# What does collision mean when dealing with elementary particles?

## Main Question or Discussion Point

What does "collision" mean when dealing with elementary particles?

Can someone point me to some online references that will get me started in understanding what is meant by a particle collision?

The issues I don't understand-
Leptons and quarks are supposed to be point particles. Do they interact only via their electric charge? They can't "physically" bounce off each other...

Does the extended nature of the hypothetical string make the idea of collisions any easier?

Is a collision required for an electron to interact with a positron and create energy/photons?

Are these questions all simple mindedly classical and made senseless by Quantum Mechanics?

Thanks
Richard

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These are all good questions, but there's no simple answer, as you've probably guessed.

One thing to keep in mind is that to describe these interactions in a precise way, you have to forget the "point particle" picture in your head and really think of fundamental particles as extended quantum fields - the "wave" part of the wave-particle duality you've probably heard of. That means that particles can interact even if they are separated by some distance, so long as their respective fields overlap (think of two magnets that are passing closely to each other, if that helps). The closer they get, the greater the "overlap" of their fields, and the greater the likelihood of an interaction.

Now, when you want the full picture, you describe the specific interaction, i.e. electromagnetic, weak, strong, etc. in terms of exchanged particles. For example, for a purely electromagnetic interaction, the colliding particles exchange photons. How many and how strongly they are exchanged depends on how close the particles get.

In the Standard Model, the electromagnetic interaction gets mixed with the weak interaction, so you speak of the electro-weak interaction, and the exchanged particles include the Z0 and the W+ and W- particles. For the strong interaction, gluons are exchanged.

If you do a search for the Standard Model of Particle Physics, you'll find many helpful explanations of all this.

malawi_glenn