- #1
Hepth
Gold Member
- 464
- 40
I know I should know this, but I have a quick question.
Let's say we have a diagram:
Lets assume:
1 = "quark"
3 = "antiquark"
2 = W boson
4 = photon
q = same quark flavor as "3"
Time flows from left to right.
Now let's say I start writing the diagram at point 2:
[tex]
W_\mu \bar{u}_1 \gamma^\mu (1- \gamma_5) \frac{i}{\not q - m_3} (- i e \gamma^\alpha)\epsilon^{*}_\alpha v_3
[/tex]
I think that's right.
For every dirac spinor propagator I write [tex]\frac{i}{\not q - m}[/tex]
Now if I want to write out WHAT "q" is, I have to choose a direction of momentum flow. Is it convention, or by rule, that I drew the diagram from top to bottom, so the direction of "q" is up (against the direction of writing the amplitude).
so [tex] q = p_3- p_4[/tex]
Where both p1 and p3 are flowing IN and p2 and p4 are flowing OUT.
Or would it be down, and if not then why? What decides which way I write the momentum flow for the propagator as if I choose differently I get a different result.
Let's say we have a diagram:
Code:
1-->----------2
|
| <- "q"
v
|
3--<----------4
Lets assume:
1 = "quark"
3 = "antiquark"
2 = W boson
4 = photon
q = same quark flavor as "3"
Time flows from left to right.
Now let's say I start writing the diagram at point 2:
[tex]
W_\mu \bar{u}_1 \gamma^\mu (1- \gamma_5) \frac{i}{\not q - m_3} (- i e \gamma^\alpha)\epsilon^{*}_\alpha v_3
[/tex]
I think that's right.
For every dirac spinor propagator I write [tex]\frac{i}{\not q - m}[/tex]
Now if I want to write out WHAT "q" is, I have to choose a direction of momentum flow. Is it convention, or by rule, that I drew the diagram from top to bottom, so the direction of "q" is up (against the direction of writing the amplitude).
so [tex] q = p_3- p_4[/tex]
Where both p1 and p3 are flowing IN and p2 and p4 are flowing OUT.
Or would it be down, and if not then why? What decides which way I write the momentum flow for the propagator as if I choose differently I get a different result.