Dear members, please help me to understand clearly what we are talking about when the topic is the collision, scattering or absorption event of two particles. It is clear that there is a "qiuck" interaction between them. Also I know that we can consider the cross section For example let's try to collide two electrons. What is not clear for me that what kind of force takes place when this quick change (collision) happens? 1. Electrostatic force. When they are close to each other, this force became so strong that will behave like a collision force. Clear, but is this the only force that represents the collision? And what if we try to collide particles with different polarity (proton+electron) or neutral ones? 2. Strong interaction. I assume that this is working even with neutral particles. Is this the force that is necessary to take place when we are talking about collision? 3. Exclusion principle. Maybe this prevents two fermions to occupy the same quantum state -- and by this they can't be the same place at a time, so they will scatter on each other. 4. Any of the above interactions. It doesn't matter what kind of force, if their movement changing fast by any force, we are talking about a collision. 5. Other? Thank you!