Distinguishing between interactions (decays and collisions)

[Soffie]

I always struggle to know which force (strong nuclear, electromagnetic, weak, gravity) is responsible for an interaction. For example,

pi+ + pi- = neutron + pi0

I would say its strong force responsible, because quarks are involved. But the pions are also charged, so how do I know it's not electromagnetic force?

Or for example, take Beta decay. How would I know just from looking at the equation that it's the weak interaction? I know the exchange particle is a boson and thus it would be weak, but in an equation you are not given the exchange particle. Is there a general set of rules/characteristics of interaction equations through which you can tell which force is responsible?

Thanks

 

[Orodruin]

pi+ + pi- = neutron + pi0

This breaks baryon number and violates conservation of angular momentum. The process cannot occur.

 

[Soffie]

This breaks baryon number and violates comservation of angular momentum. The process cannot occur.

How do you know it's a strong interaction from the equation? (baryon no. only conserved if strong interaction) is it because quarks are involved?
But in beta decay it involves quarks and leptons, and it's a weak decay. How do you distinguish interactions from one another?

 

[Vanadium 50]

You have to start out by determining if a reaction is allowed or not. The reaction you wrote down cannot happen by any interaction.

 

[Soffie]

Ok, so how about this interaction as an example:

e+ + e- = J/psi

How do I tell just from the equation what interaction is responsible?

 

[Vanadium 50]

Start with:

Can it be strong?
Can it be electromagnetic?
Can it be weak?

 

[mfb]

To expand the last answer: The strong interaction is the strongest interaction. If an interaction is possible via the strong interaction, this will (nearly always) dominate.
If something cannot happen via the strong interaction, then the electromagnetic interaction is the strongest remaining option. Same here.
If both strong and electromagnetic interaction cannot mediate the process, but the process is still possible, then it will happen via the weak interaction. This is always the case if neutrinos are involved, if individual leptons appear or disappear (but not in pairs), or if quark flavors change.

 

[dukwon]

(baryon no. only conserved if strong interaction)

Not true: it's conserved by every Standard Model interaction. You might be confusing it with quark flavour, which is conserved by the strong and electromagnetic forces.

 

[Orodruin]

Not true: it's conserved by every Standard Model interaction.

Just to be complete, this does depend on what you consider a SM "interaction". For perturbative processes, it is true that the SM preserves baryon number. However, it should be mentioned that baryon number is generally not preserved and broken by non-perturbative processes related to the vacuum configuration that preserve B-L, but not B+L. Of course, I strongly doubt that this is what the OP had in mind and just as I am writing this I am realising that I was the one who brought up baryon number conservation in the first place ... (In my defence - the proposed interaction would also break B-L and Lorentz invariance ...)