Gluons are assigned with a mass

Asked by Alexis Kalogeropoulos on 2018-06-19

Hello

I would like for your feedback as I see something strange - What I do

import model mssm-full
define p = g u c d s u~ c~ d~ s~
define j = g u c d s u~ c~ d~ s~
define p = g u c d s u~ c~ d~ s~ b b~
define j = g u c d s u~ c~ d~ s~ b b~
define stop = t1 t1~
define char = x1- x1+
define l+ = e+ mu+
define l- = e- mu-
define vl = ve vm vt
define vl~ = ve~ vm~ vt~
generate p p > t1 x1- @1
add process p p > t1~ x1+ @2

but then, looking at the LHE I see that the gluon as assigned with a mass

<event>
 4 2 +2.0901467e+00 2.56758000e+02 7.81653100e-03 1.11665700e-01
       -5 -1 0 0 0 501 -0.0000000000e+00 +0.0000000000e+00 +1.0873512647e+03 1.0873622599e+03 0.0000000000e+00 0.0000e+00 1.0000e+00
       21 -1 0 0 501 502 +0.0000000000e+00 -0.0000000000e+00 -6.2693849868e+01 6.2693849868e+01 4.8899170000e+00 0.0000e+00 -1.0000e+00
 -1000006 1 1 2 0 502 +1.5747485633e+02 -3.3561346253e+01 +4.0782235789e+02 4.8191676661e+02 2.0000000000e+02 0.0000e+00 0.0000e+00
  1000024 1 1 2 0 0 -1.5747485633e+02 +3.3561346253e+01 +6.1683505695e+02 6.6813934312e+02 2.0000000000e+02 0.0000e+00 1.0000e+00
<scales pt_clust_3="14000.00000" pt_clust_4="14000.00000"></scales>
<clustering>
<clus scale=" 256.758"> 2 3 1 -1</clus>
<clus scale=" 256.758"> 2 3 1 -1</clus>
</clustering>
</event>

Do you have any idea why (I am using v2_6_0)?

Thanks

Alexis

Question information

Language:
English Edit question
Status:
Solved
For:
MadGraph5_aMC@NLO Edit question
Assignee:
No assignee Edit question
Solved by:
Olivier Mattelaer
Solved:
2018-06-20
Last query:
2018-06-20
Last reply:
2018-06-20

Hi,

The first problem is a physical one, you setup your beam as a 5 flavor PDF but with a 4 flavor model.
Then some symmetry are broken which lead to this issue.
I will try to fix this in 2.6.4 but in the mean time, you should run this in a consistent way.

Cheers,

Olivier

> On 19 Jun 2018, at 11:37, Alexis Kalogeropoulos <email address hidden> wrote:
>
> New question #670308 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/670308
>
> Hello
>
> I would like for your feedback as I see something strange - What I do
>
> import model mssm-full
> define p = g u c d s u~ c~ d~ s~
> define j = g u c d s u~ c~ d~ s~
> define p = g u c d s u~ c~ d~ s~ b b~
> define j = g u c d s u~ c~ d~ s~ b b~
> define stop = t1 t1~
> define char = x1- x1+
> define l+ = e+ mu+
> define l- = e- mu-
> define vl = ve vm vt
> define vl~ = ve~ vm~ vt~
> generate p p > t1 x1- @1
> add process p p > t1~ x1+ @2
>
> but then, looking at the LHE I see that the gluon as assigned with a mass
>
> <event>
> 4 2 +2.0901467e+00 2.56758000e+02 7.81653100e-03 1.11665700e-01
> -5 -1 0 0 0 501 -0.0000000000e+00 +0.0000000000e+00 +1.0873512647e+03 1.0873622599e+03 0.0000000000e+00 0.0000e+00 1.0000e+00
> 21 -1 0 0 501 502 +0.0000000000e+00 -0.0000000000e+00 -6.2693849868e+01 6.2693849868e+01 4.8899170000e+00 0.0000e+00 -1.0000e+00
> -1000006 1 1 2 0 502 +1.5747485633e+02 -3.3561346253e+01 +4.0782235789e+02 4.8191676661e+02 2.0000000000e+02 0.0000e+00 0.0000e+00
> 1000024 1 1 2 0 0 -1.5747485633e+02 +3.3561346253e+01 +6.1683505695e+02 6.6813934312e+02 2.0000000000e+02 0.0000e+00 1.0000e+00
> <scales pt_clust_3="14000.00000" pt_clust_4="14000.00000"></scales>
> <clustering>
> <clus scale=" 256.758"> 2 3 1 -1</clus>
> <clus scale=" 256.758"> 2 3 1 -1</clus>
> </clustering>
> </event>
>
> Do you have any idea why (I am using v2_6_0)?
>
> Thanks
>
> Alexis
>
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Hi Olivier

