MadGraph5_aMC@NLO

fail to generate four jet event in electron-positron collider with cut on invariant mass of jet pair

Asked by user1905

Hi All,
I am trying to generate four jet events using following syntax:
e^- e^+ \to j j j j QED<=99 QCD=0
 I apply the cut on the maximum invariant mass of jet pair at 80.0 GeV. without the cut, the events are getting generated but after this cut, the code fails. The version of madgraph i am using is 2.6.7. I paste some line of error generated below.

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://cp3.irmp.ucl.ac.be/projects/madgraph/wiki/FAQ-General-14

Thanks .

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Olivier Mattelaer
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Best Olivier Mattelaer (olivier-mattelaer) said : 		#1

Hi,

So you can look at the various log file (for each channel) and check
If it fails due to no phase-space (smin > shat) or fails since it "just" does not find any point passing the cuts.

In the first case, this means that you ask something impossible, in the second something too hard for MG5aMC.
The solution is likely to ask softer cuts.

Cheers,

Olivier

> On 29 May 2023, at 14:40, amir subba <email address hidden> wrote:
>
> New question #706824 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/706824
>
> Hi All,
> I am trying to generate four jet events using following syntax:
> e^- e^+ \to j j j j QED<=99 QCD=0
> I apply the cut on the maximum invariant mass of jet pair at 80.0 GeV. without the cut, the events are getting generated but after this cut, the code fails. The version of madgraph i am using is 2.6.7. I paste some line of error generated below.
>
> 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://cp3.irmp.ucl.ac.be/projects/madgraph/wiki/FAQ-General-14
>
> Thanks .
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Revision history for this message
user1905 (userinhep) said : 		#2
Revision history for this message
Olivier Mattelaer (olivier-mattelaer) said : 		#3

Hi,

Since version 2.6.7 is not supported anymore.
I can not test with your version but the folllowing code works with our Long Term Stable version
generate e+ e- > 4 j
output
launch
set lep 250
set mmjj 80

Of course the following one:
generate e+ e- > 4 j
output
launch
set lep 250
set mmjjmax 80

Does not work since you forbid any pair of jet to be above 80 and there you do not have any phase-space available.

Cheers,

Olivier

> On 29 May 2023, at 19:10, amir subba <email address hidden> wrote:
>
> Question #706824 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/706824
>
> Status: Answered => Open
>
> amir subba is still having a problem:
> I dont think the cut is that hard. In the center of mass energy of 250
> GeV, asking a pair of jets with invaraiant mass of 80 GeV is not that
> hard. Although, the code works fine if we work on the 5 falvor scheme.
> But in that case the final state is just b-quark.
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

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Olivier Mattelaer (olivier-mattelaer) said : 		#5

You do impose the following inequality
(p1+p2)^2 = 2 p1*p2 < 80**2
(p1+p3)^2 = 2 p1*p3 < 80**2
(p1+p4)^2 = 2 p1*p4 < 80**2
(p2+p3)^2 = 2 p2*p3 < 80**2
(p2+p4)^2 = 2 p2*p4 < 80**2
(p3+p4)^2 = 2 p3*p4 < 80**2
You can write
250**2 = (p1+p2+p3+p4)**2 = 2 p1p2 + 2 p1p3 +2 p1p4+2 p2p3+2 p2p4+2 p2p4
Now using the equility that you impose on the invariant mass of a pair of jet:
250**2 = 62500 < 6*80**2 = 38400
This is clearly impossible, and this is why MG5aMC returns zero cross-section for that cut.

Cheers,

Olivier

(p1+p2)^2 = 2 p1*p2 < 80**2

Revision history for this message
user1905 (userinhep) said : 		#6

Thanks Olivier Mattelaer, that solved my question.