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Interaction between Neutrons/Neutrinos

  1. Apr 27, 2013 #1

    I was wondering if there is any evidence on how (free/not within nucleus) neutron/neutron interactions work. I know there is QCD which states they attract (but maybe only in a nucleus?), but that's theory - has this been experimentally proven at all ? I'm only interested in the free particle scenario as the rules within a nucleus may be entirely different when protons come into play.
    Same question applies for Neutron/Anti-neutron, Neutrino/Neutrino, Neutrino/Anti-Neutrino interactions. I only found theoretical predictions on these interactions (or nothing at all) and sometimes they were even contradictory.
    I think QCD definetly does not act on neutrinos/anti-neutrinos and I'm really having trouble believing that neutrino/anti-neutrino pairs would not attract each other even tho they annihilate upon collision. Something smells foul here I think ;)
    Maybe somebody could clarify this to me ?

    Thanks and cheers !
  2. jcsd
  3. Apr 27, 2013 #2


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    QCD cannot be applied except in a bounded domain due to confinement. The theory states that the quark components of neutrons (and protons) in the nucleus are bound together by the strong force mediated by the gluons. The individual quarks have color charges dictating their color interaction. That interaction is confined within the nucleons with some slight neutral dipolar (or tripolar?) residual effects extending beyond. (consider how two objects can magnetically attract though they have no net magnetic charge.)

    This is the part of the strong force responsible form nuclear binding. This picture is simplistic given the quantum nature of the interactions at this scale. But it is also necessarily short ranged so doesn't apply at all in the free particle scenario you demand. The strong force has no effect on neutrinos and the other leptons as they are color neutral.

    The only free particle interactions between neutrinos or neutrons given their separation which excludes weak and strong forces and their EM charge neutrality would be their gravitational attraction and some multipole moment electromagnetic interaction. They will behave like very very weak bar magnets and may have some slight electrical polarizability. I don't believe these could ever be measurable in the free particle scenario, especially given the elusive nature of neutrinos and the limited lifespan of free neutrons.

    As to empirical evidence rather than theory, you have to understand that we cannot build a color multimeter the way we do with electric charge and so we can't do the same sort of pith ball experiments to establish direct observation of strong forces the way we do with EM. What we do is analyze how the predictions of the theory relate to large numbers of particle collision experiments. That's what those big colliders are for. In so far as the theory can provide any tractable calculations of specifics we get good agreement with standard model predictions and the results of experiments. That is indirect empirical evidence.
  4. Apr 27, 2013 #3
    Thank you for your reply.
    Some questions remain, tho.
    So this means that when considering a neutrino/antineutrino-pair (to leave QCD/strong-force out of the game) which is separated by some distance (beyond weak force range), both particles would only feel the extremely low attractive gravity force and nothing else ? Which in turn would mean that the annihilation of the pair is extremely unlikely and therefore neutrinos/anti-neutrinos would probably have by far the highest particle-counts compared to the other particles. Is this the case ? And if not, why not ?
    Last edited: Apr 27, 2013
  5. Apr 27, 2013 #4
    I hope my questions are not too nooby, but I really struggle finding consistent information about neutrino/neutrino interaction on the web =/
  6. Apr 27, 2013 #5


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    xortdsc, First of all, the interactions you're talking about involve only neutral particles and are extremely difficult if not impossible to observe experimentally.

    The neutron-neutron interaction and the possible existence of a "dineutron" bound state has nevertheless received a fair amount of speculation. (This, by the way, is still within the realm of nuclear physics, not QCD.) The nuclear force is believed to be charge-independent, which means p-p, n-p and n-n forces are the same, only the Coulomb force in p-p is different.

    The force does, however, depend on the relative spin orientation of the two particles. If the spins are parallel, we call it the triplet state. If they are antiparallel, we call it the singlet state. The interaction in the triplet state is observed to be attractive, while the interaction in the singlet state is repulsive.

    A proton and neutron can be in the triplet state, and form a stable bound state (the deuteron). But due to the exclusion principle, two neutrons will find themselves in the singlet state and will not be attracted.
  7. Apr 27, 2013 #6
    parallel spins attract and anti-parallel spins repel ? I thought it was the other way around. Isn't it the reason why 2 electrons can share an atomic orbital, because they must be anti-parallel (not having same spin -> not being in the same quantum state) ? i'm confused now... that spin thing is quite confusing to me anyway.
    lets say we have 2 electrons. can they be in the same place with anti-parallel spin ? and would the repulsive force between 2 nearby electrons with anti-parallel spin be as great as with paralell spins or how does it differ ? can the parallel/anti-parallel spin combinations be seen as a "force modulator" or as an "additional force" ? there is clearly quite a gap in my knowledge...
  8. Apr 27, 2013 #7


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  9. Apr 27, 2013 #8


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    Neither. Spins don't attract OR repel. It's just that the nuclear force happens to be different depending on the particle spins.

    Both nucleons and electrons have spin 1/2, but the forces between them are quite different. We were talking about the nuclear force between protons and neutrons. The nuclear force gets pretty complicated, but it's an observed fact that two nucleons with parallel spins attract, while ones with antiparallel spins repel. With two electrons there's only the Coulomb force which is always repulsive (like charges) and does not depend on spin.

    Both nucleons and electrons are fermions - that is they obey Fermi-Dirac statistics. Which means, as you say, that two of them can't occupy the same state. You can put two identical fermions in the same place only if their spins are different (antiparallel, i.e. singlet state). And this is what I said before:

    Be clear that the exclusion principle itself is a law, and not a force(!) People often mistake that. The exclusion principle says the particles must be in the singlet state, while it's the nuclear force in the singlet state which does the repelling.
  10. Apr 27, 2013 #9


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    The reason for the reversed spins of orbital electrons or nucleons is principally the manifestation of Pauli's exclusion principle given their fermionic statistics. They an have identical momentum/position states provided there is one remaining difference, their spins.

    The spin also corresponds with the magnetic moments of the particles so there is also magnetic dipole interactions. Indeed when we say we measure a particle's spin we actually observe its magnetic moment as it interacts with a non-uniform magnetic field.
  11. Apr 28, 2013 #10

    thank you all very much for clarifying. Also the linked thread gave quite some insight. I gotta digest that information.
    But just to be clear: There is no force (discovered yet) which results in the pauli exclusion principle ? So it's a somewhat "hardwired" rule to make sense of the behaviour of fermions without a deep understanding why it is that they won't overlap ? I guess scientists would agree that there should be some sort of force resulting in this principle as just the principle by itself is sort of unsatisfying as it lacks explaination ?

  12. Apr 28, 2013 #11


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    Nope, and nope. The exclusion principle is well understood, and requires no explanation in terms of a force. It's a rigorous consequence of quantum field theory - that all integer spin particles must be bosons, and spin-half particles must be fermions. This is quite satisfying! :smile:
  13. Apr 28, 2013 #12
    Hmm, and it's not problematic that all processes can be described as interactions except for the exclusion principle ?
    I mean I'm sure the principle works well and is enough to make predictions, but from a more-less philosophical perspective is seems very unelegant, isn't it ?
    But I guess in QM there is plenty of philosophically rather unpleasing stuff, like the particle/wave-duality for example. By knowing in which situation something must be treated as a particle or a wave may work really nicely, but is sort of incomplete if one cannot explain why it behaves sometimes like this and sometimes like that and how it transforms between the states. Tho, maybe it's just my lack of knowledge. That's why I'm here after all. :)
    Last edited: Apr 28, 2013
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