Majorana Neutrinos: Evidence of Particle vs. Antiparticle?

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    Majorana Neutrinos
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Discussion Overview

The discussion revolves around the nature of Majorana neutrinos and whether they can be considered their own antiparticles. Participants explore implications of this concept in the context of specific reactions, such as neutrinoless double beta decay and the interaction of neutrinos with protons.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation

Main Points Raised

  • One participant questions whether the observation that neutrinos and antineutrinos behave differently in certain reactions indicates that neutrinos are not their own antiparticles, thus not Majorana particles.
  • Another participant suggests that the reaction is possible but unlikely due to the small mass of the neutrino, which leads to different helicity states for neutrinos and antineutrinos.
  • A participant expresses confusion about why reaction rates would differ if neutrinos and antineutrinos are the same particle, seeking clarification on the suppression of one reaction over the other.
  • It is proposed that while neutrinos and antineutrinos are the same particle, they can arrive at a proton in different states, similar to how different polarizations of light can interact differently with materials.
  • A participant raises a concern about the implications for neutrinoless double beta decay, questioning how the requirement for left-handed and right-handed states can be reconciled if they are the same particle.

Areas of Agreement / Disagreement

Participants express differing views on the implications of neutrinos being their own antiparticles, with no consensus reached on whether the observed differences in reaction rates support or contradict the Majorana nature of neutrinos.

Contextual Notes

Participants note the dependence on helicity states and the small mass of neutrinos, which may influence the behavior of neutrinos and antineutrinos in reactions. The discussion highlights unresolved questions regarding the conditions under which certain processes occur.

Malamala
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Hello! I don't know much about this topic so I am sorry if my question is silly. As far as I understand if neutrinos are Majorana particles, one consequence is that neutrinos are their own antiparticles. This can be observed, for example, in neutrinoless double beta decay. However, if we take the following reaction: $$\nu+p\to e^++n$$ we know from experiment that when ##\nu## is what we identify as an antineutrino the reaction is observed, but when ##\nu## is what we call a neutrino, the reaction doesn't take place. If the neutrino and antineutrino were the same particles, shouldn't both reaction take place equally often? Isn't this a clear evidence that neutrino is not its own antiparticle and hence not a Majorana particle? Of course I am missing something but I am not sure what. Can someone enlighten me please? Thank you!
 
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The process would be possible but extremely unlikely, suppressed by the small mass of the neutrino relative to its energy. What we call antineutrino would be a neutrino with opposite helicity*, and due to the small mass the two are nearly independent even if neutrinos are Majorana particles.

*I hope I remember that correctly
 
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mfb said:
The process would be possible but extremely unlikely, suppressed by the small mass of the neutrino relative to its energy. What we call antineutrino would be a neutrino with opposite helicity*, and due to the small mass the two are nearly independent even if neutrinos are Majorana particles.

*I hope I remember that correctly
Sorry, I am a bit confused. If the neutrino and anti neutrino would be the exactly same particle, wouldn't the reaction rates be the same, as they are the same particle? Why would we get a further suppression for one over the other?
 
They are the same particle but they are arriving at your proton in different states.

It's a bit similar to light which has two polarizations. Same particle (photons), but you can have systems that let one polarization pass and not the other.
 
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mfb said:
They are the same particle but they are arriving at your proton in different states.

It's a bit similar to light which has two polarizations. Same particle (photons), but you can have systems that let one polarization pass and not the other.
Oh, I think I understand. But why don't we have the same argument for neutrinoless double beta decay? In principle we would need at one vertex a LH neutrino and at the other a RH antineutrino (in order for them to interact weakly). Given that they are the same particle (i.e. same line in a Feynman diagram), they can't be both LH and RH at the same time. So shouldn't neutrinoless double beta decay not take place by the same argument that the above reaction doesn't take place?
 

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