CP violation

Under CP violation (C for engl batch charge or charge conjugation charge conjugation; . P for parity Parity ) is the violation of the CP invariance. The latter states that the physical relationships and regularities in a system should not change when all particles are replaced by their antiparticles, while all spatial coordinates are mirrored.

Discovery and history

Apparent P- invariance and parity violation

According to the normal everyday experience, the physics in a mirror world should not differ from its original. That is, each process that is observed in a mirror should be, can be realized by suitable experimental arrangement in the normal world (P- invariance ).

As early as 1956 Tsung- Dao Lee postulated and Chen Ning Yang, however, that the weak interaction, the subject of the beta decay violates the symmetry point. In the same year this parity violation was confirmed by Chien- Shiung Wu in Wu experiment. The weak interaction preference is given to the left-handedness (which means " turning to the left" of the spin of elementary particles ) versus right-handedness. Only left-handed particles and right-handed antiparticles take part in it. On the electromagnetic and strong interactions take the left - and right-handed particles participate with equal strength.

The parity violation can be well illustrated with neutrinos that interact only weakly and can occur only as a left-handed neutrinos and right-handed antineutrinos. Under the parity transformation ( " reflection point " ) is a right-handed neutrino but from a left-handed neutrino.

Apparent C- invariance and C- violation

Physicists has been known since the 1930s that, for every elementary antiparticles, which carries exactly the same mass but opposite charge. Originally, the theory and observations talked that all interactions and decays of antiparticles proceed exactly as with normal particles, so that it is invariant under charge conjugation, short C- invariant, are. The electromagnetic and the strong interaction receive C. For example, the Coulomb's law is invariant under the charge conjugation. The weak interaction violates C, however, which can illustrate to neutrinos as well. Under the charge conjugation is a left-handed neutrino a left-handed antineutrino, which is not observed experimentally.

Apparent CP invariance

One exchanges in addition to mirroring also particles antiparticles (C P), so there is no longer a contradiction to the above situation, because it is from a left-handed neutrino under the CP transformation, a right-handed antineutrino.

Discovery of CP violation

1964 discovered the American physicist James Christenson, James Cronin, Val Fitch and René Turlay (Nobel Prize in physics for Cronin and Fitch 1980) a tiny irregularity in the decay of heavy neutral K- mesons ( kaons ) that a violation also of the combined CP symmetry close left. In studying the decays of mesons in an experiment at the Alternating Gradient Synchrotron at Brookhaven National Laboratory ( pions ) were at a rate of about 2 ‰, the CP violating decays of the meson into two charged π - mesons observed.

Linking with the standard model

In the standard model of particle physics, the cause of CP violation is related to the quark sector with the production of the quark masses. Quarks get their mass by coupling to the Higgs field, which necessarily have a mass mixing matrix occurs, which is named after the physicists Nicola Cabibbo, Makoto Kobayashi and Toshihide Maskawa ( CKM matrix ). 1972 Kobayashi and Maskawa showed that this matrix is complex, if existing in nature in three or more quark families. In three families, according to the theory of a complex phase in the CKM matrix is unitary ago exactly. This phase is responsible for the CP violation in the Standard Model. Thus, in particle reactions CP violation but can occur at all, certain requirements must be met. So it must in particular be at least two competing processes that lead from the same initial state to the same final state, so that there is an interference. Three types of CP violation are known:

• Known CP violation in particle -antiparticle mixing, as well as indirect CP violation

• CP violation in the decay known, also known as direct CP violation

• CP violation in the interference between mixing and decay

The CP violation remains a very active area of ​​research, since tests of the Standard Model with a great sensitivity to the presence of new physics are possible in a number of other channels. So also the study of CP violation in the system of neutral mesons such perspectives. Precision measurements of CP violation in the system of the B and Bs mesons are a focus of the physics program of the LHCb experiment at the Large Hadron Collider at CERN. In building located in Japan, the accelerator SuperKEKB at KEK, where the successor of the Belle experiment Belle II, is expected to start the data acquisition end of 2016.

In principle, CP violation should also be possible in the strong interaction, for example, would lead to a relatively large electric dipole moment of the neutron. Experimental evidence for such a CP violation but there is no. This gap is referred to as "strong CP problem".

Based on the observation of neutrino oscillations and the associated realization that neutrinos are not massless, there must also be a mixing matrix, (sometimes just MNS matrix ) indicate the PMNS matrix, which by Bruno Pontecorvo, Ziro Maki, Masami Nakagawa and Shoichi Sakata is named. As with the CKM matrix could also be a source of CP violation, which has not yet been observed.

Conservation of CPT invariance

In contrast to the ( " doubly - combined " ) CP violation in quantum field theory for a fundamental theorem of Wolfgang Pauli and Gerhart Lüders at all events, the ( " triple combination " ) CPT symmetry under all circumstances invariant (T = time reversal, ie reversal of the direction of motion, the orbital and spin angular momenta and the transition to the complex conjugate, i - > i).

Cosmological necessity of CP violation

Andrei Sakharov noted in the 1960s that the CP violation is one of the conditions to ensure that it can give the observed large dominance of matter over antimatter in the Universe ( baryon asymmetry ). Our current understanding of the Big Bang assumes that he produced particles and antiparticles in the same amount. In the baryogenesis then came the now observed imbalance. What is the exact mechanism, however, is controversial. Experiments have to clarify their origin is the CP violation and whether it is sufficiently large to generate sufficient matter can.