Nature is not symmetric under mirror reflections. For example, a mirror image experiment of neutron decay will yield different results. This is a fundamental characteristic of nature. Nature has only a "left hand" & no "right hand".
One of the starkest facts showing parity violation is the non-existence of right-handed neutrinos. (A right-handed neutrino is one whose angular momentum points in the same direction as its velocity). In other words, a neutrino current is always left circularly polarized.
In contrast, photons come in both right-handed and left-handed varieties.
Parity violation can also be found in less exotic places like the cesium atom.
Parity violation is an asymmetry which leads to a number of constraints. One of the constraints is that each generation of quarks and leptons must come in pairs. For example, the up quark must come with the down quark to cancel the anomaly arising from the left-handed electron neutrino. This is a condition that parity violating quantum fields must obey for consistency.
String theory yields a more fundamental and deeper understanding as to the origin of parity violation.
Last updated: 03 March, 2000
