# Homework Help: Particle Physics; how to tell which is the most likely interaction in decays/collisio

1. Dec 5, 2009

### kylie14

I'm struggling a bit with my module on particle physics. This is only part of the problem set, but I need it for the rest (which is asking whether or not some particular decays are possible).

My problem is: How can you tell which is the most likely interaction for certain processes?

For example,

a positron and electron annihilating and producing a quark anti-quark pair;
the quark pair then goes on to hadronize.

I'm not really sure which are the most likely processes responsible but my guess is:

weak for the first part,
strong for the second part.

Is is possible that the second part is also weak?

Also, am I right in saying that processes that go via the strong force have to conserve strangeness, but ones that go via weak do not have to?

Any help would be much appreciated,
Thankyou!

2. Dec 5, 2009

### diazona

Re: Particle Physics; how to tell which is the most likely interaction in decays/coll

Generally the "strongest" interaction that can be responsible for a particular process will be the dominant mode by which it proceeds. So any process that can occur by gluon exchange (the strong force) will usually go that way. Otherwise, it will proceed by photon exchange (EM force) if it can, and if not then by a W boson (weak force).

For your particular situation, what intermediate particle is involved when the positron and electron annihilate? That tells you what kind of process it is.

And you are correct in saying that strong interactions conserve strangeness but weak interactions don't have to. This is because weak interactions are the only ones that can change the flavor of a quark, just like strong interactions are the only ones that can change the color of a quark.

3. Dec 5, 2009

### kylie14

Re: Particle Physics; how to tell which is the most likely interaction in decays/coll

Thankyou very much, that helps me a quite a lot.

I'm not told the intermediate particle unfortunately, the question just asks which is the most likely.

Going from your reasoning that means the most likely in interaction for the first part would therefore be EM? And the second part is most likely via the strong interaction?

So then if that's the case I'd have to check for conservation of strangeness for when the quark pair hadronizes. Thankyou for also clearing up that part for me too by the way.

4. Dec 5, 2009

### diazona

Re: Particle Physics; how to tell which is the most likely interaction in decays/coll

The "meat" of the task of analyzing a particle reaction is to figure out what the intermediate particle(s) is/are. I didn't want to just give it away, but you should be able to figure it out. Usually the easiest way is to draw Feynman diagrams representing the possible interactions.
Sounds reasonable.

5. Dec 6, 2009

### kylie14

Re: Particle Physics; how to tell which is the most likely interaction in decays/coll

Oh, I see! Thankyou, I'll try to draw it and see what I get.

6. Dec 6, 2009

### kylie14

Re: Particle Physics; how to tell which is the most likely interaction in decays/coll

Brilliant, thanks, I'm sorted! I think I have enough now to be able to figure it out in general.