# Confusion regarding interactions and its relation to the strong/weak force

I'm not using the template because, again, it's not a question I need help with, it's understanding the topic so I can actually do the homework. :P

First of all, is interaction completely different to decay? Because all hadrons interact by the strong interaction, and yet they can decay into leptons (i.e. in Beta + or - decay) and I thought leptons only felt the weak interaction. So do hadrons "interact" by the strong interaction (and by "interaction", I'm guessing it means they feel the force), but they decay by the weak interaction? Because this implies that all decay is by the weak interaction - the strong interaction can't change quark type, so surely nothing can decay into another product if there's no changing of quark type at all? However, I've been told that mesons can "interact" with baryons via the strong interaction and change a proton to a neutron and vice versa. Why is that the strong interaction? I know it involves hadrons but there's been a quark change - u to d or vice versa!? That's what the weak interaction does. ---> VERY confusing!

Also, if Beta decay involves a proton or neutron decaying, then surely that's the weak interaction? But I thought hadrons didn't feel the weak interaction? And does the W boson actually decay into an electron/neutrino pair or does it exchange charge/momentum with the pair? I know the W boson mediates the weak force, so is it released as a by-product when the baryon changes, and then decays?

Very confused. :(

tiny-tim
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Hi!

So then why does this occur via the strong interaction:

+ve pion + neutron ---> proton

?!

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tiny-tim
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hi physics369!
+ve pion + neutron ---> proton

i'm confused

there seem to be some quarks missing on the RHS …

what is the whole equation in u's and d's, and are any of them altered?

Oh, okay. So the quarks themselves aren't changed, but they're just re-arranged so that you get different particles? Is this called decay?

tiny-tim
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Is this called decay?

dunno what other people do

i wouldn't call it decay​

vela
Staff Emeritus
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I would call that an interaction. A decay is when you have a single entity, like a particle or nucleus, that falls apart into lower-energy constituents. Here you have two particles interacting.

All of the fundamental interactions can mediate decays, though you can safely ignore gravity. To identify which particular interactions can be responsible for a certain decay, you have check that it follows the various rules for the different interactions.
+ve pion + neutron ---> proton
This hypothetical reaction looks wrong because, as tiny-tim noted, there's stuff missing on the RHS. Without knowing what else came out of the mess, you can't say whether it was due to the strong, weak, or electromagnetic interaction. On the LHS, the pion consists of an up quark and a down antiquark, and the neutron, one up and a down. The pion's up quark can join the up and down quark of the neutron to form a proton, but you still have the down quark and antiquark left over. Those two could bind together to form a neutral pion, in which case, you'd say the interaction was mediated by the strong interaction. Those two could also annihilate with each other, but in doing so, they'll produce a photon or Z. If the interaction produces a photon, you'd say it was mediated by the electromagnetic interaction; if you got a Z instead, which would invariably then decay into something else, you'd say it went through the weak interaction.

I want to note that even when you do know the end products, it's possible that it can get there through different channels. For example,

$$\pi^+ + n \rightarrow p + e^- + e^+$$

can occur through both the electromagnetic and weak interactions.

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Okay, so can leptons be produced by a strong nuclear decay? Can you give me an example of a strong nuclear decay?

vela
Staff Emeritus
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Leptons don't carry color charge, so they don't interact via the strong force. If you see leptons, you know the weak or electromagnetic force was involved.

The decay of the rho meson to two pions occurs via the strong interaction.

Thank you. :)

So when you say "involved" that means both in the reactants, and the products?

I'm dealing with the proton, neutron, pions (+/-/0) and kaons (+/-/0) and their antiparticle companions - so for the purpose of my exam, can I say that all hadrons that I'm dealing with are:

- produced by either strong interaction, or by high-energy lepton/anti-lepton collsions (weak interaction)
- interact by the strong interaction
- decay by the weak interaction (e.g. Beta decay or pions/kaons decaying into muons/antimuonneurtinos)

And leptons (electron, muon, tau - and their corresponding neutrinos - as well as all of the aforemtioned particles' antiparticle companions):

- Are produced by the weak interaction (e.g. Beta decay)
- Are only affected/interact by the weak interaction
- Can't decay because their fundamental

vela
Staff Emeritus
Homework Helper
Thank you. :)

So when you say "involved" that means both in the reactants, and the products?
You can draw complicated Feynman diagrams which involve more than one interaction, but generally speaking, you're usually only looking at the simplest diagrams, which involve only one of the forces.
I'm dealing with the proton, neutron, pions (+/-/0) and kaons (+/-/0) and their antiparticle companions - so for the purpose of my exam, can I say that all hadrons that I'm dealing with are:

- produced by either strong interaction, or by high-energy lepton/anti-lepton collsions (weak interaction)
- interact by the strong interaction
- decay by the weak interaction (e.g. Beta decay or pions/kaons decaying into muons/antimuonneurtinos)

And leptons (electron, muon, tau - and their corresponding neutrinos - as well as all of the aforemtioned particles' antiparticle companions):

- Are produced by the weak interaction (e.g. Beta decay)
- Are only affected/interact by the weak interaction
- Can't decay because their fundamental
I'd be generally wary of making generalized statements because there's usually some case which doesn't fit. It would be better if you understood the basic rules that the interactions always obey rather than trying to come up with these rules of thumb which may not always work. In other words, if you know the basic vertices, it's usually pretty easy to draw up a Feynman diagram for an allowed decay or interaction.

I can tell you right now that I can think of counterexamples to most of what you wrote above. For example, the most common way a neutral pion decays is into two photons; time-reversed, you get two photons interacting to produce a pion; electrons can be produced by pair production; muons decays into lighter leptons; and so on.

You might want to download the Particle Data Book from the Particle Data Group. It's a convenient reference for particle physics.

http://pdg.lbl.gov/

Can someone tell me why this is the strong force if quarks were changed?

Pion- + p ---> K+ + Sigma-

Surely there would have to be a quark change in that:

d(u) + uud ---> u(s) + dds

Where (u) refers to it being anti-up

?!?!

vela
Staff Emeritus