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Chirality of the neutrino

But what mean those terms?... especially for the neutrino.

• Chirality: Look at your right hand. It is an assymetric object. Its mirror image is your left hand. If you try to superimpose your two hands turned to you, you find no way to do that. That's chirality! Something is chiral when it is not superposable to its mirror image. Many things in nature are chiral.

  One distinguish left chiral objects from right chiral objects. This difference, in general arbitrary, can be based on the rotation of the polarity plane of a light beam passing through a test material. Using a certain type of sugar called dextrose, the polarity plane rotates to the right, when the light beam is looked from the back. Dextrose is then said to be of right chirality. That's the rule. Most of the life molecules turn the polarity plane to the left: they are said to be of left chirality. But amino-acids, which are supposed to be at the origin of life, are of right chirality. This difference is a great mystery which could be linked to the assymetry of weak interactions, thus linked to the neutrinos.
  
  

• Helicity: One of the main properties of a particle is its spin. It figures the self-rotation of the particle. If the projection of the spin on the particle velocity direction is opposite to it, the particle is said to be of left helicity. On the contrary, it is said to be of right helicity.

  Theoretically, a zero mass neutrino can only have a left helicity. It is very peculiar to the weak interaction, which produces only left helicity neutrinos. A neutrino could be of left or right chirality. But a zero mass neutrino having a left helicity would be always of left chirality... The terms "left" or "right" (often subscripted L and R) refer to the left or right chirality. In the limit of zero mass they correspond to the left or right helicity.

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Last update: 26/06/1999: http://wwwlapp.in2p3.fr/neutrinos/achiralite.html

Didier Verkindt