Which model should I be using then ? I see that only the mssm-full actually worked for me...

Hi,

You should use one with the b mass set to zero.
You can edit your model for having that, or use a restrict file/...
that's up to you.

Cheers,

Olivier

> On 19 Jun 2018, at 11:52, Alexis Kalogeropoulos <email address hidden> wrote:
>
> Question #670308 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/670308
>
> Status: Answered => Open
>
> Alexis Kalogeropoulos is still having a problem:
> Hi Olivier
>
> Which model should I be using then ? I see that only the mssm-full
> actually worked for me...
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Hi again

So, since the mssm model is not part of the latest MG releases, I copied it from an older version to the models dir. Now, doing

import model mssm-no_b_mass

in the and I get a message

Command "import /afs/cern.ch/work/a/alkaloge/MCT/CMSSW_8_1_0/src/GenLHEfiles/Run2Mechanism/MG5_aMC_v2_6_3/proc_card_mg5.dat" interrupted in sub-command:
"output singlestop -nojpeg" with error:
ZeroDivisionError : float division by zero
Please report this bug on https://bugs.launchpad.net/mg5amcnlo
More information is found in 'MG5_debug'.
Please attach this file to your report.

I ve tried like a simple ttbar with 'sm' model and it works fine, so I guess it is just the fact I ported an older model ? In any case, can you advice how to use the mssm no_b_mass with latest MG versions ?

Thanks again

Hi Alexis,

Actually the mssm is still present within the code, we just rename it
to make explicit something that was implicit before (and that was creating a LOT of issue)

So the mssm model, is not the MSSM-SLHA2 model.
You can make a diff, and you will see that they are 100% identical.

The "mssm" model was handle in a weird way by MG5aMC due to pytha6 support.
In order for PY6 to shower correctly such model, we were force to use a SLHA1 param_card (even if the model was SLHA2 compliant since FR author did not want to provide a SLHA1).

So we were force to provide to the user a SLHA1 card, card that we were translating to SLHA2 for
the evaluation of the matrix-element.
You can understand WHY it is creating so many problem.

With py6 not supported anymore and py8 handling SLHA2 correctly. It was time to not support anymore
such trick. I would therefore strongly suggest that you drop the "mssm" model and use the "MSSM-SLHA2" model instead. You can still provide a slha1 input file and the use the command:
"update to_slha2" (at the time of the edition of the card obviously) to have kind of the same behaviour.
(But at least now you are in control and you can (i.e. you should) check if the conversion is correctly occuring)

This being said:

If i do this
> import model MSSM_SLHA2-no_b_mass
> generate p p > t1 x1- @1
> add process p p > t1~ x1+ @2
> output

I do not have any issue.

