Process generating and syntax

Hi Huanian,

>And the corss section that Madgraph 5 shows is around 0.0006 pb. But I calculate using Cross section * Branch_ratio is
>about 0.0015 pb. I do not know what will be the problem. Is there something wrong with the syntax for this process?

Two reason for that
1) by default the run_card has some cut on the sm particles (like a minimum pt for jet/lepton)
You can put (in the run_card.dat)
   F = cut_decays ! Cut decay products
In order to not apply those cuts if the particles are originated from a decay.

2) By default they are also a cut on the invariant mass of the decaying particles.
this is control by the line (in the run_card.dat)
  15 = bwcutoff ! (M+/-bwcutoff*Gamma)
This is important since you neglect other diagram that produces the same final states. If your production particles are off-shell
those diagram are usually interfere in a non negligible. Therefore the (very) off-shell production leads to un-physical result that you shouldn't use for your analysis.
This is of course process dependent
p p > w+ , w+ > e+ ve is fine with an infinite bwcutoff
but
p p > z, z > e+ e- is meaningless with an infinite value (p p > a > e+ e- is important as soon as you are off-shell)

>And I have another issue. I generate p p > t1 t1~ the cross section is about 0.237pb; then I generate p p > t1 t1~, (t1~ > x1- b~) >the cross section is about 0.185 pb; generating p p > t1 t1~, (t1 > n2 t) the cross section for this is about 0.05 pb. But I generate
>p p > (t1~ > x1- b~) (t1 > n2 t) the cross section is still around 0.185. I do not know what is going on here.

Are you using v4 format proc_card? since this syntax is only supported in retro-compatibility mode.
I've run the same test in MG5
(put the following in a text file and lauch it via ./bin/mg5 PATH)

import model mssm
generate p p > t1 t1~
output
launch -f # return 1.311 +- 0.001488 pb
generate p p > t1 t1~, t1 > n2 t
output
launch -f # return 0.1502 +- 0.0001913 pb
generate p p > t1 t1~, t1~ > x1- b~
output
launch -f # return 0.8652 +- 0.00119 pb
generate p p > t1 t1~, t1~ > x1- b~, t1 > n2 t
output
launch -f # return 0.09904 +- 0.0001286 pb

As you can see, this is working fine.
you can send me your proc_card with your error (<email address hidden>) if you want me to investigate a bit more.

Cheers,

Olivier

Hi Huanian,

>And the corss section that Madgraph 5 shows is around 0.0006 pb. But I calculate using Cross section * Branch_ratio is
>about 0.0015 pb. I do not know what will be the problem. Is there something wrong with the syntax for this process?

Two reason for that
1) by default the run_card has some cut on the sm particles (like a minimum pt for jet/lepton)
You can put (in the run_card.dat)
   F = cut_decays ! Cut decay products
In order to not apply those cuts if the particles are originated from a decay.

2) By default they are also a cut on the invariant mass of the decaying particles.
this is control by the line (in the run_card.dat)
  15 = bwcutoff ! (M+/-bwcutoff*Gamma)
This is important since you neglect other diagram that produces the same final states. If your production particles are off-shell
those diagram are usually interfere in a non negligible. Therefore the (very) off-shell production leads to un-physical result that you shouldn't use for your analysis.
This is of course process dependent
p p > w+ , w+ > e+ ve is fine with an infinite bwcutoff
but
p p > z, z > e+ e- is meaningless with an infinite value (p p > a > e+ e- is important as soon as you are off-shell)

>And I have another issue. I generate p p > t1 t1~ the cross section is about 0.237pb; then I generate p p > t1 t1~, (t1~ > x1- b~) >the cross section is about 0.185 pb; generating p p > t1 t1~, (t1 > n2 t) the cross section for this is about 0.05 pb. But I generate
>p p > (t1~ > x1- b~) (t1 > n2 t) the cross section is still around 0.185. I do not know what is going on here.

Are you using v4 format proc_card? since this syntax is only supported in retro-compatibility mode.
I've run the same test in MG5
(put the following in a text file and lauch it via ./bin/mg5 PATH)

import model mssm
generate p p > t1 t1~
output
launch -f # return 1.311 +- 0.001488 pb
generate p p > t1 t1~, t1 > n2 t
output
launch -f # return 0.1502 +- 0.0001913 pb
generate p p > t1 t1~, t1~ > x1- b~
output
launch -f # return 0.8652 +- 0.00119 pb
generate p p > t1 t1~, t1~ > x1- b~, t1 > n2 t
output
launch -f # return 0.09904 +- 0.0001286 pb

As you can see, this is working fine.
you can send me your proc_card with your error (<email address hidden>) if you want me to investigate a bit more.

Cheers,

Olivier

Hi,

>Thank you very much for your help. For the second reason you mention, what you mean is this line "15 = bwcutoff ! >(M+/-bwcutoff*Gamma)" will block the off-shell process? And what does the number 15 mean?

Please read the FAQ:
https://answers.launchpad.net/madgraph5/+faq/2014
If something is still not clear after, don't hesitate to ask me, I will then change the FAQ to clarify that point.

>The same situation for w decay. I generate p p > w+ w-, the cross section is about 72 pb. But I also generate p p > w+ w-, w- > j j, the > cross section is about 34.38 pb, which is supposed to be 48 pb.

Yes sure this is for the same reason
1) presence of cuts for the jets which makes the cross-section drops
2) bwcutoff=15 so you don't have the full tail of the w, which reduces the cross-section as well.

Cheers,

Olivier