Neutrinos as majorana particles or where are left handed antineutrinos?


by roberto85
Tags: antineutrinos, handed, majorana, neutrinos, particles
roberto85
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Dec16-11, 01:56 PM
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Hi everyone, this has been bothering me for a while and even though i've done some light reading on this topic im struggling to understand it.

I know that theory states that we do not see left handed antineutrinos or right handed neutrinos and this is where cp violations comes from. But since the neutrino has no charge, how would we be able to distinguish from a left handed antineutrino and a left handed neutrino? So could it be that in fact left handed antineutrinos do exist but they are just exactly the same as left handed neutrinos... which means that neutrinos don't actually have antiparticles at all. Or that the differing handedness between neutrinos is in fact their only distinguishing feature and we should in fact just define left handed neutrinos as neutrinos and right handed antineutrinos as the antineutrino. Could it be that neutrinos are the maverick of the standard model... i don't understand this?
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Bill_K
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Dec16-11, 03:11 PM
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roberto85, You shouldn't let things like this bother you, maybe we can talk it out.

The simple long-standing theory is that there only exist left-handed neutrinos and right-handed antineutrinos. At least these are the only ones that couple to other particles by the weak interaction. The interaction violates C and violates P, but preserves the combination CP.

How do we tell a neutrino from an antineutrino? By letting it hit something. For example, try to observe the reaction ν + p → n + e+. Well, you have to get the ν from somewhere, and you find that the ν's that come from n → p + e- + ν do produce this reaction, while the ones that come from p → n + e+ + ν do not. Showing that they are different particles.

More recently it's been learned that neutrinos must have a small mass, implying that right-handed neutrinos and left-handed antineutrinos exist as well, although they have not yet been seen directly. They don't participate in the weak interaction.
jtbell
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Dec16-11, 03:56 PM
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Quote Quote by Bill_K View Post
maybe we can talk it out.
Maybe we should change your username to Dr. Bill.

roberto85
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Dec16-11, 03:57 PM
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Neutrinos as majorana particles or where are left handed antineutrinos?


Quote Quote by Bill_K View Post
roberto85, You shouldn't let things like this bother you, maybe we can talk it out.

The simple long-standing theory is that there only exist left-handed neutrinos and right-handed antineutrinos. At least these are the only ones that couple to other particles by the weak interaction. The interaction violates C and violates P, but preserves the combination CP.

How do we tell a neutrino from an antineutrino? By letting it hit something. For example, try to observe the reaction ν + p → n + e+. Well, you have to get the ν from somewhere, and you find that the ν's that come from n → p + e- + ν do produce this reaction, while the ones that come from p → n + e+ + ν do not. Showing that they are different particles.

More recently it's been learned that neutrinos must have a small mass, implying that right-handed neutrinos and left-handed antineutrinos exist as well, although they have not yet been seen directly. They don't participate in the weak interaction.
In the above example it would have to be that one of the neutrinos was left handed and the other was right handed, which is what we use to distinguish them as being either a neutrino or an antineutrino respectively. But since a right handed neutrino would, i assume, look and behave exactly the same as a right handed antineutrino.. how do we know whether we have ever seen a right handed neutrino. Because currently our only way to distinguish the two types of neutrinos we see is by their handedness, but in the standard model we dont call particles with opposite handedness as antiparticles from each other. I think that it's a nomenclature thing which has been added to complete the patterns we see in the standard model, but i think by taking observation out of context of the standard model we should not call right handed antineutrinos as such and instead just call them right handed neutrinos which is what they are since there is no defining feature which makes them an antiparticle. Unless... there is more information i am missing.. which did make me think of virtual neutrinos since these would be created as particle antiparticle pairs. But im sure we've not seen virtual neutrinos directly but perhaps they are indirectly implicated in calculation of the electron dipole moment? Maybe this issue can offer clues to the anomalous muon dipole moment?

I understand the case for having antiparticles of neutrinos so that it fits in with the rest of the fermions in the standard model. But could it be that actually, since it is the only chargeless fermion, it does not have an antiparticle and it in fact is only one particle which comes in a left handed form and a right handed form (which we call antineutrinos). I think i need to read further about these majorana neutrinos.

I feel like i have reached a logical conclusion but i suspect i am missing something.. but i do wonder whether the naming convention of neutrino particles is not correct?
Bill_K
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Dec16-11, 04:45 PM
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There was an early attempt at describing neutrinos as Majorana particles. The crucial experimental result which killed the idea was the absence of neutrinoless double beta decay. See the Wikipedia article.
roberto85
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Dec16-11, 05:01 PM
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Quote Quote by Bill_K View Post
There was an early attempt at describing neutrinos as Majorana particles. The crucial experimental result which killed the idea was the absence of neutrinoless double beta decay. See the Wikipedia article.
Yes, to be honest i don't quite like the see saw mechanism of the majorana neutrino theory. But i still think there's something mysterious about neutrinos we have yet to discover, with the weak sector cp violation being the smoking gun. Its on my list of things to keep an eye on ;)
AdrianTheRock
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Dec17-11, 04:28 AM
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Quote Quote by roberto85 View Post
...I understand the case for having antiparticles of neutrinos so that it fits in with the rest of the fermions in the standard model. But could it be that actually, since it is the only chargeless fermion...
Standard Model neutrinos are not chargeless - they just don't have electric or colour charges. They do, however, have weak charges (otherwise they wouldn't interact with the W or Z bosons).

Electric charges are derived from the basic electroweak charges in accordance with the formula

Q = I3W + (YW / 2)

where I3W is called weak isospin and YW weak hypercharge.

(Left-handed) neutrinos have I3W = +1/2 and YW = -1, which add up to zero electric charges. RH anti-neutrinos have -1/2 and +1 respectively.

RH neutrinos and LH anti-neutrinos do have charges of 0 and 0 respectively, and are thus sterile (though the term "sterile neutrino" is more commonly used when referring to additional neutrino flavours that also have zero charges).
roberto85
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Dec17-11, 02:29 PM
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Quote Quote by AdrianTheRock View Post
Standard Model neutrinos are not chargeless - they just don't have electric or colour charges. They do, however, have weak charges (otherwise they wouldn't interact with the W or Z bosons).

Electric charges are derived from the basic electroweak charges in accordance with the formula

Q = I3W + (YW / 2)

where I3W is called weak isospin and YW weak hypercharge.

(Left-handed) neutrinos have I3W = +1/2 and YW = -1, which add up to zero electric charges. RH anti-neutrinos have -1/2 and +1 respectively.

RH neutrinos and LH anti-neutrinos do have charges of 0 and 0 respectively, and are thus sterile (though the term "sterile neutrino" is more commonly used when referring to additional neutrino flavours that also have zero charges).

Cool, i had read a little about sterile neutrinos and read something about sterile neutrinos possibly being neutral heavy leptons (NHL).. oh i see where this links in with the seesaw mechanism. Okay, i'm not so dismissive of the seesaw mechanism now especially since
NHL's might be a good candidate for dark matter. All very interesting, thanks for helping me clear up this issue that had been bothering me for some time.


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