SM process cross section

Hi Huanian,

> Recently I am running some Standard Model backgrounds. But the cross section does not match with the theorectical Leading-order predictions. For example, I have the process (p p > t t~ z), the cross section that Madgraph provides is 679.5 fb, while the Leading-order prediction is 1100 fb.

Do you know from where this number comes from?
My guess is that this number comes from a matched generation p p > t t~ z + p p > t t~ z j +…
We know that for t t~ production the t t~ j samples is around 50% of the total productions, so this might be the case
for tt~z production as well.
Therefore only a matched computation can reach the expected value.
For my information for which center of mass did you run?

> And also the cross section does mot match inside Madgraph. If I let t t~ z decay, so my process is below:
> generate p p > t t~ z, (t > b w+, w+ > j j), (t~ > b~ w-, w- > l- vl~), z > j j
> add process p p > t t~ z, (t > b w+, w+ > l+ vl), (t~ > b~ w-, w- > j j), z > j j
>
> The cross section that Madgraph gives is 114.5 fb, however, my calculation is 679.5 * BR (2*0.216*0.676*0.7) = 138.9 fb.

They are two reasons for this
1) We take into account the off shell effects and we therefore expect to have deviation to the naive estimation.
The difference between the naive estimate and our more precise computation should be of order Gamma/M
So in this case 1% (top) +1% (anti-top) + 2% (Z) +2*2% (W) = 8%
2) We apply (by default) a cut on on-shell particles to avoid to have too far off-shell top/anti-top/z since such contributions are not gauge invariant.
this is the bwcutt that you will find in the run_card.

Cheers,

Olivier

On Sep 26, 2013, at 8:36 PM, Huanian Zhang <email address hidden> wrote:

> New question #236422 on MadGraph5:
> https://answers.launchpad.net/madgraph5/+question/236422
>
> Hi professor,
>
> Recently I am running some Standard Model backgrounds. But the cross section does not match with the theorectical Leading-order predictions. For example, I have the process (p p > t t~ z), the cross section that Madgraph provides is 679.5 fb, while the Leading-order prediction is 1100 fb.
>
> And also the cross section does mot match inside Madgraph. If I let t t~ z decay, so my process is below:
> generate p p > t t~ z, (t > b w+, w+ > j j), (t~ > b~ w-, w- > l- vl~), z > j j
> add process p p > t t~ z, (t > b w+, w+ > l+ vl), (t~ > b~ w-, w- > j j), z > j j
>
> The cross section that Madgraph gives is 114.5 fb, however, my calculation is 679.5 * BR (2*0.216*0.676*0.7) = 138.9 fb.
>
> So could you please offer me some clue about this ont-matching issue? Thank you very much and have a good day.
>
> Huanian Zhang
>
>
> --
> You received this question notification because you are a member of
> MadTeam, which is an answer contact for MadGraph5.

Hi Olivier,

Thank you very much for your hlep. I think I already fix the first problem by changing the scale. But I sitll have some cross section problems regarding other processes.

1) generate p p > t t~ w+ ren scale and fac scale = 91.188 GeV
add process p p > t t~ w-

The cross section from literature(1204.5678) is (290+550)fb, while the Madgrph gives 676.7 fb. No any cuts other than bwcutt applied in the run_card.

generate p p > t t~ w+, (t > b w+, w+ > j j), (t~ > b~ w-, w- > l- vl~), (w+ > j j)
add process p p > t t~ w-, (t > b w+, w+ > j j), (t~ > b~ w-, w- > l- vl~), (w- > j j)
add process p p > t t~ w+, (t > b w+, w+ > l+ vl), (t~ > b~ w-, w- > j j), (w+ > j j)
add process p p > t t~ w-, (t > b w+, w+ > l+ vl), (t~ > b~ w-, w- > j j), (w- > j j)
add process p p > t t~ w+, (t > b w+, w+ > j j), (t~ > b~ w-, w- > j j), (w+ > l+ vl)
add process p p > t t~ w-, (t > b w+, w+ > j j), (t~ > b~ w-, w- > j j), (w- > l- vl~)

The cross sectin that Madgraph gvies is 169.6fb, while simple estimatino only gives 133.59 fb.

