How exactly does the pion decay through the weak force?

[Simfish]

So the pion(+)n is made up of an up quark and a down antiquark. And somehow, it emits a W+ boson, which then turns into a muon and a muon-neutrino.

From the Wiki article, "W bosons can decay to a lepton and neutrino or to an up-type quark and a down-type quark."

Does this mean that the reaction can go in both directions? Meaning that a pion(+) could turn into a W+ boson, which then turns back into a pion(+)?

And how do the up quark and down antiquark interact to produce that W+ boson?

 

[Vanadium 50]

Does this mean that the reaction can go in both directions? Meaning that a pion(+) could turn into a W+ boson, which then turns back into a pion(+)?

How could you tell? The initial state and the final state are the same. How would this be different from the pion just sitting there?

As far as "do the up quark and down antiquark interact to produce that W+ boson", what kind of answer are you looking for?

 

[Simfish]

How could you tell? The initial state and the final state are the same. How would this be different from the pion just sitting there?

Perhaps there is a phase where you can measure the W+ boson (if that's possible). Alternatively, this could explain why some pions take longer than "usual" to decay. (as opposed to other particles that decay through an irreversible route)

As far as "do the up quark and down antiquark interact to produce that W+ boson", what kind of answer are you looking for?

Basically, I want to know if pion decay is phenomenological. Or in other words, why do up quarks interact with down antiquarks to produce the W+ boson?

 

[Bill_K]

why do up quarks interact with down antiquarks to produce the W+ boson?

This is one of the basic assumptions that go into the standard model, namely that the weak interaction can be described by W_μj^μ where j^μ is the weak current. The weak current is a sum of terms, each term containing a pair of fermions, either quark plus antiquark or lepton plus neutrino.

I guess the real answer to 'why' is that ultimately no one knows why, but if we make that assumption it agrees with experiment.

 

[edguy99]

So the pion(+)n is made up of an up quark and a down antiquark. And somehow, it emits a W+ boson, which then turns into a muon and a muon-neutrino.

From the Wiki article, "W bosons can decay to a lepton and neutrino or to an up-type quark and a down-type quark."

Does this mean that the reaction can go in both directions? Meaning that a pion(+) could turn into a W+ boson, which then turns back into a pion(+)?

And how do the up quark and down antiquark interact to produce that W+ boson?

Informative picture here: http://www.animatedphysics.com/baryon_decay/muon_spin.jpg of the main particles - Pion/Muon/Electron

The boson is around for a much smaller timescale, a high concenetration of energy, around just long enough for the pion to change into a muon.