How Do Neutrinos Differ from Anti-Neutrinos?

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Discussion Overview

The discussion centers on the differences between neutrinos and anti-neutrinos, exploring theoretical aspects and properties related to their helicity, charge, and lepton number. Participants examine the implications of these differences in the context of particle physics.

Discussion Character

  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant, Glenn, questions the characteristic differences between neutrinos and anti-neutrinos, noting that antimatter typically has opposite charge characteristics compared to matter, while neutrinos have no charge.
  • Another participant suggests that the primary difference is in helicity, stating that a "right-handed" neutrino would correspond to an anti-neutrino.
  • A later reply references a quote discussing the "CP" transformation, indicating that it maps a left-handed neutrino to a right-handed anti-neutrino, but notes that this transformation does not commute with the Hamiltonian due to a complex phase in the mass matrix that distinguishes particles from anti-particles.
  • Another participant argues that helicity is not a good quantum number for massive particles like neutrinos, as it can change depending on the reference frame. They assert that the only distinguishing feature between neutrinos and anti-neutrinos is lepton number, which can vary based on the nature of the neutrino, particularly if it is a Majorana particle.

Areas of Agreement / Disagreement

Participants express differing views on the significance of helicity and lepton number in distinguishing neutrinos from anti-neutrinos. There is no consensus on the primary characteristics that differentiate them.

Contextual Notes

Participants highlight limitations in the definitions and implications of helicity and lepton number, particularly regarding the behavior of massive particles and the nature of Majorana particles. These aspects remain unresolved within the discussion.

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

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.
 
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.
 

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