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I know that the continuous exchange of gluons between quarks is what holds hadrons together, and that the exchange of pions between nucleons is what creates the strong residual force. However, how exactly does the pion mediate the residual strong force--is it emitted by one nucleon and absorbed by another, equivalently emitting and absorbing the gluons that hold the pion together? How is the pion formed in the first place? The way I reasoned was: the quarks in two nucleons get close enough to each other to pull them away from the quarks in their parent nucleons; it is energetically favorable to form new particles instead of pulling the quarks apart, so pions are formed.

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  • $\begingroup$ The meson theory of nuclear forces may help scholarpedia.org/article/… .It uses quantum field theory to construct a model ( search yukawa ) long before quarks were discussed (1935). $\endgroup$
    – anna v
    Feb 10, 2022 at 5:03
  • $\begingroup$ An analogous question: how do photons mediate the electromagnetic force? How are the photons formed in the first place? If you understand that, it's basically the same thing. $\endgroup$
    – Chris
    Feb 10, 2022 at 6:51

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Here is how I visualize this process. Please tell me if it is incorrect and I will delete:

we imagine two nucleons sitting right next to one another. Note that a pion has two quarks inside it and the nucleons have three each. A quark that just happens to come close to the "surface" of its nucleon at the same time another quark happens to do the same thing right next door will "see" the other quark and be influenced by it. At the moment the two quarks are thus "looking" (i.e., throwing gluons back and forth) at one another from neighboring nucleons, one could claim that those two quarks constitute a temporary pion being shared in some sense between the two nucleons, which would tend to hold the nucleons together. Note that because the quarks are each confined in their respective nucleons, we cannot reach down in there and actually pluck that pion-like thing away for further study as if it were a real, live pion.

Of course the real physics is far, far more complicated and subtle than this crude model; please let me know if this is a sensible (although hand-waving) way to imagine how pion exchange glues nucleons together.

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  • $\begingroup$ I think that's a very nice explanation! Helped me understand the meaning of "virtual Pions". $\endgroup$
    – Connor
    Apr 27, 2022 at 13:01

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