I I don't understand this Feynman Diagram

Question:
Draw the lowest-order Feynman diagrams for the e+e- --> W+W-process

The answer gives three diagrams. I understand the first two, but the third makes no sense to me. Here it is:

WeirdFeynmanDiagram.png



So this is a t-channel Feynman diagram. As far as I can tell regarding how these types of Feynman diagrams are read, the following processes are taking place:

e- --> ve + W-
e+ + ve- --> W+

But this makes no sense to me. How can an electron decay into a neutrino and a W boson? And how can a positron and a neutrino combine to form a W boson?

Am I simply reading this wrongly?
 
33,027
8,806
There are no decays involved. An electron, a (virtual) neutrino and a W boson couple. That coupling is allowed. For a decay the W and the neutrino would have to be real (leave the Feynman diagram).

Splitting the diagram into those subprocesses doesn't work well because you assign particle properties to things that are not actual particles.
 

nrqed

Science Advisor
Homework Helper
Gold Member
3,531
178
Question:
Draw the lowest-order Feynman diagrams for the e+e- --> W+W-process

The answer gives three diagrams. I understand the first two, but the third makes no sense to me. Here it is:

View attachment 108644


So this is a t-channel Feynman diagram. As far as I can tell regarding how these types of Feynman diagrams are read, the following processes are taking place:

e- --> ve + W-
e+ + ve- --> W+

But this makes no sense to me. How can an electron decay into a neutrino and a W boson? And how can a positron and a neutrino combine to form a W boson?

Am I simply reading this wrongly?
well, the best way to think about it is to not order the time in the intermediate states because a Feynman diagram is really a combination of several time ordered diagrams. If you insist in thinking of it this way, then this Feynman diagram is a combination of what you described (where you put the vertex [itex]e^- W^- [/itex] vertex occurs first) plus the time ordered diagram for which the other vertex occurs first.

But there is no contradiction here. You ask "how can an electron decay into a neutrino and a W boson?" We are dealing with an off-shell neutrino here (it is a virtual neutrino) so there is no problem with that reaction.
 

haushofer

Science Advisor
Insights Author
2,199
545
Just to add: Maybe you look upon these Feynmandiagrams too literally as processes. The actual process is given by the full amplitude. Feynmandiagrams are terms in its Taylor expansion.
 

Want to reply to this thread?

"I don't understand this Feynman Diagram" You must log in or register to reply here.

Related Threads for: I don't understand this Feynman Diagram

Replies
6
Views
4K
Replies
2
Views
3K
Replies
2
Views
3K
Replies
1
Views
1K

Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving
Top