# What is the dominant interaction in elastic collision pi- + neutron -> pi- + neutron?

What is the dominant interaction in elastic collision pi- + neutron -> pi- + neutron? Is it strong nuclear interaction or electromagnetism?

Edit: sorry I meant to say "elastic". There is no matter - energy transformation.

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Sorry, as you wrote it, $\pi^-+n \to \pi^-+n$ is clearly and inevitably an elastic collision because the final and initial particles coincide, no other are produced, so the kinetic energy has to be conserved because the total energy is conserved. There is no inelastic collision given by the equation you wrote. If you added $+X$ to the right-hand side, I am pretty sure that as long as the excess of energy would be well enough the pion mass, $X$ would be pion-like and dominated by the strong force. For low energies, no new strongly interacting particles can be produced, only photons. –  Luboš Motl Nov 10 '11 at 7:36
But even if the extra final particles emitted were photons, it doesn't really mean that the interaction only depends on the electromagnetic force. One may draw diagrams - potentially greater one numerically - which include the strong-force vertices, rearrange the quarks etc. in various ways, and emit a photon from one of them. The electromagnetic interaction has to be included (at least 1 vertex) because photons don't interact strongly; but the strong vertices may be included as well. –  Luboš Motl Nov 10 '11 at 7:39
user6076 make up your mind. the title says "elastic" and in the question "inelastic". I have replied to the elastic question. –  anna v Nov 10 '11 at 11:57

Elastic scattering by the optical theorem is connected to the total cross section. The total cross section is dominated by strong interaction particle creation byproducts if enough energy is available.

If the pion is very slow ( low energy)it will decay before it interacts. The nucleus coulomb barrier will be important for a small part of the phase space of increasing energy, i.e. the electron volt range. In the MeV range, where one has a chance to have a pion beam, it is the strong interaction that will take over. There will be an energy region where they are equal.

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