cross sections beyond SM

Asked by Alan Hernández on 2020-06-07

Hi! I'm new in madgraph, I´m interested in cross-sections in new models so I created an UFO file with Feynrules, I just introduced the chormomagnetic parameter: at. I would like to know how it affects the top-top production cross-section. I get the following result in the SM at 14TeV and MT=172.9 GeV

Cross section (pb)= 599.5 ± 0.93

then, I used the following .txt file

import model SMext_UFO
generate p p > t t~
output CMDM
launch -n run_at+mz
set ebeam1 7000
set ebeam2 7000
set MT 172.9
set at -.00193
launch -n run_atSM
set at 0
launch -n run_at+mz2
set at -.00193

so I get:

Cross section (pb)= 599.5 ± 0.93
Cross section (pb)=598.3 ± 0.82
Cross section (pb)= 598.2 ± 0.92

The first result is the same that the SM case and I think it should be different since there is a new factor at.
For the second result at=0 so I think It should be the SM result.
for the last result, I used the same value for "at" that in the first case, so I think both results should be the same.

I don't understand it, what I am missing?

and extra question: How different should be the madgraph results and the measurements in ATLAS or CMS?? for example for tt production cross-section?

thanks in advance

Alan

Question information

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MadGraph5_aMC@NLO Edit question
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Last query:
2020-06-10
Last reply:
2020-06-10

This question was reopened

Hi,

> Cross section (pb)= 599.5 ± 0.93
> Cross section (pb)=598.3 ± 0.82
> Cross section (pb)= 598.2 ± 0.92

Those number are given by a Monte-Carlo method. So they are associated to a statistical error which is printed on those lines. According to the see that you use for the computation of the integral, you will get slightly different result. In madgraph we change the seed for you by default and this is why run 1 and run 3 does not provide the exact same result (but compatible result).

> and extra question: How different should be the madgraph results and the measurements in ATLAS or CMS?? for example for tt production cross-section?

Those result are LO accurate result. The total cross-section is affected by huge theoretical uncertainty at that order. (Such uncertainty are computed by default if you have a python compatible installation of lhapdf). So the total measured cross-section is typically quite different from the one predicted above.
On the other hand, the shape of the distribution have smaller theoretical error.

Note that going to NLO accuracy (which we also support) reduces such theoretical uncertainties and the cross-section starts to be reliable.

Cheers,

Olivier

> On 8 Jun 2020, at 01:01, Alan Hernández <email address hidden> wrote:
>
> New question #691204 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/691204
>
> Hi! I'm new in madgraph, I´m interested in cross-sections in new models so I created an UFO file with Feynrules, I just introduced the chormomagnetic parameter: at. I would like to know how it affects the top-top production cross-section. I get the following result in the SM at 14TeV and MT=172.9 GeV
>
> Cross section (pb)= 599.5 ± 0.93
>
> then, I used the following .txt file
>
> import model SMext_UFO
> generate p p > t t~
> output CMDM
> launch -n run_at+mz
> set ebeam1 7000
> set ebeam2 7000
> set MT 172.9
> set at -.00193
> launch -n run_atSM
> set at 0
> launch -n run_at+mz2
> set at -.00193
>
> so I get:
>
> Cross section (pb)= 599.5 ± 0.93
> Cross section (pb)=598.3 ± 0.82
> Cross section (pb)= 598.2 ± 0.92
>
> The first result is the same that the SM case and I think it should be different since there is a new factor at.
> For the second result at=0 so I think It should be the SM result.
> for the last result, I used the same value for "at" that in the first case, so I think both results should be the same.
>
> I don't understand it, what I am missing?
>
> and extra question: How different should be the madgraph results and the measurements in ATLAS or CMS?? for example for tt production cross-section?
>
> thanks in advance
>
> Alan
>
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Alan Hernández (alaban7) said : #2

Thanks!

Alan Hernández (alaban7) said : #3

I am trying to go NLO with the chromomagnetic moment of the top quark, so I have to renormalize the model. I followed some tutorials, then I use WriteFeynArtsOutput to get the counterterms but I got some warnings

"Part specification \
{{-F(9,{e1x2}),F(9,{e2x2}),V(4,{e3x2}),V(4,{e4x2})},Join[0,0,2]}[[2,1,\
1]] is longer than depth of object."

I think it is releated to ttgg vertex induced by the chromomagnetic Lagrangian, I tried to delete all the terms -F(9,{e1x2}),F(9,{e2x2}),V(4,{e3x2}),V(4,{e4x2})} in the .mod file and it was working fine but when I tried

p p > t t~

in madgraph I am getting the error:

Poles do not cancel, run cannot continue

so I think it is because of the vertex ttgg delated. How can I fix it?

For this you would need help the FeynRules authors.

Cheers,

Olivier

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