CP violation in K decays: Experimental aspects

Abstract

CP violation was originally discovered in neutral K mesons. Over the last few years, it has also been seen in B mesons, and most of the research in the field is currently concentrating on the B system. However, there are some parameters which could be best measured in kaons. In order to see to which extent our present understanding of CP violation within the framework of the CKM matrix is correct, one has to check for possible differences between the K system and the B system. After an historical overview, I discuss a few of the most important recent results, and give an outlook on experiments that are being prepared. 1 The discovery of CP violation Symmetries are a salient feature of our world, but so is the breaking of approximate symmetries. Still, for a long time physicists believed that at the level of elementary particles, a high level of symmetry should prevail. In particular, it was expected that all fundamental interactions should be symmetric under the discrete transformations of spatial inversion (parity transformation P), substitution of antiparticles for particles (charge conjugation C), and time inversion (T). However, in 1956 Lee and Yang concluded from experimental data that the weak interactions might not be invariant under spatial inversion, in other words that parity might be violated. This was then explicitly shown in an experiment by Wu in 1957 [1]. For a few years, physicists were inclined to believe that although parity was broken, this symmetry violation was exactly compensated by the charge symmetry violation, and that the symmetry under a combined charge and parity transformation (CP) was exactly conserved. An obvious example was the helicity of the neutrino, which was always observed to be negative (‘left-handed neutrino’). Parity transformation would transform it into a right-handed neutrino, which has not been observed in nature. In other words, it appears that parity is maximally violated. However, by performing charge conjugation in addition, one arrives at the right-handed anti-neutrino, which does exist in nature. However, only a few years after the discovery of parity violation, it turned out that this so-called ‘CP symmetry’ was also violated, although to a much smaller extent than parity itself. In an experiment carried out at the Brookhaven Alternating Gradient Synchrotron (AGS), Christenson, Cronin, Fitch and Turlay found out that the longer-lived of the two neutral kaons, the K 0 L, which frequently decays into three pions and should therefore be assigned odd parity, in rare cases decayed into a parity-even twopion state [2]. For some physicists, this was hard to believe and a number of possible explanations were looked at [3] before it was accepted that CP had to be broken at the per mil level. While at first CP violation was regarded by some physicists as a sort of unwelcome guest and an unnecessary complication of nature, it later turned out that it is in fact vital for our very existence! According to the Big Bang model of the origin of the universe, particles and antiparticles were at first produced in equal numbers. We know that at present, however, the universe contains almost no antimatter. How could matter survive and not be annihilated right away by antimatter, in which case the universe would now be a fairly dull place made up largely of photons, without much of a structure and without physicists wondering about it? In 1967 Andrei Sakharov found three necessary conditions for creating such a ‘baryon asymmetry’ [4]: