# Neutrinos and Anti-Neutrinos

1. Jan 12, 2004

### Glenn

What are the characteristic differences between the two?

I was under the impressions that antimatter and matter had the same characteristics except for an opposite charge. But a neutrino has no charge.

Thanks,
Glenn

2. Jan 12, 2004

### Lonewolf

The only difference, as far as I'm aware, is in their helicity. So basically, a "right handed" neutrino would be an anti-neutrino.

3. Jan 12, 2004

Staff Emeritus
Quote from a post by Lobos Motl on today's s.p.r.:
It is only the combined "CP" transformation whose candidate operator can be found: it maps a left-handed neutrino into a right-handed antineutrino, which is OK. However even this combined "CP" transformation, although you can define such candidate operators, does not commute with the Hamiltonian because the mass matrix for quarks (and - as we know today - probably also for the leptons) contains a complex phase that physically distinguishes particles from anti-particles.

4. Jan 13, 2004

### Yustas

Helicity is not a good quantum number for a massive particle (like a neutrino). Since it is roughly speaking defined as a projection of a particle's spin onto its momentum, one can always switch to a reference frame that is moving faster then the particle. In this frame you'll observe opposite helisity. So helicity is not Lorentz-invariant.

Neutrinos are interesting precisely because they have no electric charge. The only thing that distinguishes neutrino and antineutrino is a lepton number. And even that depends on "what it is." For example, if neutrino is a so-called Majorana particle, then it is its own antiparticle (transforms into itself under the CP-transformation), just like a photon or neutral pion.