CP symmetry

Definition: CP symmetry is a form of symmetry which exists when a system is invariant (unchanging) under both charge conjugation (C) and parity (P). In other words, if you took the universe and swapped every particle about an axis or center (parity), and then replaced every particle with its antiparticle (switched charges from positive to negative and vice versa), the laws of physics would operate exactly the same way.

In our universe, parity is not preserved under either C or P individually. However, when both operations C and P (written as CP, or PC) are implemented, the laws of physics operate basically the same way. In the words of Roger Penrose (The Road to Reality), "CP sends a particle's zig into its antiparticle's zag, and vice versa."

CP Violation

In 1964, James Cronin and Val Fitch discovered a particle decay (the K⁰ meson) which is non-invariant under the CP operation, meaning that CP symmetry is not quite invariant - though very close. This work earned Fitch and Cronin the 1980 Nobel Prize in Physics.

This symmetry is also frequently related to a time reversal operation (T). When combined together, these three operations form a CPT symmetry, which does appear to be completely invariant.

CP Violation and Quarks

In 1972, Makoto Kobayashi and Toshihide Maskawa applied a principle of broken symmetry to explain CP violation, which resulted in a total of six types of quarks, even though the quark model at the time contained only three quarks. A fourth quark had been hypothesized in 1970, and evidence in 1974 helped support it. The fifth quark was discovered in 1977. It was not until 1994 that the sixth and final quark was experimentally observed.

For their work with this model, Kobayashi and Maskawa earned half of the 2008 Nobel Prize in Physics.

Also Known As: CPT symmetry,CPT theorem,CP violation