simulate fixed-target experiment

Hi,

You will need to weaken the default cut since they are set for lhc. I.e. higher than your centre of mass energy.

Now you can not do 2 to one since it is kinematcaly not possible to have the three particles on shell

Cheers,

Olivier

Ps: If I were you I would check if Pythia 8 does not dedicated mode (more accurate) for such generations

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________________________________
From: <email address hidden> <email address hidden> on behalf of Jui-Lin Kuo <email address hidden>
Sent: Wednesday, October 9, 2019 9:37:31 PM
To: Olivier Mattelaer <email address hidden>
Subject: [Question #684891]: simulate fixed-target experiment

New question #684891 on MadGraph5_aMC@NLO:
https://eur03.safelinks.protection.outlook.com/?url=https%3A%2F%2Fanswers.launchpad.net%2Fmg5amcnlo%2F%2Bquestion%2F684891&amp;data=02%7C01%7C%7C96b99a684d384d22523e08d74cf020d4%7C7ab090d4fa2e4ecfbc7c4127b4d582ec%7C0%7C0%7C637062468027618001&amp;sdata=NTt9np576sJrFmZxyPM9qgahAztWoYcqSCvoMUotL5w%3D&amp;reserved=0

Hello,

I'm trying to estimate the meson production by pp collision in the fixed-target experiment.
I generate the processes p p > j and p p > j and set ebeam1 400.0, ebeam2 0.938.
The rest parameters are the same as default parameters.
However, at the parton level, I cannot get an estimation of the cross section and it reports no events.

Please let me attach the information I got here.

  === Results Summary for run: run_01 tag: tag_1 ===

     Cross-section : nan +- nan pb
     Nb of events : 0

store_events
INFO: Storing parton level results
No event detected. No cleaning performed! This should allow to run:
    cd Subprocesses; ../bin/internal/combine_events
  to have your events if those one are missing.

I also try to generate process p p > u u~ or p p > d d~ with same setting as previous run and I got the following information.

Survey return zero cross section.
   Typical reasons are the following:
   1) A massive s-channel particle has a width set to zero.
   2) The pdf are zero for at least one of the initial state particles
      or you are using maxjetflavor=4 for initial state b:s.
   3) The cuts are too strong.
   Please check/correct your param_card and/or your run_card.
Zero result detected: See https://eur03.safelinks.protection.outlook.com/?url=https%3A%2F%2Fcp3.irmp.ucl.ac.be%2Fprojects%2Fmadgraph%2Fwiki%2FFAQ-General-14&amp;data=02%7C01%7C%7C96b99a684d384d22523e08d74cf020d4%7C7ab090d4fa2e4ecfbc7c4127b4d582ec%7C0%7C0%7C637062468027618001&amp;sdata=emGafblSuNpEKqdKy1Dj4NvUOlpUMfC7DlTOTAobocw%3D&amp;reserved=0
quit

I'm wondering if there are some parameters other than ebeam that I need to change in order to get a reasonable estimation.

Many thanks,
Jui-Lin

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Hi Olivier,

Thank you very much for the answer and the suggestion.
I change the cuts and now I could get a reasonable estimation.

Can you elaborate more why I cannot do the 2 to 1 process but the 2 to 2 process is feasible, please?
The subprocess are either g q -> q or q q -> g, and the center of mass energy is higher than the masses of these light quarks and gluons, so it seems to me that it is possible to make three particles on-shell. (Maybe I miss some details...)

Many thanks,
Jui-Lin

Well for 2>1 you have
p1+p2 = p3

the onshell condition is
p1^2 = 0
p2^2 = 0
p3^3 = 0

so if you take the first equation and you square it, you have

(p1+p2)^2 = p3^2 =0
p1^2 +p2^2 +2*p1*p2 = 0
2*p1*p2 =0

now if you expess p1 as (e, 0, 0, -e) and p2 as (e, 0,0,e) [this assume that you are in the center of mass but this is not a restriction of the argument.

then you have
2**(e^2+e^2) =0
therefore you should have e=0, i.e your process is not interesting.

Cheers,

Olivier

> On 10 Oct 2019, at 12:34, Jui-Lin Kuo <email address hidden> wrote:
>
> Question #684891 on MadGraph5_aMC@NLO changed:
> https://eur03.safelinks.protection.outlook.com/?url=https%3A%2F%2Fanswers.launchpad.net%2Fmg5amcnlo%2F%2Bquestion%2F684891&amp;data=02%7C01%7C%7C4a621bca43d3489efe7b08d74d6d7ac0%7C7ab090d4fa2e4ecfbc7c4127b4d582ec%7C0%7C0%7C637063005667870175&amp;sdata=6vGD9RlI160iirF15GrMj5CaAR1SrVSMrTKmgiiJ1HI%3D&amp;reserved=0
>
> Status: Answered => Open
>
> Jui-Lin Kuo is still having a problem:
> Hi Olivier,
>
> Thank you very much for the answer and the suggestion.
> I change the cuts and now I could get a reasonable estimation.
>
> Can you elaborate more why I cannot do the 2 to 1 process but the 2 to 2 process is feasible, please?
> The subprocess are either g q -> q or q q -> g, and the center of mass energy is higher than the masses of these light quarks and gluons, so it seems to me that it is possible to make three particles on-shell. (Maybe I miss some details...)
>
> Many thanks,
> Jui-Lin
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Hi Olivier,

Thanks for the explanation.

Cheers,
Jui-Lin