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Neutrino interaction/detection?

  1. Feb 3, 2012 #1
    I'm aware a neutrino does not 'interact' with other particles nor have charge.

    But in the Super-K detector, neutrinos are said to be detected through the 'charged particles they generate when they occasionally interact with other particles.'

    How exactly does this occur? I mean, what does the word interaction really refer to here? Collision? And how are the charged particles generated?

    I hope I've made my question clear :-)

    Many thanks in advance!
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  3. Feb 3, 2012 #2


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    This isn't true. Neutrinos don't carry electric charge or color charge, so they don't interact through the electromagnetic interaction or through the strong interaction. However, neutrinos do interact with other particles through the weak interaction. As its name says, this interaction is weak, so neutrinos only rarely collide with atoms, but these interactions do occur, and this is how the detectors work.
  4. Feb 3, 2012 #3
    So, -- pardon my ignorance :-) -- but what prevents them from colliding more often? Their size?
  5. Feb 3, 2012 #4


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    When neutrinos collide with atoms, in those few cases that this happens, it knocks out a relativistic electron. In fact, this electron has such high energy that its speed in that medium (usually water) is faster than the speed of light in that medium. This creates a Cerenkov radiation. It is this radiation that is detected at many of the neutrino detectors around the world, including at SuperK.

  6. Feb 3, 2012 #5
    I think size is one factor. Neutrinos are so small that the space between atoms is like a asteroid roaming through our solar system. The space between particles is enormous to the neutrino, and there is no "gravity" to pull them closer to any other "planets". And if string theory is right, neutrinos may be a superstring, which makes their size even smaller; the equivalent of you and me roaming our entire observable universe trying to hit one planet.
  7. Feb 3, 2012 #6
    @ZapperZ, @mjacobsca - Thanks a lot. I think that quite clarifies everything!
  8. Feb 3, 2012 #7


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    This is a bit misleading. It has nothing to do with "size", because these things (including electrons an other elementary particles) are considered as "point-particle". It has everything to do with the type of interactions that each of the particle can undergo. As has been stated, neutrino only undergoes the weak interaction (and gravity, but it has such a weak mass that we can't detect such a thing... yet!). Point particles such as electrons interact predominantly via electromagnetic interactions, which is significantly stronger.

    Last edited: Feb 3, 2012
  9. Feb 5, 2012 #8
    Yes it is to do with the type of interaction but it does also have allot do do with size(of the spacing between the nucleus and the electrons). As I am sure you are aware atoms are > than 99.9% empty space, and thus it is extreamly unlikely that a neutrino will come into contact with THE NUCLEUS NOT AN ELECTRON( witch is even harder to detect) and then this interaction can be detected.
  10. Feb 5, 2012 #9


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    You missed the point.

    One can argue that the electron is the "same size" as the neutrino. By default, it too should see all those empty space. Yet, we can detect electron easily, but not neutrino.

    By logic, the "size" here doesn't matter! It must be predominantly due to something else!

  11. Feb 5, 2012 #10

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    Not so, profesrchaos - neutrino-nuclei interactions are more common than neutrino-electron interactions. As Zz says, size is not the most relevant factor.
  12. Feb 5, 2012 #11
    sorry i believe one has been misunderstood, I do state(however not clearly) that neutrino nucleus interactions occur more frequently than neutrino electron interactions, i just outline that neutrino nucleus interactions(the more common) are incredibly unlikely to occur due to the fact that atoms are so very empty and the fact neutrinos are hard to detect is they have no charge and i believe they are more of a point particle than an electron is(they are smaller) so the factors are the do not relay interact unless with collusion with say nucleus/electron witch is highly unlikely due to size of them and spacing in atom. This is why CERN fire billions of these at geneva and only a few compared to the amount fired are detected
  13. Feb 5, 2012 #12


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    Electron-nucleon and muon-nucleon interactions are both more likely than neutrino-nucleon interactions, because electrons and muons can interact via both the electromagnetic and weak interactions, whereas neutrinos can interact only via the weak interaction.

    These have all been studied at accelerators, using high-energy beams of electrons, muons and neutrinos impinging on suitable targets.
  14. Feb 5, 2012 #13
    How can something be "more of a point particle"? This doesn't make much sense, because both the neutrino and the electron are both modeled to be point particles, that is both are infinitely small, and one is no bigger or smaller than the other! We can't attribute a size to a point. Also, CERN is firing Protons against Protons at the LHC, not electrons at nuclei, or neutrinos at nuclei.

    The point is, neutrinos interact only through weak interactions, while the electrons interact through both weak and electromagnetic interactions, where the electromagnetic interaction is much stronger than the weak, so that is why neutrinos are hard to detect compared to electrons.
  15. Feb 5, 2012 #14
    "CERN is firing Protons against Protons at the LHC, not electrons at nuclei, or neutrinos at nuclei." It is a given that CERN firers protons against protons, but then the collisions release particles, some of which are called neutrinos, these are then directed (fired) at the big detector at Geneva( a place). In physics we can call a star a point source of light and we can say that a light bulb is more of a point source that is what i was getting at with the electron and the neutrinos. And yes they are both modeled to be point particles but if you understood why the higgs gives the neutrino smaller mass you would understand their relative sizes unless your going to tell me that an electron is much more dense than a neutrino
    Last edited by a moderator: Feb 5, 2012
  16. Feb 5, 2012 #15
    I work for one of these detectors (ATLAS) at CERN, and we do not direct neutrinos anywhere. Yes, they are produced in the collisions, but we do not get to direct them in any way after that (at least not in the collisions at ATLAS). They simply leave the detector undetected, and the only way we know that they were there is by calculating the missing energy that they took away with them.

