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Neutron Neutron collision

  1. Oct 28, 2011 #1

    I have recently read a book about particle physics and was wondering
    what could happen if two neutrons hit each other and what energy level would be required for what ever happens?
    (Since the Pauli exclusion principle does not allow two neutrons to be at the same time and space).

    If a neutron splits into a proton, electron and an anti-neutrino
    why is the electron not flying into the proton or building a two particle cloud with the proton until thy might fall apart?

    please help the noob and thx,

  2. jcsd
  3. Oct 28, 2011 #2


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    OK, several things. First of all the Pauli principle only says that the overall wavefunction of the two particles is antisymmetric. This includes the spin, so if the particles have opposite spin (spin up and spin down) they are allowed to be at the same point in space.

    Secondly, most collisions between particles are indirect, that is they exchange a virtual particle. Inside nuclei, where neutrons constantly interact, they can be thought of as exchanging pions.

    Third, neutrons aren't elementary. On a more fundamental level a neutron-neutron collision would actually be between a pair of quarks.

    Regarding the second question, the neutron's mass is 939.5 MeV/c2 while the proton's mass is 938.2 MeV/c2. This leaves 1.3 MeV/c2 worth of kinetic energy to propel the decay products rapidly apart.
  4. Oct 28, 2011 #3
    Hi Bill_K, that is so kind of you, thank you.

    2) I also looked up some Feynman diagrams and my question is solved. I should have asked, what happens, when an electron gets captured by a proton.

    Lets say two neutrons with the same spin or a different spin that were created by stripping of the proton, even thought they are unstable, would collide. What could happen? The only Feynman diagram I could find was:

    Nah, I will just have to understand quark interaction. Is there a good book?

    udd + udd -> ?


    Last edited: Oct 28, 2011
  5. Oct 28, 2011 #4


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    It can't! Not unless it arrives with at least 1.3 MeV (see above) of kinetic energy, in which case they can turn back into n + νe. Lacking this much energy, they could elastic scatter, or conceivably form a hydrogen atom.

    What would happen if two neutrons collide? Again at low energy they could just scatter: n + n → n + n. With plenty of energy available they could produce anything you can think of, just so's the total charge is conserved (zero) and the baryon number (two). Pions are popular. So for example,

    n + n → p + p + π- + π- + π- + π+
  6. Nov 1, 2011 #5
    Neutrons are strongly interacting, thus, colliding neutrons will most likely produce whatever hadrons are above threshold, whatever ones have masses lower than the available energy. That includes pions, as Bill_K had mentioned.

    With energetic-enough collisions, the quarks and gluons in them start acting like more-or-less independent entities, colliding with each other more-or-less separately. However, the quarks and gluons that emerge then form hadrons.
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