| CP symmetry refers to the fact that physical processes
in nature occur in precisely the same manner if all particles were converted
to their antimatter opposites using the CP transformation. Explicitly,
the C operation reverses all additive quantum numbers such as electric
charge, hypercharge, strangeness, etc., while the P transformation
"inverts" the coordinate system and the orientation of all objects in it:
x -> -x, y-> -y, z-> -z. That is, a particle traveling to the right in
the +x direction finds itself traveling to the left (still in the +x direction)
after a P transformation. More significantly, P reverses
the relationship between the intrinsic angular momentum (spin) of a particle
and the direction of its velocity. If the spin is aligned with the velocity,
the particle is referred to as having "positive helicity." If the spin
is anti-parallel to the velocity direction, the particle has "negative
helicity." Under a P transformation, the velocity direction is reversed
but the spin direction is not (as spin is a purely internal quantum number);
thus a positive helicity
particle -> negative helicity and vice versa. So under a CP transformation, a negative helicity proton becomes a positive helicity antiproton. It was first predicted - and subsequently confirmed by
experiment (in the 1950’s) - that positive and negative helicity particles
interact differently: a negative helicity electron will scatter off a nucleus
and transform into a neutrino, but a positive helicity electron will not.
However, a positive helicity positron will, just like the negative helicity
electron. This is an example of CP symmetry. The symmetry was believed
to be exact until 1964, when an experiment by Fitch et al. showed that
for K0 mesons, CP symmetry breaks down 0.2% of the time.
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