Violation of CP symmetry

Whereas both P and C symmetries are individually violated by nature, the combination CP invariance—the exchange of particles with their corresponding antiparticles followed by a reflection in a mirror— was thought to hold absolutely until 1964, when J. W. Cronin, Val L. Fitch, and colleagues showed that CP symmetry was violated in the decay of neutral kaons. This was the one and only violation of CP symmetry that had been observed until March 2001, when the decay of neutral B mesons also violated this symmetry. But these two violations mean that CP invariance is not absolute. With this violation of CP symmetry and with the possibility of baryon nonconservation, it is possible to explain why matter dominates the universe. The explanation, however, is speculative and not very satisfying. 

Figure 28.15. Violation of CP symmetry measured by BELLE in 2006 difference of the decay time distribution between B-mesons and anti-B-mesons.

In 1964 Cronin and Fitch [14, 15] showed experimentally by studying the decay of kaons that the combined operation CP is not an exact symmetry of Nature. This discovery is even more perplexing when viewed in the context of time evolution. If the (so far unchallenged) CPT theorem is to hold, then violations of joint CP symmetry imply violations of T symmetry (with T reversing the motion of particles). The laws of physics are therefore not the same when the time changes direction. 

Even if allowed by the CPT theorem, the non-conservation of CP symmetry was hard to accept - not least because it was not consistent with the Standard Model. In 1972, Kobayashi and Maskawa predicted that CP violation would be consistent with the Standard Model provided that three generations of quarks exist (and only two were known at the time). The subsequent discoveries of the r lepton by Pearls (1975) and of the top and bottom quarks at the Fermilab confirmed the existence of a third family, this resulted in the incorporation of the CP violation into the Standard Model. 

The conditions required for a non-symmetric Universe were first put forward by Sakharov [16] they include non-conservation of the baryon number, C and CP symmetry violation, and the existence of a period of thermal non-equilibrium during the evolution however, the present limits on the proton lifetime (1033 years) are inconsistent with the first condition, and the small degree of CP symmetry violation displayed by kaons is not compatible with the second condition. 

After the discovery of parity violation the CP symmetry, i.e., the invariance of the physical laws against the simultaneous transformation of charge and space reflection, was still assumed to be exact. However, in 1964 Cronin and Fitch (Nobel Prize 1980) discovered (Christenson et al. 1964) that the weak interaction violates that as well, although this violation is tiny, not maximal, like that of the P invariance. CP violation makes it possible to differentiate between a world and an antiworld and may be related to the matter - antimatter asymmetry. 

After the discovery of the combined charge and space symmetry violation, or CP violation, in the decay of neutral mesons [2], the search for the EDMs of elementary particles has become one of the fundamental problems in physics. A permanent EDM is induced by the super-weak interactions that violate both space inversion symmetry and time reversal invariance [11], Considerable experimental efforts have been invested in probing for atomic EDMs (da) induced by EDMs of the proton, neutron, and electron, and by the P,T-odd interactions between them. The best available limit for the electron EDM, de, was obtained from atomic T1 experiments [12], which established an upper limit of de < 1.6 x 10 27e-cm. The benchmark upper limit on a nuclear EDM is obtained from the atomic EDM experiment on Iyt,Hg [13] as d ig < 2.1 x 10 2 e-cm, from which the best restriction on the proton EDM, dp < 5.4 x 10 24e-cm, was also obtained by Dmitriev and Senkov [14]. The previous upper limit on the proton EDM was estimated from the molecular T1F experiments by Hinds and co-workers [15]. 

A permanent EDM of a stable atomic or molecular state can arise only when both P and T invariance are broken (see Fig.l). It is often said that polar molecules possess permanent EDMs and exhibit a linear Stark effect. However, the Stark effect exhibited by the polar molecule is not really linear for sufficiently small E at zero temperature, and moreover, it violates neither P nor T symmetry [10]. We emphasize that a permanent EDM that exhibits a linear Stark effect even for an infinitesimally weak is a genuine signature of P and T violation or CP violation... 

I note in passing that apart from the effects due to parity nonconservation, also effects that arise from nonconservation of the symmetry with respect to simultaneous spatial and temporal inversion, so-called VT-odd effects, or to simultaneous charge conjugation and spatial inversion, denoted CT -violating effects, received particular attention especially for diatomic molecules. Readers interested in VT- or CP-violating effects in molecular systems are referred to the book of Khriplovich [42] and to the reviews [32,43]. 

The elementary interactions violate the symmetry under the combined transformation consisting of spatial reflection (P) followed by charge conjugation (C) - the so-called CP symmetry. 

For some years it was surmised that all interactions possess these symmetries, but an experiment in 1956 showed that weak interactions violate parity, as seen by the fact that radioactive decay particles are emitted with a large left-right asymmetry [16], Within a decade, an experiment on the decay of kaon particles showed that strong interactions are also not symmetrical, having a small asymmetry under the combined operation CP [17], There is a theorem for the type of theories used to describe particle interactions (local, Lorenz-invariant field theories) that they must be invariant under the triple reflection CPT. Since CP symmetry is broken, this theorem seems to indicate that T symmetry is also broken at the same level. These symmetry (and asymmetry) properties have played an important role in developing the standard model of particle interactions, which describes electromagnetic, weak, and strong interactions. 

Even though the CP violation is allowed by the Standard Model, its predicted effect seems to be too small to explain the excess of matter (and the existence of our coino-matter Universe). This means that our way of understanding these CP violations and perheaps the Standard Model itself must be incomplete. Therefore a lot of effort goes into research on fundamental symmetries and setting bounds on their violations. 

The weak interactions that cause atomic PNC violate not only the symmetry of parity, P, but also the symmetry of charge conjugation, C. However, the product of these, CP, is conserved. Because any quantum field theory conserves CPT, where T is time reversal this is equivalent to saying that T is conserved. However, even this symmetry is known to be violated. To date, this incompletely understood phenomenon has been seen in only two systems, the neutral kaon system, and, quite recently, the neutral B meson system. However, as noted already in the 1950 s by Ramsey and Purcell [62], an elementary particle possessing an intrinsic electric dipole moment also violates T invariance, so that detection of such a moment would be a third way of seeing T noninvariance. 

In keeping with the current interest in tests of conservation laws, we collect together a Table of experimental limits on all weak and electromagnetic decays, mass differences, and moments, and on a few reactions, whose observation would violate conservation laws. The Table is given only in the full Review of Particle Physics not in the Particle Physics Booklet. For the benefit of Booklet readers, we include the best limits from the Table in the following text. Limits in this text are for CL=90% unless otherwise specified. The Table is in two parts Discrete Space-Time Symmetries, i.e., C, P, T, CP, and CPT and Number Conservation Laws, i.e., lepton, baryon, hadronic flavor, and charge conservation. The references for these data can be found in the the Particle Listings in the Review. A discussion of these tests follows. 

CP invariance The symmetry generated by the combined operation of changing charge conjugation (Q and parity (P). CP violation occurs in weak interactions in kaon decay and in B-mesons. See also CPT theorem time reversal. 

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