Majorana Neutrinos: Evidence of Particle vs. Antiparticle?

In summary, the process of neutrinoless double beta decay is suppressed by the small mass of the neutrino relative to its energy.
  • #1
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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  • #2
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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  • #3
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?
 
  • #4
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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  • #5
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?
 

1. What are Majorana neutrinos?

Majorana neutrinos are hypothetical particles that are their own antiparticles. This means that they have the same properties as their antiparticles, making them unique in the world of particle physics.

2. How are Majorana neutrinos different from other neutrinos?

Unlike other neutrinos, which are fermions and have distinct particle and antiparticle states, Majorana neutrinos are considered to be their own antiparticles. This means that they do not have a distinct particle and antiparticle state, and are instead their own unique particle.

3. What evidence is there for the existence of Majorana neutrinos?

There is currently no direct evidence for the existence of Majorana neutrinos. However, there have been several experiments and observations that suggest their existence, such as the observation of neutrinoless double beta decay and the existence of neutrino oscillations.

4. How are Majorana neutrinos relevant to the Standard Model of particle physics?

The Standard Model of particle physics does not currently include Majorana neutrinos, as they are not predicted by the model. However, if they are proven to exist, it would require a modification of the Standard Model to account for their unique properties.

5. What implications would the discovery of Majorana neutrinos have?

The discovery of Majorana neutrinos would have significant implications for our understanding of the universe and the laws of physics. It could help explain the asymmetry between matter and antimatter, and could also shed light on the nature of dark matter. It could also lead to new developments in particle physics and potentially open up new avenues for research and discovery.

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