Elastic Collisions of Particles

[RuthlessOptimism]

Hello I have a few basic questions regarding the elastic collisions of particles.

 

Basically my questions are: "are collisions between subatomic particles, for example: electrons, perfectly elastic?". Also: "are they perfectly elastic all of the time?" and finally regarding any answer I get "is there actual data, for example: from particle collider experiments, that actually proves / or supports this?".

 

Thank you.

[studiot]

Do electrons collide?

[swansont]

Do electrons collide?

 

 

Yes, though it's not like hard-sphere scattering. You will have an impact parameter, which is the point of closest approach.

 

——

 

It's not going to be perfectly elastic because accelerating a charged particle will give you radiation. But it's often a good approximation. Where else is the energy going to go?

 

http://hypernews.slac.stanford.edu/slacsite/aux/HiPPP/scattering/

[RuthlessOptimism]

Thank you Swansont that link was very helpful. I was kind of thinking that some energy would end up being emitted as photons. I've read a little bit about cherenkov radiation before (the simplified popular mechanics version), so I knew decelerated particles will give off photons, but I did not know accelerated ones did to.

 

So what is still weird and interesting to me about say a "collision" between two charged particles with the same polarity of charge is that if they collide head on, then bounce back the way they came they have to go through deceleration (of their original velocity), then acceleration (to their new velocity going the opposite way), so why don't they emit a photon or photon(s) twice?

[robinpike]

Thank you Swansont that link was very helpful. I was kind of thinking that some energy would end up being emitted as photons. I've read a little bit about cherenkov radiation before (the simplified popular mechanics version), so I knew decelerated particles will give off photons, but I did not know accelerated ones did to.

 

So what is still weird and interesting to me about say a "collision" between two charged particles with the same polarity of charge is that if they collide head on, then bounce back the way they came they have to go through deceleration (of their original velocity), then acceleration (to their new velocity going the opposite way), so why don't they emit a photon or photon(s) twice?

 

I suppose most 'collisions' would be glancing rather than a specific head-on bouncing back collision. But even so, a glancing collision between two electrons would cause de-acceleration on approach due to the electrons having negative charge, and then acceleration as they pass the point of closest encounter and their negative charges push the electrons apart. So does even a glancing encounter cause two photons to be emitted?

Edited April 4, 2016 by robinpike

[timo]

Not sure if that is obvious to everyone: The meaning of elastic in the context of particle physics is not to be understood in the sense of rubber balls bouncing off some wall (or another rubber ball). In particle physics the term "elastic scattering" refers to a process X -> Y where the particle content of Y is the same as X and no excited states are created in Y.

-> Some processes are elastic, e.g. {e-, e-} -> {e-, e-} (note that elementary particles cannot have excited states).

-> If new particles are created, the process is not elastic, e.g. {e+, e-} -> {2 photons}.

-> If input and output particles are identical but excited states are created, the process is explicitly called inelastic (e.g. deep inelastic proton scattering). The creation of an excited state usually is followed by a later decay step that will also create new particles.

-> Particle physics experiments tend to focus more on the inelastic processes, since that is the ones that create new particles or excited states that give insight into the objects' structures.

Edited April 4, 2016 by timo

[swansont]

 

I suppose most 'collisions' would be glancing rather than a specific head-on bouncing back collision. But even so, a glancing collision between two electrons would cause de-acceleration on approach due to the electrons having negative charge, and then acceleration as they pass the point of closest encounter and their negative charges push the electrons apart. So does even a glancing encounter cause two photons to be emitted?

 

 

I don't know that the number of photons is specified, but the acceleration is linked to the amount of energy that is emitted. (Also, deceleration is just a subset of acceleration — acceleration is change in velocity per unit time)