madspin and decay chain

Hi,

> firstly, It's about madspin. I generate the process e+ e- > h z.and then i use madspin by input decay z > l+ l- in the block . I dont"t use any final state cut. and the BW- cutoff is default value.Then the result is the cross section of e+ e- > h z and mutiplied by the branch rate of z to l+l-. it"s right?

That’s correct.

> Secondly, i want to use the decay chain. The way which use is the narrow width approximation(NWA)? I generate e+ e- > h z,z > l+ l-. The run_card.dat is just as above in madspin. But the result is 37% lower than that in madspin,which is impossible. And then i set the parameter of BW-CUTOFF to 1000, and the result is also lower than that in madspin about 33%. So i don't know how to solve it.

Did you set cut_decays on False in the run_card?
This is likely the reason, you have cut in this case on your final state lepton.

> I think madspin is spin correlation and the difference between madspin and NWA will not large. So what's the problem about it and how to set the parameter of bw-cutoff in order to compare them? Or other parameter that independent?

Note that the decay-chain does not use the NWA for the cross-section computation (MadSpin does). i.e. the decay chain syntax does not assume that the total cross-section is production times branching ratio.

Cheers,

Olivier

> On 29 Apr 2017, at 11:48, niujj <email address hidden> wrote:
>
> New question #630269 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/630269
>
> hello professor，
>
> I have some problems on madspin and the decay chain.
>
> firstly, It's about madspin. I generate the process e+ e- > h z.and then i use madspin by input decay z > l+ l- in the block . I dont"t use any final state cut. and the BW- cutoff is default value.Then the result is the cross section of e+ e- > h z and mutiplied by the branch rate of z to l+l-. it"s right? and i don't know how to reset the parameter of BW- cutoff.
>
> Secondly, i want to use the decay chain. The way which use is the narrow width approximation(NWA)? I generate e+ e- > h z,z > l+ l-. The run_card.dat is just as above in madspin. But the result is 37% lower than that in madspin,which is impossible. And then i set the parameter of BW-CUTOFF to 1000, and the result is also lower than that in madspin about 33%. So i don't know how to solve it.
>
> I think madspin is spin correlation and the difference between madspin and NWA will not large. So what's the problem about it and how to set the parameter of bw-cutoff in order to compare them? Or other parameter that independent?
>
> Waiting for your help!
> sincerely
> niujj
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Hi Olivier,

Thanks for your help, but I still have some doubt on it.

Firstly ,I'm sure that i do set cut_decays on False in the run_card in the decay-chain. And when i have one more look at the run.parameter, i found

Apply pt/E/eta/dr/mij/kt_durham cuts on decay products or not
# (note that etmiss/ptll/ptheavy/ht/sorted cuts always apply)

Is the reason for etmiss/ptll/ptheavy/ht/sorted cuts ? And if there is other reasons?

Secondly, you said the decay-chain does not use the NWA for the cross-section computation (MadSpin does ). What's the method about decay-chain? And is there other way also only use NWA?

sincerely
niujj

Hi Niujj,

> Is the reason for etmiss/ptll/ptheavy/ht/sorted cuts ? And if there is
> other reasons?

Please take a look at the following FAQ:
https://answers.launchpad.net/mg5amcnlo/+faq/2442
It is likely to cover your problem.

> Secondly, you said the decay-chain does not use the NWA for the cross-section computation (MadSpin does ). What's the method about decay-chain? And is there other way also only use NWA?

We integrate the full matrix element over the full phase-space.
The decaying particle have an additional phase-space cut which force those particle to be on shell up to BWcut times the width.

Cheers,

Olivier

> On 29 Apr 2017, at 15:48, niujj <email address hidden> wrote:
>
> Question #630269 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/630269
>
> Status: Answered => Open
>
> niujj is still having a problem:
> Hi Olivier,
>
> Thanks for your help, but I still have some doubt on it.
>
> Firstly ,I'm sure that i do set cut_decays on False in the run_card in
> the decay-chain. And when i have one more look at the run.parameter, i
> found
>
> Apply pt/E/eta/dr/mij/kt_durham cuts on decay products or not
> # (note that etmiss/ptll/ptheavy/ht/sorted cuts always apply)
>
> Is the reason for etmiss/ptll/ptheavy/ht/sorted cuts ? And if there is
> other reasons?
>
>
> Secondly, you said the decay-chain does not use the NWA for the cross-section computation (MadSpin does ). What's the method about decay-chain? And is there other way also only use NWA?
>
> sincerely
> niujj
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.