A brief look back at a discovery that surprised the world of particle physics 50 years ago.
In the summer of 1964, at the International Conference on High-Energy Physics (ICHEP) in Dubna, Jim Cronin presented the results of an experiment studying neutral kaons at Brookhaven National Laboratory. In particular, it had shown that the long-lived neutral kaon can decay into two pions, which implied the violation of CP symmetry – a discovery that took the physics community by surprise. The news was greeted with some scepticism and met a barrage of questions. Everyone wanted to be satisfied that nothing had been overlooked, and that all other possibilities had been considered carefully and ruled out. People need not have worried. Cronin, together with Val Fitch, visiting French physicist René Turlay and graduate student Jim Christenson, had spent months asking themselves the same questions, testing and cross-checking their results thoroughly. There was, in the end, only one conclusion that they could draw from their observations: CP symmetry was not a perfect symmetry of nature. Only when the researchers were completely satisfied did they make their findings known to the physics community. It is testament to their patience and the quality of their work that the result was so robust to scrutiny. It was 15 years later that Cronin and Fitch received the 1980 Nobel Prize in Physics for the discovery.
The announcement of a broken symmetry was not new to the physics community, having first occurred only a few years previously, when the maximal non-conservation of parity (P) in the weak interaction was discovered by Chien-Shiung Wu and her colleagues in 1957, following the proposal by Tsung-Dao Lee and Chen-Ning Yang that parity violation might explain puzzles in the decays of charged kaons. The disturbing conclusion that the laws of physics depend on the frame of reference was evaded, however, because experiments soon showed that symmetry under charge-conjugation (C) was also maximally violated. Therefore, as long as the combined operation, CP, was a good symmetry, the possibility of an absolute distinction between left-handed and right-handed co-ordinate systems would be prevented, being compensated exactly by the asymmetry between particles and antiparticles. CP invariance had already been suggested as the means to restore symmetry conservation by Lev Landau, and by Lee and Yang, so the situation seemed to be resolved neatly.
No elegant alternative was available to replace CP invariance
When the news came in 1964 that CP was also a broken symmetry, it was harder to accept, because no elegant alternative was available to replace CP invariance. There was also the issue of the treasured CPT theorem: if CPT holds, then CP violation implies violation of time-reversal (T) symmetry. The discovery of CP violation led to the unsettling conclusion that the microscopic laws of physics do indeed allow absolute distinctions between left- and right-handed co-ordinate systems, between particles and antiparticles, and between time running forwards and backwards.
By the early 1960s, the neutral kaon system had already proved to be a rich testing ground for new physics. Its “strange” behaviour had been a matter for scrutiny since its discovery in cosmic rays in 1946. Neutral kaons were found to be produced copiously through the strong interaction, while their long lifetimes suggested decays via the weak interaction. In 1953, Murray Gell-Mann assigned the K0 a “strangeness” quantum number, S = 1, which was conserved by the strong force but not by the weak force. This implied that there must exist a distinct anti-K0, K0, with S = –1. However, because both the K0 and K0 appeared to decay to two pions, the distinction between the particles was blurred somewhat. The situation prompted Gell-Mann and Abraham Pais to propose, in 1955, that the states of definite mass and lifetime, labelled K1 and K2, were instead an admixture of the two particles, and were even and odd, respectively, under the CP transformation. Under the assumption of CP invariance, the K2 was forbidden to decay to two pions. This gave it a much longer lifetime than the K1, as observed.
The primary motivation for the experiment at Brookhaven was to study a phenomenon peculiar to the kaon system called regeneration (see box). Fitch, an expert on kaons, had approached Cronin, who with Christenson and Turlay had built a state-of-the-art spectrometer based on spark chambers, which could be operated with an electronic trigger to select rare events. It was just what was needed for further tests of regeneration. Finding a “new upper limit” for K2 decaying to 2π was a secondary consideration, listed under “other results to be obtained”. The experiment was approved for 200 hours of run-time, and about half of this was devoted to the “CP invariance run”, across five days towards the end of June 1963. Turlay began the analysis of the CP run in the autumn. By the time it was complete, early in 1964, it was clear that 2π decays were present, with 45±10 events, corresponding to about one in 500 of K2 decays to charged modes. In the conclusion of their seminal paper, published in July 1964, the team stated: “The presence of a two-pion decay mode implies that the K2 meson is not a pure eigenstate of CP” (Christenson et al. 1964).
