 | CP-violation: Encyclopedia II - CP-violation - CP Violation
CP-violation - CP Violation
In 1964, James Cronin and Val Fitch provided clear evidence that CP symmetry could be broken, too, winning them the 1980 Nobel Prize. Their discovery showed that weak interactions violate both the charge-conjugation symmetry C between particles and antiparticles and at the same time also violate P or parity. The discovery shocked particle physics and opened the door to questions still at the core of particle physics and of cosmology today. The lack of an exact CP symmetry, but also the fact that it is so nearly a symmetry created a great puzzle.
It was discovered in 1964 by the group of Cristenson, Cronin, Fitch and Turlay in a kaon decay experiment that CP symmetry was violated, and only a weaker version of the symmetry could be preserved by physical phenomena, which was CPT-symmetry. Besides C and P, there is a third operation, time reversal (T), which corresponds to reversal of motion. Invariance under time implies that whenever a motion is allowed by the laws of physics, the reversed motion is also an allowed one. Therefore, the combination of CPT is thought to constitute an exact symmetry of all types of fundamental interactions. Because of the CPT-symmetry, a violation of the CP-symmetry is equivalent to a violation of the T-symmetry. CP violation implied nonconservation of T, provided that the long-held CPT theorem was valid. In this theorem, regarded as one of the basic principles of quantum field theory, charge conjugation, parity, and time reversal are applied together.
Recently, a new generation of experiments, including the BaBar Experiment at the Stanford Linear Accelerator Center (SLAC) and the Belle Experiment at the High Energy Accelerator Research Organisation (KEK), Japan, have observed CP violation using B mesons. Before these experiments, it was a logical possibility that all CP violation was confined to kaon physics. These experiments dispelled any doubt that the interactions of the Standard Model violated CP.
The CP violation of the Standard model is incorporated by including a complex phase in the CKM matrix. A necessary condition for the appearance of the complex phase, and thus for CP-violation, is the presence of at least three generations of quarks.
There is no experimentally known violation of the CP-symmetry in Quantum Chromodynamics. The strong CP problem is the question of why no such violation is detected even though the theory in principle allows for it.
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 Adapted from the Wikipedia article "CP Violation", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki |
