[immijimmi]

More specifically: What separates a neutrino from an antineutrino? Lepton number cannot be considered as it is not a physical property of the particle: it is a value used to balance equations, and is assigned to them because they are either particles or antiparticles. They are electrically neutral and have no colour charge. The one property that could be different for them and their counterparts is the direction of spin. However, can't a neutrino spin in any direction anyway? I'm pretty sure there isnt a set of specific directions neutrinos are allowed to spin in, and a corresponding set of opposite directions for the antineutrinos. Surely that would require some explanation for the limitation, and because i can't think of anything that would back this up i'm going to assume the that that isn't the case. Therefore there is either no difference between a neutrino and an antineutrino, OR the difference is relative, and i'm not going into detail on that unless i have to because it's hard to put into words and isnt necessarily needed. So, if there really isn't a difference. If any of the above is incorrect, please give an explanation or i'll end up really confused.
If not, we can at least say the neutrinos are all their own antiparticles.

What do you think?

This post has been edited by immijimmi: 2 January 2012 - 03:16 AM

[swansont]

immijimmi, on 2 January 2012 - 03:15 AM, said:

More specifically: What separates a neutrino from an antineutrino? Lepton number cannot be considered as it is not a physical property of the particle: it is a value used to balance equations, and is assigned to them because they are either particles or antiparticles. They are electrically neutral and have no colour charge. The one property that could be different for them and their counterparts is the direction of spin. However, can't a neutrino spin in any direction anyway? I'm pretty sure there isnt a set of specific directions neutrinos are allowed to spin in, and a corresponding set of opposite directions for the antineutrinos. Surely that would require some explanation for the limitation, and because i can't think of anything that would back this up i'm going to assume the that that isn't the case. Therefore there is either no difference between a neutrino and an antineutrino, OR the difference is relative, and i'm not going into detail on that unless i have to because it's hard to put into words and isnt necessarily needed. So, if there really isn't a difference. If any of the above is incorrect, please give an explanation or i'll end up really confused.
If not, we can at least say the neutrinos are all their own antiparticles.

What do you think?

When neutrinos were thought to be massless, the difference was in their chirality — they are exclusively (or now strongly) left-handed: the spin and the linear momentum vectors are anti-aligned, with anitineutrinos being right-handed.
http://hyperphysics..../neutrino3.html
I think that we are still in the middle of nailing down details, because neutrino oscillations are a relatively recent observation, and the best model has yet to be developed or chosen.

We can't say that they are their own antiparticles until there is experimental confirmation, e.g. double beta decay.

[immijimmi]

swansont, on 2 January 2012 - 12:13 PM, said:

When neutrinos were thought to be massless, the difference was in their chirality — they are exclusively (or now strongly) left-handed: the spin and the linear momentum vectors are anti-aligned, with anitineutrinos being right-handed.
http://hyperphysics..../neutrino3.html
I think that we are still in the middle of nailing down details, because neutrino oscillations are a relatively recent observation, and the best model has yet to be developed or chosen.

We can't say that they are their own antiparticles until there is experimental confirmation, e.g. double beta decay.

I understand that, but what if a neutrino is emitted to right from a particle that is moving to the left at the same speed? To the observer the neutrino would not be moving and it would be impossible to tell its helicity. From what i've read in the article you linked helicity is affected by how the particle is moving relative to the observer, and although it would be hard to move faster than a neutrino moving close to c, it would not be impossible and some neutrinos can move slower than this aswell.

This post has been edited by immijimmi: 2 January 2012 - 06:29 PM

[swansont]

immijimmi, on 2 January 2012 - 06:26 PM, said:

I understand that, but what if a neutrino is emitted to right from a particle that is moving to the left at the same speed? To the observer the neutrino would not be moving and it would be impossible to tell its helicity. From what i've read in the article you linked helicity is affected by how the particle is moving relative to the observer, and although it would be hard to move faster than a neutrino moving close to c, it would not be impossible and some neutrinos can move slower than this aswell.

I suspect some of this depends on the nature of neutrino oscillations and the source of mass. That's not my area of physics.

[guenter]

The point is that in case the neutrino is massless one can define only observers who see it from "one side". From this perspective neutrinos are lefthanded and antineutrinos are righthanded, as according to the standard model. If however they possess mass, then neutrinos having a velocity < c can be overtaken and thus look righthanded. If left/right handed depends on a choosen frame of reference, then the left handed neutrino is identical with the righthanded neutrino. This at least is my understanding.

[swansont]

guenter, on 3 January 2012 - 06:30 PM, said:

The point is that in case the neutrino is massless one can define only observers who see it from "one side". From this perspective neutrinos are lefthanded and antineutrinos are righthanded, as according to the standard model. If however they possess mass, then neutrinos having a velocity < c can be overtaken and thus look righthanded. If left/right handed depends on a choosen frame of reference, then the left handed neutrino is identical with the righthanded neutrino. This at least is my understanding.

Mine as well. What I don't know is what kind of problems you get with the loss of conservation of lepton number, and how the oscillations affect that. Is there some symmetry that is broken, and how does it happen?

[questionposter]

Wait, we don't know of anti-neutrinos yet?

http://en.wikipedia....i/Anti-neutrino

Why does a science forum use wikipedia as a reference anyway?

[guenter]

swansont, on 3 January 2012 - 07:44 PM, said:

Mine as well. What I don't know is what kind of problems you get with the loss of conservation of lepton number, and how the oscillations affect that. Is there some symmetry that is broken, and how does it happen?

Good question, I would love to know the answer. To my knowledge the experimental results regarding neutrino oscillations offer an explanation how they acquire mass, but not why the lepton number isn't conserved. The conservation of this number is restricted to the SM. In the frame of most of the GUT-theories the lepton number is not conserved, e.g. not with the predicted proton decay. In these theories only the difference of baryon- and leptonnumber is conserved. However the proton decay isn't detected yet, so it's speculation. But inspite of this the neutrino oscillations indicate that the SM will have to be modified. Perhaps and hopefully the LHC will tell more within some years.

[swansont]

questionposter, on 4 January 2012 - 06:50 AM, said:

Wait, we don't know of anti-neutrinos yet?

I'm at a loss as to how you came to this conclusion.

[questionposter]

swansont, on 4 January 2012 - 10:48 AM, said:

I'm at a loss as to how you came to this conclusion.

swansont, on 2 January 2012 - 12:13 PM, said:

I think that we are still in the middle of nailing down details, because neutrino oscillations are a relatively recent observation, and the best model has yet to be developed or chosen.

We can't say that they are their own antiparticles until there is experimental confirmation, e.g. double beta decay.

If we knew that neutrinos had an anti-matter counterpart, we'd know for certain they couldn't be their own anti-particle, just like how we know an electron isn't its own counterpart because we discovered the positron.

This post has been edited by questionposter: 4 January 2012 - 10:28 PM

[DrRocket]

questionposter, on 4 January 2012 - 06:50 AM, said:

Why does a science forum use wikipedia as a reference anyway?

Because some people ask endless inane questions without first doing the simple research that ought be expected of them and wiki provides a ready reference that one can use without having to write a textbook. I like to use wiki articles after I have satisfied myself that the article is correct (not all of them are correct, by a long shot).

[immijimmi]

DrRocket, on 5 January 2012 - 12:31 AM, said:

Because some people ask endless inane questions without first doing the simple research that ought be expected of them

I did put the research in beforehand and i know that neutrinos are meant to have corresponding antineutrinos. What I was asking is what the true physical difference is between the two, and that was something I didn't manage to find.

The current view is that neutrinos have mass because they must have mass for oscillations to occur. Furthermore, mass is the only difference between the three types of neutrino AND if they didn't have it, they would constantly move at c which defies observations.

This post has been edited by immijimmi: 5 January 2012 - 06:11 AM

[DrRocket]

immijimmi, on 5 January 2012 - 06:05 AM, said:

I did put the research in beforehand and i know that neutrinos are meant to have corresponding antineutrinos. What I was asking is what the true physical difference is between the two, and that was something I didn't manage to find.

The current view is that neutrinos have mass because they must have mass for oscillations to occur. Furthermore, mass is the only difference between the three types of neutrino AND if they didn't have it, they would constantly move at c which defies observations.

You apparently did a bit of research. But you are not questionposter and you did not question the use of wiki articles in providing an explanation.

The mass of neutrinos is thought to be exceedingly small. It is only because of this mass, which as you note is required for neutrinos to oscillate between types, that they do not travel at c. Prior to the discover of the neutrino oscillations particle physics texts treated the neutrino as massless, traveling at c. I am not aware of any experiments that have actually measured a neutrino speed below c, and in fact there is one that purports to have evidence of superluminal travel by neutrinos (though I think most people expect that there is a mistake somewhere).

The difference between particles and anti-particles lies in a change in sign of certain quantum numbers -- charge and chirality.

Assuming that neutrinos are indeed massive, chirality is not the same as helicity.

That is the "true physical difference" between a particle and an anti-particle.

This post has been edited by DrRocket: 5 January 2012 - 06:30 AM

[immijimmi]

DrRocket, on 5 January 2012 - 06:27 AM, said:

You apparently did a bit of research. But you are not questionposter and you did not question the use of wiki articles in providing an explanation.

The mass of neutrinos is thought to be exceedingly small. It is only because of this mass, which as you note is required for neutrinos to oscillate between types, that they do not travel at c. Prior to the discover of the neutrino oscillations particle physics texts treated the neutrino as massless, traveling at c. I am not aware of any experiments that have actually measured a neutrino speed below c, and in fact there is one that purports to have evidence of superluminal travel by neutrinos (though I think most people expect that there is a mistake somewhere).

The difference between particles and anti-particles lies in a change in sign of certain quantum numbers -- charge and chirality.

Assuming that neutrinos are indeed massive, chirality is not the same as helicity.

That is the "true physical difference" between a particle and an anti-particle.

Thanks, that's all the answer I needed.

You're right that neutrinos have very small mass, electron neutrinos have less than 3 eV in rest energy.
Haven't neutrinos been observed at just under c coming from the Sun, though? I'm sure i've heard that somewhere.

[swansont]

immijimmi, on 5 January 2012 - 06:35 AM, said:

Haven't neutrinos been observed at just under c coming from the Sun, though? I'm sure i've heard that somewhere.

I'm not sure how you could determine that; I can't think of a way to time tag an event that produced them, and the speed differences are exceedingly small. But scientists are clever so if you have a link I'd like to read about it. The detection of neutrinos (e.g. from SN1987a) is also indeterminate, because it takes a little time for the photons to make it out of the supernova so the neutrinos arrive first. But that gives you some information; SN1987a was 168,000LY away, and the time difference was a few hours. If you know the time delay of the photons you could get an approximate speed of the neutrinos. I think you're looking at roughly a day out of 168,000 years, or a part in 10^8.

[immijimmi]

swansont, on 5 January 2012 - 11:01 AM, said:

I'm not sure how you could determine that; I can't think of a way to time tag an event that produced them, and the speed differences are exceedingly small. But scientists are clever so if you have a link I'd like to read about it. The detection of neutrinos (e.g. from SN1987a) is also indeterminate, because it takes a little time for the photons to make it out of the supernova so the neutrinos arrive first. But that gives you some information; SN1987a was 168,000LY away, and the time difference was a few hours. If you know the time delay of the photons you could get an approximate speed of the neutrinos. I think you're looking at roughly a day out of 168,000 years, or a part in 10^8.

sorry, i dont have a link. I think i heard it in a documentary about the neutrino research at SNOLAB.

[immijimmi]

Found it!! It isnt helicity because they don't travel at c so it's technically a variable. Apparently it's chirality. I had to look in the wiki page for sterile neutrinos to find that gem, though.

Credit to DrRocket for the original suggestion of chirality though.

This post has been edited by immijimmi: 20 January 2012 - 04:51 PM

[Widdekind]

I understand, that an anti-neutrino has a "Weak hyper-charge" of +1, whereas a neutrino has a "Weak hyper-charge" of -1. Naively unifying W+EM (hyper)charges, into a "charge doublet" (Q_W,q_EM); then anti-neutrinos would have the vector (0,+1); neutrinos (0,-1). Via absorption of W⁺ bosons, anti-neutrinos can be "charged up" into anti-electrons (+1,+1). Via absorption of W^- bosons, neutrinos can be "charged down" into electrons (-1,-1).

I understand, that Weak hyper-charge is a "good quantum number", i.e. conserved in every interaction; and, that anti-neutrinos & neutrinos are exactly as distinct, as anti-electrons & electrons (or anti-matter & matter) ?

I understand, that neutrinos, as fermions (s = 1/2), can exist in two states, i.e. "right-handed" (s_z = +1/2) and "left-handed" (s_z = -1/2), where the z-axis is defined to be parallel to the particle's linear momentum. Likewise for anti-neutrinos. If so, then RH neutrinos are no more equivalent, to LH anti-neutrinos, then "spin-up" electrons are to "spin-down" anti-electrons. I understand, that the Dirac equation implies the existence of four species of each fundamental particle, i.e. RH/LH for M/AM, none of which represent exactly the same quantum states.

This post has been edited by Widdekind: 22 January 2012 - 02:10 PM

[IM Egdall]

immijimmi, on 6 January 2012 - 03:55 PM, said:

sorry, i dont have a link. I think i heard it in a documentary about the neutrino research at SNOLAB.

See http://neutrinoscien...-for-party.html

[Widdekind]

swansont, on 5 January 2012 - 11:01 AM, said:

I'm not sure how you could determine that; I can't think of a way to time tag an event that produced them, and the speed differences are exceedingly small. But scientists are clever so if you have a link I'd like to read about it. The detection of neutrinos (e.g. from SN1987a) is also indeterminate, because it takes a little time for the photons to make it out of the supernova so the neutrinos arrive first. But that gives you some information; SN1987a was 168,000LY away, and the time difference was a few hours. If you know the time delay of the photons you could get an approximate speed of the neutrinos. I think you're looking at roughly a day out of 168,000 years, or a part in 10^8.

I understand, that neutrinos are generated, exclusively by Weak interactions; and, therefore, neutrinos are generated, exclusively into Weak eigenstates , which are super-posed "mixtures", of the canonical mass eigenstates . Somehow, those different mass eigenstate components, of the neutrino's wave-function, evolve at different rates, due to said different masses; and that the resulting "beat frequencies" are observed, as neutrino oscillations.

How should this be understood ? For, in the rest-frame of a massive neutrino, surely the various components, of its wave-function, would not separate apart; rather, the wave-function would surely remain more-or-less "clumped together". And if so, then would not the wave-function, of that same neutrino, in some other reference frame, in which the neutrino was moving rapidly, be the "clumped together" wave-function, of the particle, in its own rest-frame, relativistically "boosted" by some velocity near that of light ?

If so, is not the constraint, on the various components, of a neutrino's multi-faceted wave-function, that they all must be propagating, with the same velocity ? You mentioned SN1987a -- did those neutrinos arrive "all together in one pulse", or did they arrive gradually, over hours / days ? For, if they arrived as a "clump", then surely their wave-functions had not become "smeared through space", as would surely be the case, if the various components, of the neutrino wave-functions, were propagating at different speeds. I.e. if a neutrino is a little like a "motorcade of three cars", then all three "cars" "drive" at the same speed ? Again, otherwise, if not, then one of the "cars" would pull ahead; one of the "cars" would lag behind; and the neutrinos would become "smeared out thru space", with one "flavor" of "car" detected for the first few hours/days; then the next "flavor"; then the next.

IM Egdall, on 22 January 2012 - 03:50 PM, said:

See http://neutrinoscien...-for-party.html

The neutrinos generated by SN1987a were electron neutrinos. Whereas, the neutrinos generated for the CERN-SG experiments, were muon / tauon neutrinos, i.e. different "flavors" of neutrinos, from those generated by SN1987a. Could that "flavor" difference account for the discrepancies ?