2) generate p p > t t~ h ren scale and fac scale = 236.5 GeV. No any cuts except bwcutt applied in the run_card.
 the corss section from Madgraph is 475.3 fb.

generate p p > t t~ h, (t > b w+, w+ > l+ vl), (t~ > b~ w-, w- > j j), h > b b~ higgs mass = 125 GeV.
add process p p > t t~ h, (t > b w+, w+ > j j), (t~ > b~ w-, w- > l- vl~), h > b b~

this process has 120.7 fb from Madgraph, while the simple estimation gives 80.09fb.

In both cases, the madgraph cross sections exceed the simple estimation largely. Thank you very much and have a good day.

Huanian

Hi,

I don't see the number that you quote from paper 1204.5678.
I'm looking in table 1.
for 14TeV I have:
416 + 206 (+-4% from pdf)

The order of magnitude seems close to your MG results (676.7) and then need to check what are the cuts/ exact mass/ electroweak scheme/...
in order to have perfect agreement.

For the decay, a precise answer to all effect to take into account is written here:
https://answers.launchpad.net/madgraph5/+question/236617

Cheers,

Olivier

On Sep 30, 2013, at 8:46 PM, Huanian Zhang <email address hidden> wrote:

> Question #236422 on MadGraph5 changed:
> https://answers.launchpad.net/madgraph5/+question/236422
>
> Status: Answered => Open
>
> Huanian Zhang is still having a problem:
> Hi Olivier,
>
> Thank you very much for your hlep. I think I already fix the first
> problem by changing the scale. But I sitll have some cross section
> problems regarding other processes.
>
> 1) generate p p > t t~ w+ ren scale and fac scale = 91.188 GeV
> add process p p > t t~ w-
>
> The cross section from literature(1204.5678) is (290+550)fb, while the
> Madgrph gives 676.7 fb. No any cuts other than bwcutt applied in the
> run_card.
>
>
> generate p p > t t~ w+, (t > b w+, w+ > j j), (t~ > b~ w-, w- > l- vl~), (w+ > j j)
> add process p p > t t~ w-, (t > b w+, w+ > j j), (t~ > b~ w-, w- > l- vl~), (w- > j j)
> add process p p > t t~ w+, (t > b w+, w+ > l+ vl), (t~ > b~ w-, w- > j j), (w+ > j j)
> add process p p > t t~ w-, (t > b w+, w+ > l+ vl), (t~ > b~ w-, w- > j j), (w- > j j)
> add process p p > t t~ w+, (t > b w+, w+ > j j), (t~ > b~ w-, w- > j j), (w+ > l+ vl)
> add process p p > t t~ w-, (t > b w+, w+ > j j), (t~ > b~ w-, w- > j j), (w- > l- vl~)
>
> The cross sectin that Madgraph gvies is 169.6fb, while simple
> estimatino only gives 133.59 fb.
>
>
> 2) generate p p > t t~ h ren scale and fac scale = 236.5 GeV. No any cuts except bwcutt applied in the run_card.
> the corss section from Madgraph is 475.3 fb.
>
> generate p p > t t~ h, (t > b w+, w+ > l+ vl), (t~ > b~ w-, w- > j j), h > b b~ higgs mass = 125 GeV.
> add process p p > t t~ h, (t > b w+, w+ > j j), (t~ > b~ w-, w- > l- vl~), h > b b~
>
> this process has 120.7 fb from Madgraph, while the simple estimation
> gives 80.09fb.
>
>
> In both cases, the madgraph cross sections exceed the simple estimation largely. Thank you very much and have a good day.
>
> Huanian
>
> --
> You received this question notification because you are a member of
> MadTeam, which is an answer contact for MadGraph5.