    I think you are missing the idea. You can't talk about the density of electrons or neutrinos, the concept is completely irrelevant, because as far as we know, they are both infinitely small. Also, the mass of a particle has nothing to do with its size. The coupling of particle to the Higgs has nothing to do with their size. The reason you can talk about the relative sizes of stars and lightbulbs is because they actually take up some finite volume, while point particles, by definition, have no volume, so you can't talk about one being bigger than the other.
  17. Feb 5, 2012 #16
    It seems rather obvious when you put it that way!

    @cbetanco - So you account any of (all) the missing mass to neutrinos?
  18. Feb 5, 2012 #17
    Perhaps you are thinking of the CNGS beam? CNGS uses neutrinos created in the Super Proton Synchotron which are directed* towards the Gran Sasso lab (not to Geneva). While the Super Proton Synchotron is also used as the final acceleration stage for the protons that collide in the ATLAS and CMS detectors, the neutrinos created in those collisions are not the same ones directed to Gran Sasso. The LHC and CNGS are two experiments using some of the same apparatus.

    *As always, in reality it's a little more complicated than that. One can't simply catch neutrinos and shoot them at something ;)

    Also, hi everyone. New here. :)
  19. Feb 5, 2012 #18
    I appear to be in the minority here, but I don't believe in infinitely small, point-particles. String theory suggests that the point particles are individual strings, with sizes at the scale of the Planck Length, and whether string theory is correct or not, I believe that quarks, electrons, and neutrinos do have a size, and are not infinitely small. So perhaps the weak interaction is the only thing pulling neutrinos into other matter, but I'm not ready to discount size as a factor until someone proves string theory wrong.
  20. Feb 5, 2012 #19
    I totally agree, other theory's also suggest this M theory for example
  21. Feb 5, 2012 #20
    There is strong argument against the finite size.

    Deep inelastic Scattering experiments at SLAC (at least at the energies involved) shows particles to be pointlike.

    Maybe their size is finite, but that would probably require probing the plank scale, which may never be done.

    I am content to this of particles of zero volume, either way that does not mean that the previous size argument is wrong.

    Atoms are mostly empty and even though the neutrinos have zero volume it is still possible to set a scale of distance between the nuclei and the neutrino, which helps the size argument.
  22. Feb 5, 2012 #21
    There is a relevant concept of size when it comes to reactions such as neutrino collisions with electrons or nuclei - but the size in question is not a property of any individual particle!

    The likelihood of an interaction between one particle, let's say a, and another particle b at which a stream of a particles is 'fired' or directed, is expressed mathematically by a measure known as the cross-section for the reaction in question. The cross-section is in units of area (length squared) and can naively be thought of as the size of the target area that the incoming particle a has to 'hit' for an interaction with b to occur.

    The size of the cross-section depends on the interaction we have in mind, so in general will depend on the identities of both a and b. For example the cross-section for an interaction between a photon and a neutrino will be zero (ignoring high-energy quantum corrections) because photons and neutrinos do not interact directly. In a lot of cases the cross-section also varies according to other variables, most commonly the centre-of-mass energy of the particles. Weak interactions at low energies are a case in point: the lower the COM energy the further off mass shell the virtual W/Z boson will be and hence the lower the probability of the interaction occuring.

    Because neutrinos only interact weakly, their cross-sections for interactions with electrons or nuclei are correspondingly small. Because neutrinos are more likely to interact with nuclei than electrons, the cross-sections for neutrino-nuclei interactions will be larger than those for neutrino-electron collisions.
    Last edited: Feb 5, 2012
  23. Feb 5, 2012 #22


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    You are kidding, right? String theory right now is "not even wrong"!

    This "prove" thing works both ways. Show a VALID (i.e. experimentally verified) theory in which there IS a size to such thing. In other words, find something that has the same degree of validity as QED!

    Based on what we know now, size isn't a factor. We already have a plausible reason for why neutrinos interact weakly without invoking String. Beyond that, you are making speculative assertion that would violate the PF Rules.

  24. Feb 5, 2012 #23
    Well, yes and no.

    For certain well known interaction, I would say yes, for example for a muon in the detector decaying into a neutrino and a charged pion (this is not the only decay mode for the muon, just the most likely), you would detect the pion, and the neutrino would be known to be there from the calculated missing energy.

    But missing energy does not always imply a neutrino. One example would be in the case of a theory with extra spatial dimensions, in which some of the energy of your interactions "leaks" into the other spatial dimensions, and hence is lost in your detector (but this is a speculative theory).

    This is highly speculative and has not a shred of experimental backing as of yet. All current experimental evidence points to elementary particles being point like. So I think it's a bit mute to argue about what you THINK the structures of particles might be, since there is absolutely no evidence that particles have any structure at all. I think the OP was interested in how we detect neutrinos, which you can give a satisfactory answer without ever having to invoke the idea of a finite size of elementary particles, which is not how the standard model treats them.
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