Cheers,

Olivier

> On 19 Jun 2018, at 12:37, Alexis Kalogeropoulos <email address hidden> wrote:
>
> Question #670308 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/670308
>
> Alexis Kalogeropoulos posted a new comment:
> Hi again
>
> So, since the mssm model is not part of the latest MG releases, I copied
> it from an older version to the models dir. Now, doing
>
> import model mssm-no_b_mass
>
> in the and I get a message
>
> Command "import /afs/cern.ch/work/a/alkaloge/MCT/CMSSW_8_1_0/src/GenLHEfiles/Run2Mechanism/MG5_aMC_v2_6_3/proc_card_mg5.dat" interrupted in sub-command:
> "output singlestop -nojpeg" with error:
> ZeroDivisionError : float division by zero
> Please report this bug on https://bugs.launchpad.net/mg5amcnlo
> More information is found in 'MG5_debug'.
> Please attach this file to your report.
>
> I ve tried like a simple ttbar with 'sm' model and it works fine, so I
> guess it is just the fact I ported an older model ? In any case, can you
> advice how to use the mssm no_b_mass with latest MG versions ?
>
> Thanks again
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Hi,

I have taken a (quick) look at the reason why the mass is wrongly assign.
And actually as you can also see in the example events that you provide, the momentum is correctly set.
( I means that P^2 is zero for the gluon) So the problem is here quite easy to fix.

So you can have a quick work around by taking your lhe file and recompute all the mass according to P^2 and your output will be the expected one. (You obviously still mix up 4 and 5 flavour but that is your choice)

Cheers,

Olivier

Hi Oliver

Thanks for your follow-up - Actually, I am already using a b_zero_mass scheme, and the output now looks fine (as expected) - Pythia anyways reassigns the b-quark mass later on correctly for all I care.

Cheers

Alexis

Hi,

Excellent, this is the correct choice to my point of view.
I have in any case provide a patch solving such issue and will put it below in case someone else face such issue before the next release.

Thanks,

Olivier

=== modified file 'Template/LO/SubProcesses/unwgt.f'
--- Template/LO/SubProcesses/unwgt.f 2017-06-22 08:03:29 +0000
+++ Template/LO/SubProcesses/unwgt.f 2018-06-20 11:50:48 +0000
@@ -513,6 +513,10 @@

       double precision stot,m1,m2
       common/to_stot/stot,m1,m2
+
+ INTEGER IMIRROR
+ INTEGER IPROC
+ COMMON/TO_MIRROR/IMIRROR, IPROC
 c
 c Data
 c
@@ -641,6 +645,13 @@
             pb(4,isym(j,jsym))=pmass(j)
          enddo
       endif
+
+ if (IMIRROR.eq.2.and.pmass(1).ne.pmass(2)) then
+c Note that in this context isym(1,jsym) should never be "2" since the mass differ
+ pb(4,isym(1,jsym))=pmass(2)
+ pb(4,isym(2,jsym))=pmass(1)
+ endif
+
 c
 c Add info on resonant mothers
 c

=== modified file 'UpdateNotes.txt'
--- UpdateNotes.txt 2018-06-20 08:18:36 +0000
+++ UpdateNotes.txt 2018-06-20 11:52:03 +0000
@@ -2,6 +2,8 @@

 2.6.4 ()
       OM: Fix a bug in auto-width when mass are below QCD scale.
+ OM: Fix a bug for g b initial state where the mass in the lhe file was not always correctly assigned
+ Note that the momentum was fine (i.e. in the file P^2 was not equal to the mention M but to the correct one)

 2.6.3 (15/06/18)
       OM: When importing model, we now run one additional layer of optimisation:

> On 20 Jun 2018, at 13:13, Alexis Kalogeropoulos <email address hidden> wrote:
>
> Question #670308 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/670308
>
> Status: Answered => Solved
>
> Alexis Kalogeropoulos confirmed that the question is solved:
> Hi Oliver
>
> Thanks for your follow-up - Actually, I am already using a b_zero_mass
> scheme, and the output now looks fine (as expected) - Pythia anyways
> reassigns the b-quark mass later on correctly for all I care.
>
> Cheers
>
> Alexis
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.