Informal Madgraph tutorial

Go to http://madgraph.hep.uiuc.edu/

and from 'download source' download 'Madgraph II' (Madgraph link doesn't appear to work).  This downloads both Madraph and MadEvent, but we'll only be concerned with Madgraph for now.  Inside the MadgraphII folder, you can compile the executable mg2 by typing 'make'.

Here is a example of running the mg2 executable from the command line to create a fortran function that returns the matrix element squared for the process e+e- > A (photon > dd~ (d quark and d antiquark)  Here is the subroutine 'see_dd' that is created in the file ee_dd.f

Here is corresponding subroutine for e+e-> A > mu+mu-

<- My mistake I should have made it mu- mu+ so these would be same
<-GAD is EM coupling of d quark, GAL is EM coupling of muon

 

How Madgraph handles couplings and color flow

------------------------------------------------

It might be instructive to look at the difference between the e+e->A>dd~ and e+e->A>mu+mu- subroutines. Here's a diff of the two files matrix.f for e+e- > A > dd~ and e+e- > A > mu+mu-:

<-Color factor

< C FOR PROCESS : e+ e- -> d d~
---
> C FOR PROCESS : e+ e- -> mu+ mu-
10c10
< C Crossing 1 is e+ e- -> d d~
---
> C Crossing 1 is e+ e- -> mu+ mu-
146c146
< C FOR PROCESS : e+ e- -> d d~
---
> C FOR PROCESS : e+ e- -> mu+ mu-


182,183c182,183
< DATA (CF(i,1 ),i=1 ,1 ) / 3/
< C T[3,4]
---
> DATA (CF(i,1 ),i=1 ,1 ) / 1/
> C


189,190c189,190
< CALL OXXXXX(P(0,3 ),ZERO ,NHEL(3 ),+1*IC(3 ),W(1,3 ))
< CALL IXXXXX(P(0,4 ),ZERO ,NHEL(4 ),-1*IC(4 ),W(1,4 ))
---
> CALL IXXXXX(P(0,3 ),ZERO ,NHEL(3 ),-1*IC(3 ),W(1,3 ))
> CALL OXXXXX(P(0,4 ),ZERO ,NHEL(4 ),+1*IC(4 ),W(1,4 ))


192,193c192,193
< CALL IOVXXX(W(1,4 ),W(1,3 ),W(1,5 ),GAD ,AMP(1 ))
< JAMP( 1) = +AMP( 1)
---
> CALL IOVXXX(W(1,3 ),W(1,4 ),W(1,5 ),GAL ,AMP(1 ))
> JAMP( 1) = -AMP( 1)
 

 

Single Top

Now for something a little for interesting, complicated, and relevant.

Here is the matrix.f file for ug > t > e+vebb~d  and the .gif file for the Feynman diagrams