MadGraph5_aMC@NLO

An error message while processing scattering events

Asked by John

Hi,

I am trying to run scattering of polarzied electron positron in the same helicity states in the standard model.

In the middle of simulations after the launch command, the madgraph outputs

Error detected in "generate_events run_01"
write debug file /home/yc654/MadGraph5_v1_4_8_4/PROC_sm_0/run_01_tag_1_debug.log
If you need help with this issue please contact us on https://answers.launchpad.net/madgraph5
MadGraph5Error : 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.
    3) The cuts are too strong.
    Please check/correct your param_card and/or your run_card.

Opposite helicity states cases run fine but only the same helicity states produces this problem.
(Note- the latest version of the madgraph is used)

John

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Olivier Mattelaer (olivier-mattelaer) said : 		#1

Hi Johan,

Could you send to me (omatt __AT__ illinois.edu)
all the files present in your
/home/yc654/MadGraph5_v1_4_8_4/PROC_sm_0/Cards directory?
This will allow me to reproduce your run and then I will be able to give you an appropriate answer.

Cheers,

Olivier

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Olivier Mattelaer (olivier-mattelaer) said : 		#2

Hi John,

(Sorry I just remark that I wrote Johan in my previous answer, too use to that name ;-))

Thanks for the cards.
I think that this is perfectly normal.
If you make the sum over the four helicity you obtain:
8.15 + 2.702 + 0 + 0
RL + LR + RR + LL
which is indeed 4 times more than a standard run [average over the helicity] 2,717.
By looking at the diagram, you can convince yourself that this is indeed physical that the result is zero.

This message is printed each time that we evaluate a zero cross-section, so nothing to worry about here.

Cheers,

Olivier

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