During the year that followed, there was feverish activity in both the experimental and theoretical communities. The discovery of CP violation raised many questions about its origins, and the size of the effect. In particular, it was unclear from experiment whether the effect was occurring in the kaon decays (direct CP violation) or in neutral kaon mixing (indirect CP violation). Indeed, the results could be explained solely by invoking indirect CP violation, which was achieved by the simple, but ad hoc, addition of a small admixture of the CP = +1 eigenstate to the mass eigenstate of the long-lived neutral kaon. This was parameterized by the small complex parameter ε, which had a magnitude of about 2 × 10–3. The two states of distinct (short and long) lifetime were then KS = K1 + εK2 and KL = K2 + εK1 (to order ε2).
Among the many theoretical papers that followed in the wake of the discovery of CP violation was that by Lincoln Wolfenstein in August 1964, which proposed the “superweak” model. This was the minimal model, which accounted for the observed effect by adding a single CP-violating contribution to the ΔS = 2 mixing-matrix element between the K0 and the K0. There was no CP-violating contribution to the kaon decays themselves, hence the model offered a prediction that the phenomenon would be seen only as a feature of neutral kaon mixing. Alternatively, a “milliweak” theory would include direct CP-violating contributions to neutral kaon decays (ΔS = 1), as well as to the kaon mixing-matrix element. Another proposal was that the action of an all-pervading long-range vector field of cosmological origin could cause the observed decay to 2π without invoking CP violation. This was a relatively easy option to test experimentally, because it predicted that the decay rate would depend on the energy of the kaons.
The experimental confirmation of the π+π– decay of the long-lived kaon came early in 1965, from groups at the Rutherford Laboratory in the UK and at CERN. These experiments also dispensed swiftly with the vector-field proposal. There was no evidence for the variation of the decay rate with energy. Experiments were now needed to determine the CP-violating parameters η+– and η00 – the ratios of the amplitudes for the KL and KS decays into π+π– or π0π0, respectively (see box in“NA31/48: the pursuit of direct CP violation”) – the measurable quantities being the related magnitudes (|η+–|, |η00|) and phases (φ+–, φ00).
In 1964, Jack Steinberger had realized that the interference between KS and KL decaying to the same final state (π+π–) could provide a valuable way to study CP violation. Results published in 1966 from two such experiments at CERN’s Proton Synchrotron provided measurements of |η+–| and φ+–. The more difficult challenge of measuring decays to π0π0 was taken up by spark-chamber experiments at CERN, Brookhaven and Berkeley. In another experiment at CERN, a beam of KL passed along a pipe through the Heavy-Liquid Bubble Chamber (HLBC), in which the photons from the π0 decays would convert. First results from the spark-chamber experiments seemed to indicate that |η00| was much larger than |η+–|. However, in late 1968 the HLBC collaboration presented a result that was compatible with |η00| = |η+–|. After some confusion, the spark-chamber experiments confirmed this result and also measured φ00.
More refined experiments were to follow, giving more precise measurements for the different decay modes. By the time of the 13th ICHEP in London in 1974 – 10 years after the announcement in Dubna – all results agreed perfectly with the predictions of the superweak model, with no need for direct CP violation. However, a new theory that accounted for CP violation was already in the air – and with it new challenges for a new generation of experiments.
Neutral kaon mixing, oscillations and regeneration
Because the weak interaction does not conserve strangeness, second-order weak-interaction processes mediate transitions between the strangeness eigenstates K0 and K0 . Therefore, the physical particles (eigenstates of mass and lifetime) are linear combinations of K0 and K0 , and states born as one or the other “oscillate” between these two eigenstates before decaying. The two physical eigenstates are called KS and KL – short and long – reflecting their different lifetimes. Allowed to propagate for long enough, a mixed beam of neutral kaons will evolve into a pure beam of KL. Because K0 and K0 have different interactions with matter, if an initially pure KL beam enters matter, the K0 component will interact preferentially, forming a different admixture of K0 and K0 . This admixture must be different from the pure KL that entered the matter, which means that a component of KS is “regenerated” in the beam. Regeneration is not an effect of CP violation, but it is used extensively in “regenerators” in kaon experiments.