Closed apuignav closed 8 years ago
Hi, and thanks for the question! You haven't missed anything in the manual, this particular scenario is in fact missing and I shall add it. Thanks for bringing it up :)
Here is how you can draw it:
\begin{tikzpicture}
\begin{feynman}
\vertex (b1) {$b$};
\vertex [right=of b1] (b2);
\vertex [right=of b2] (b3);
\vertex [right=of b3] (b4) {$d$};
\vertex [above=2em of b1] (d1) {$\overline u$};
\vertex [above=2em of b4] (d2) {$\overline u$};
\vertex at ($(b2)!0.5!(b3)!0.5cm!-90:(b3)$) (g1);
\vertex [below=5em of b3] (g2);
\vertex [below=4em of b4] (l1) {\(\ell^{+}\)};
\vertex [below=2em of l1] (l2) {\(\ell^{-}\)};
\diagram* {
(d2) -- [fermion] (d1),
(b1) -- [fermion] (b2) -- [fermion, edge label={$u,c,t$}] (b3) -- [fermion] (b4),
(b2) -- [boson, quarter right] (g1) -- [boson, quarter right] (b3),
(g1) -- [photon, bend right] (g2),
(l1) -- [fermion, bend right] (g2) -- [fermion, bend right] (l2),
};
\end{feynman}
\end{tikzpicture}
which produces
Basically, the idea is to create another vertex where the W boson and photon lines all meet. which is achieved with
\vertex at ($(b2)!0.5!(b3)!0.5cm!-90:(b3)$) (g1);
This little piece of magic comes from Ti_k_Z and you can read more about it in section 13.5 of the Ti_k_Z manual. There are two things happening there: first, (b2)!0.5!(b3)
finds the coordinate that is half-way between (b2)
and (b3)
; the second step is (x)!0.5cm!-90:(b3)
which finds the coordinate that is 0.5cm
down the line from (x)
to (b3)
, after this line has been rotated by -90
degrees about (x)
. These two operations are then chained together so that (x)
is actually (b2)!0.5!(b3)
. The net effect is that it places a node 0.5cm
above the midpoint between (b2)
and (b3)
. (I hope that makes sense).
An alternative way to achieve the same thing would be
\vertex at ($(b2)!0.5!(b3) + (0, -0.5cm)$) (g1);
which is perhaps easier to understand, but it won't work if the graph is rotated.
As a last comment, I have to admit that I'm not completely happy with the way bosonic lines meet up. I'll see if I can fix that at some point.
I am trying to create a penguin diagram similar to
but i find myself unable to place a vertex in the middle of the curve. So far, considering only the b quark part, I have
but all attempts at adding the extra vertex have failed. Am I misunderstanding the manual or is this case not foreseen?
PS: Using
arc
hasn't helped either...