one on leave from the Centre d'études Nucleaires, Saclay) (Christensen et al. 1964). To the uninitiated, their discovery seemed of no special interest – a so-called K⁰₂ meson had been found to decay sometimes into two particles (pions), instead of into the more usual three particles (pion + muon + neutrino, three pions, etc). To many high-energy physicists, however, and specialists in weak interactions in particular, the result was startling because it seemed to disprove one of the fundamental rules that are believed to cover the behaviour of matter: CP invariance.

This rule is connected with others by the so-called CPT invariance theorem. C represents "charge conjugation", changing the wave function of a particle into that of its corresponding antiparticle (or vice-versa); P is the parity operator, reversing the sign of all the space co-ordinates; T represents changing the sign of the time co-ordinate – running time backwards. Physically, CPT invariance implies that antiparticles (or particles), viewed upside down in a mirror with time running backwards, are subject to exactly the same forces and interactions as particles (or antiparticles) in the normal world. Moreover, for strong and electromagnetic interactions, systems of fundamental particles are invariant under C, P and T operations separately: for example (considering C alone) the force between an antiproton and a positron is the same as between a proton and an electron, so that an "anti" hydrogen atom would behave exactly as a normal hydrogen atom; considering P, the electron cloud is symmetrical about the proton nucleus so that the atom has no left and right or top and bottom. For weak interactions, however, P invariance does not hold: a particle undergoing spontaneous decay does show a kind of Ieft and right side. But the "left" side is the "right" side of an antiparticle, so that until now it was accepted that the double action represented by CP introduced no change in any system. This was put in doubt by the Princeton/Brookhaven experiment.

A new force in the universe

The results were published just before the Dubna conference, and at the conference another group (from Illinois University) gave further evidence for the effect. There followed, as the rapporteur of the theoretical plenary session remarked, a great deal of agitated discussion, especially over the clinking sound of vodka glasses. Afterwards, at CERN and in many other laboratories, the discussion continued by letter and by telephone and new ideas were developed. At an early stage, three possibilities emerged: a) the experimental results were wrong; b) CP invariance could no longer be assumed to occur in all cases; c) an unknown effect transformed some of the K⁰₂ mesons in the experiment into K⁰₁ mesons, which then decayed into pions in the normal way.

The first explanation could be safely ruled out; the second was intriguing to many theoreticians, who busily began working out the possible consequences or tried to explain the cause in various ways; the third rapidly produced at least two independent postulates of a new, hitherto unsuspected, fundamental force in the universe.

At Stanford University (US), such an idea came from J S Bell, a CERN physicist on leave, and J K Perring (on leave from the Atomic Energy Research Establishment in England) (Bell and Perring 1964). At CERN, J Bernstein (on leave from New York University), N Cabibbo and T D Lee (on leave from Columbia University) evolved a similar notion. It seems that, apart from the four known forces (strong, electromagnetic, weak and gravitational), there may be a fifth, extremely weak but long-range, force that is different for antiparticles and particles. As a result, K⁰₂ mesons would sometimes be changed to K⁰₁ mesons and the experimental results could be explained without having to give up the principle of CP invariance.

• Compiled from "Last month at CERN" pp118–119.

Compiler's Note

In 1957 Richard Feynman lost a $50 bet when C S Wu and collaborators showed that weak interactions maximally violate parity P, acting only on left-handed particles (and right-handed antiparticles). Then in 1964 came the clear evidence that even the compound symmetry CP was broken in weak interactions.

Unlike parity violation, CP violation is a very small effect but this distinction between matter and antimatter may have profound consequences. Andrei Sakharov showed that CP violation (combined with other conditions very likely in the Big Bang) could account for the apparent lack of antimatter in the universe today. This led Val Fitch (of the Brookhaven experiment described here) to remark that the first evidence, ever, for CP violation was the fact that we exist.

For almost half a century, experiments and theory have failed to provide a fully satisfactory explanation of CP violation. Experiments continue in systems other than kaons and with ever-increasing accuracy, notably BELLE at KEK and BaBar at SLAC. The LHCb experiment at CERN is specially designed to study CP violation when the LHC starts up in 2008.

As far as is known, the combined CPT symmetry, a basic principle of relativistic quantum field theory, is an exact invariant in all processes. If experiments ever show that CPT is violated a lot more than $50 will be at stake.