This symmetry seems quite empirical to us because at the human scale the structure of the matter is dominated by the electromagnetic force which is known to respect parity conservation. Nothing in the links between atoms favors one side rather than another. That's why an object in a mirror looks real! Think about it next time you brush your teeth. |

What is amazing is that there is no theoretical reason to respect parity conservation. In practice all of the fundamental interactions do but one: Parity Violation (PV) is a specific feature of the weak interactions first argued by T.D. Lee and C.L. Yang in 1956 and experimentally established the same year by Miss C.S. Wu in beta-transition of polarized Cobalt nuclei. In fact it turns out that the charged weak current, acting in Beta-transitions, couples only to left-handed states (particles with projection of their spin anti-parallel to their momentum). This is a 100% violation of the parity symmetry! Mirror image of a charged weak process has never been observed. Imagine a world with a "strong weak force"... |

The SPhN participates in a program of Parity Violation experiments at JLab. The experimental method is to elastically scatter an electron beam on a hadronic target (proton or nuclei). Electron and hadrons both feel the weak interaction and can exchange a Z0 boson during the elastic process. This boson is the messenger of the weak neutral interation and have a mass of ~91.2 GeV. Extracting its contribution in the scattering gives us the new information we are seeking.

At Q

Neutrino beams could do it, they are pure weak probes. But it would take decades to accumulate the statistic...

Then you think that Parity Violation could be part of the trick, and you are right. If we can measure two mirror-image scattering processes in the same experimental conditions, the difference between the two counting rates will isolate the weak contribution. In practice we'll measure the ratio of the difference over the sum which cancels out all the errors of normalization and allow accurate measurement of small quantities. This ratio is called a parity violating asymmetry. Since the difference is proportionnal to the weak transition and the sum is dominated by the electromagnetic part, our asymmetry is basically the ratio of the two probabilities illustrated above. In the kinematical domain of our experiments (Q

To get a mirror-image of a scattering process we note that Parity operation reverses the sign of vectors like position and momentum but lets axials vectors like spin (product of two vectors) unchanged. Hence buildind a Parity symmetric scattering process would require to flip all the particles directions but not the spin. Experimentally we choose a more convenient and equivalent method which consist in keeping everything unchanged but flipping the spin. The picture is the following:

we send a polarized electron beam on an unpolarized hadronic target. Processes with the electron spin parallel (Right state) or anti-parallel (Left state) to the beam direction are related by Parity operation. The P.V. asymmetry then simply becomes the asymmetry in the Left/Right counting rates.