MadGraph reweight technique

Asked by Yue Xu on 2019-10-04

Hello,

I generated the events with an external model which contains a heavy higgs ( hh ), and the process is:
  p p > z hh, z > l- l+, hh > j j j j

There are two parameters we can change. They are fw and fww that influence the coupling between heavy higgs and vector boson and the decay width of heavy higgs.
I would like to use MadGraph to reweight the generated events with a different setting of fw and fww.

For example, I generated events with fw=600 and fw=3600 (case A). Then, I want to reweight these events with fw=800 and fww=4800 (case B). The jet pt distribution of case A is similar with the jet pt distribution of case B. The cross-section of case B is larger than the cross-section of case A in theory.

I reweight the events by command:
        ./bin/madevent rewight run_01 -f

The setting of reweight_card.dat is ( the decay width of heavy higgs is always set to auto):
       launch --rwgt_name=fw800fww4800
      set DIM6COEFF 3 8.0000e+02
      set DIM6COEFF 4 4.8000e+03
      set DECAY 254 auto

Then, I got the following output information:
           INFO: central : 7.741888-112e-05
           INFO: fw800fww4800 : 7.0747137441e-05

It seems the reweighted cross-section isn't correct and not corresponding with the MG5 cross-section.

I also tried with different settings of fw and fww. I found that the reweighted cross-section is corresponding with the MG5 cross-section sometimes, but sometimes isn't.

How do these happen? How can I get a correct reweighted cross-section?

Thanks.

Question information

Language:
English Edit question
Status:
Answered
For:
MadGraph5_aMC@NLO Edit question
Assignee:
No assignee Edit question
Last query:
2019-10-08
Last reply:
2019-10-08

Hi,

If the width is dfferent in both cases then you can have large difference in the M_JJJJ distributions and therefore huge bias in the re-weighting method.

Another stuff that you might need to check is to de-activate the re-weighting by polarization.
But note that re-weighting is not expected to always work so like this I do not see any obvious problem here.

Cheers,

Olivier
> On 4 Oct 2019, at 14:22, Yue Xu <email address hidden> wrote:
>
> New question #684695 on MadGraph5_aMC@NLO:
> https://eur03.safelinks.protection.outlook.com/?url=https%3A%2F%2Fanswers.launchpad.net%2Fmg5amcnlo%2F%2Bquestion%2F684695&amp;data=02%7C01%7C%7Cbd8aa81279a0478a233f08d748c5923a%7C7ab090d4fa2e4ecfbc7c4127b4d582ec%7C0%7C0%7C637057886171359966&amp;sdata=ICDv%2FDywrVUKlTIr0XnCwSH8eTc%2FQByM12d0606opOQ%3D&amp;reserved=0
>
> Hello,
>
> I generated the events with an external model which contains a heavy higgs ( hh ), and the process is:
> p p > z hh, z > l- l+, hh > j j j j
>
> There are two parameters we can change. They are fw and fww that influence the coupling between heavy higgs and vector boson and the decay width of heavy higgs.
> I would like to use MadGraph to reweight the generated events with a different setting of fw and fww.
>
> For example, I generated events with fw=600 and fw=3600 (case A). Then, I want to reweight these events with fw=800 and fww=4800 (case B). The jet pt distribution of case A is similar with the jet pt distribution of case B. The cross-section of case B is larger than the cross-section of case A in theory.
>
> I reweight the events by command:
> ./bin/madevent rewight run_01 -f
>
> The setting of reweight_card.dat is ( the decay width of heavy higgs is always set to auto):
> launch --rwgt_name=fw800fww4800
> set DIM6COEFF 3 8.0000e+02
> set DIM6COEFF 4 4.8000e+03
> set DECAY 254 auto
>
> Then, I got the following output information:
> INFO: central : 7.741888-112e-05
> INFO: fw800fww4800 : 7.0747137441e-05
>
> It seems the reweighted cross-section isn't corrected and not corresponding with the MG5 cross-section.
>
> I also tried with different settings of fw and fww. I found that the reweighted cross-section is corresponding with the MG5 cross-section sometimes, but sometimes isn't.
>
> How do these happen? How can I get a corrected reweighted cross-section?
>
> Thanks.
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Yue Xu (yuexu) said : #2

Hi Olivier,

What else will influence the re-weighting method except the decay width ?

 If two signal points have a similar decay width and the pt distribution of vector boson is much different, will the re-weighting work ?

Cheers,
Yue

Full discussion is given in this paper: arxiv:1607.00763<http://arXiv.org/abs/arXiv:1607.00763>

But in short if
1) you have infinite statistic
2) your rewingting hyppothesis has a phase-space fully include in the original phase-space
(be carefull here with helicity by helicity re-weighting))
3) that the re-weighitng does not affect the leading color information of the events

Then you are sure to have the correct result.
Obviously the point 1. can never be achieved and this means that if your shape are too different (in ANY possible variable) then you can face either bias and/or huge theoretical error.

Cheers,

Olivier

On 8 Oct 2019, at 18:07, Yue Xu <<email address hidden><mailto:<email address hidden>>> wrote:

Question #684695 on MadGraph5_aMC@NLO changed:
https://eur03.safelinks.protection.outlook.com/?url=https%3A%2F%2Fanswers.launchpad.net%2Fmg5amcnlo%2F%2Bquestion%2F684695&amp;data=02%7C01%7C%7C6042c1e6c19f4212b41408d74c09b110%7C7ab090d4fa2e4ecfbc7c4127b4d582ec%7C0%7C0%7C637061476819161778&amp;sdata=AOCpR%2BjTnNyfFlFdvoxCPGlfUZohgcfwY9WqM8QrsN8%3D&amp;reserved=0

   Status: Answered => Open

Yue Xu is still having a problem:
Hi Olivier,

What else will influence the re-weighting method except the decay width
?

If two signal points have a similar decay width and the pt distribution
of vector boson is much different, will the re-weighting work ?

Cheers,
Yue

--
You received this question notification because you are an answer
contact for MadGraph5_aMC@NLO.

Can you help with this problem?

Provide an answer of your own, or ask Yue Xu for more information if necessary.

To post a message you must log in.