Collision of Particles: What Force Causes Repulsion?

[quasar987]

I'm reading this chapter on collision of particles and it's bugging me that they do not explain the nature of the force responsible for the repulsion betweeen the two particle when they meet. I thought about it and figured that in the case of collision of macroscopic bodies, it was the intermolecular force that is responsible for the repulsion. I read about this force in the Feyman lectures; it is vigorously repulsive past a certain distance but attractive and varying inversely with the 7th power (!) of the distance at large distances.

But what about the force responsible for collision between neutrons? What force makes them bounce off each other?

 

[Moose352]

I'm not sure about this, but the fact that neutrons are made up quarks (which are electrically charged) could explain this. In a related note, how is the "size" of a particle defined? Is it related to the strength of the field around it, or is there something else that defines it?