MadGraph5_aMC@NLO

Loop: Large weight

Asked by tom ingebretsen carlson

Hi!

I'm doing a loop calculation and MadGraph complains about an event with a large weight.
Do you know the reason for this and is there a way to solve this problem?
Thanks!

The following warnings and errors are given:

WARNING: "One Event with large weight have been found (ratio = 1.92e+06) in channel G3 (with rel.contrib=0.483).
This is likely due to numerical instabilities. The associated job is discarded to recover.
For offline investigation, the problematic discarded events are stored in:
/home/tom/Madgraph/MG5_aMC_v2_6_5/dfdm_dfdm_j_MST_700/SubProcesses/P0_gg_dfdmdfdmxg/DiscardedUnstableEvents
Start waiting for update. (more info in debug mode)
WARNING: [Fail 5 times]
 [Errno 2] No such file or directory: '/home/tom/Madgraph/MG5_aMC_v2_6_5/dfdm_dfdm_j_MST_700/SubProcesses/P0_gg_dfdmdfdmxg/G3_1/results.dat'
INFO: Idle: 0, Running: 0, Completed: 130 [ 8h 53m ]
INFO: Idle: 0, Running: 0, Completed: 130 [ 8h 53m ]
[Fail 5 times]
 [Errno 2] No such file or directory: '/home/tom/Madgraph/MG5_aMC_v2_6_5/dfdm_dfdm_j_MST_700/SubProcesses/P0_gg_dfdmdfdmxg/G3_1/results.dat'
Cluster Error detected. Do you want to clean the queue? ("c"=continue the run anyway) [y, n, c][60s to answer]
>
use y
Command "generate_events run_01" interrupted with error:
IOError : [Fail 5 times]
  [Errno 2] No such file or directory: '/home/tom/Madgraph/MG5_aMC_v2_6_5/dfdm_dfdm_j_MST_700/SubProcesses/P0_gg_dfdmdfdmxg/G3_1/results.dat'

Thanks for the help

Cheers,

Tom

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Olivier Mattelaer (olivier-mattelaer) said : 		#1

The first question is to understand the reason of that issue.
In most case this is related to a divergence (integrable or not) of your matrix-element.

Also from your process definition, gg_dfdmdfdmxg, I have some worry that your process is not a loop-induced one (and therefore that your loop-computation is not IR and UV finite.) But I guess that you have checked that already.

If those above points are fine, then you will need to display such events, and one collaborator will check if this is indeed a numerical instabilities or not (and if something can be done).

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tom ingebretsen carlson (makronymen) said : 		#2

Okey, thank you very much for your input.

I'll have a closer look on this.

Cheers!

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tom ingebretsen carlson (makronymen) said : 		#3

Some further information:

The process is a first order loop and has no divergences. The error is only produced for one specific mass point(large mass gap) and not the other.

Process
p p > dfdm~ dfdm j [QCD]

dfdm - dirac fermion

Do you have an idea where this problem comes from?

Thanks for the help!

Tom

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Olivier Mattelaer (olivier-mattelaer) said : 		#4

Hi Tom,

Can you explain why you do not have any divergences?
And why this is a loop-induced process?

With dfdm being a fermion, you need at least one of the "p"/"j" being a gluon.
I would naively think therefore that the process without the gluon should exists (except if you have changing flavor interaction with gluon or something of that type). If you do then, this is not a loop-induced process and you should have some singularity associated with the loop.

This being said, with the information that I have here, I can not really comment on your problem.
If you need more help we would likely need more information
1) the UFO model
2) the file /home/tom/Madgraph/MG5_aMC_v2_6_5/dfdm_dfdm_j_MST_700/SubProcesses/P0_gg_dfdmdfdmxg/DiscardedUnstableEvents
3) all your card/...

This will allow us to know what you do and even potentially to try to reproduce your problem.

Cheers,

Olivier

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tom ingebretsen carlson (makronymen) said : 		#5

Hi,

the process is at 1-loop at leading order, independently of the masses. So there are no counterterms.

Yes I do have a gluon in the final state.

Okey thank you! I might get back to you with this later on. Meanwhile thanks for the inputs.

Cheers,

Tom

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