wrong color flow assigned for the interference terms in unweighted_events.lhe

Asked by Prudhvi Nikhil Bhattiprolu on 2019-09-23

Dear Madgraph team,

I am working on a new model, which essentially has only one vertex (g-g-X, where X is a new massive color singlet scalar with mass of 750 GeV (say)) in addition to the SM vertices. I want to plot/study the differential cross sections with respect to the invariant mass of 2 jets in the final state for the resonance + interference between g g > X > g g (resonance) and g g > g g (pure QCD) at the detector level (i.e. after the events are passed through Delphes). The goal is to study the effects of the interference between the QCD and resonance contributions; ultimately I want to do this at NLO, but for now I'm just trying to get the LO to work.

As a first step, I plotted d sigma/d (mjj) at parton-level obtained from unweighted_events.lhe using the syntax "generate g g > g g QCD^2==2" (which I believe does just the interference between resonance and pure QCD (gg>gg) background, please correct me if I am wrong?) and it looks as expected with positive and negative differential cross-sections in different invariant mass bins. The trouble is that Pythia8 produces an empty pythia8_events.hepmc file with no events/data. I think this is because of the wrong color flow assignment in the unweighted_events.lhe file i.e. all the entries in the columns ICOLUP(1,I) and ICOLUP(2,I) are zeroes. Is this a bug? or is this expected for the interference terms?

The reason I had thought this might have worked is that the "help generate" message gives the example of "generate p p > j j QED^2==2 QCD^2==2" to select the QED-QCD interference terms only. For this process, I do get pythia8_events.hepmc and .lhco files with events?

So, to get detector level events in my case (i.e. interference between g g > X > g g (resonance) and g g > g g (pure QCD) ), I wrote a shell script which changes the zeroes in the color flow entries in the unweighted_events.lhe file to match the color flow entries of resonance (i.e. g g > X > g g). I then split the unweighted_events.lhe into two files such that one file only has the events with positive weights and the other with negative weights only. If I then pass both .lhe files independently through Pythia8 and Delphes, I get pythia8_events.hepmc and .lhco files with events.

Finally, is there a fix for the wrong color flow assignment for the interference terms in the unweighted_events.lhe? if not, can I keep using the above procedure of changing color flow entries "by hand"?

Please let me know if anything is needed from my end.

Thank you very much!

Cheers,
Prudhvi

Question information

Language:
English Edit question
Status:
Solved
For:
MadGraph5_aMC@NLO Edit question
Assignee:
No assignee Edit question
Solved by:
Olivier Mattelaer
Solved:
2019-10-21
Last query:
2019-10-21
Last reply:
2019-09-26

The point is that I / we have no idea what is the correct leading color for this specific case.

So we did not set the color in such cas├ęs
I do not believe that your choice of color is correct. But I do not know how to check if it is or not

Cheers

Olivier

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________________________________
From: <email address hidden> <email address hidden> on behalf of Prudhvi Nikhil Bhattiprolu <email address hidden>
Sent: Monday, September 23, 2019 9:37:29 PM
To: Olivier Mattelaer <email address hidden>
Subject: [Question #684170]: wrong color flow assigned for the interference terms in unweighted_events.lhe

New question #684170 on MadGraph5_aMC@NLO:
https://answers.launchpad.net/mg5amcnlo/+question/684170

Dear Madgraph team,

I am working on a new model, which essentially has only one vertex (g-g-X, where X is a new massive color singlet scalar with mass of 750 GeV (say)) in addition to the SM vertices. I want to plot/study the differential cross sections with respect to the invariant mass of 2 jets in the final state for the resonance + interference between g g > X > g g (resonance) and g g > g g (pure QCD) at the detector level (i.e. after the events are passed through Delphes). The goal is to study the effects of the interference between the QCD and resonance contributions; ultimately I want to do this at NLO, but for now I'm just trying to get the LO to work.

As a first step, I plotted d sigma/d (mjj) at parton-level obtained from unweighted_events.lhe using the syntax "generate g g > g g QCD^2==2" (which I believe does just the interference between resonance and pure QCD (gg>gg) background, please correct me if I am wrong?) and it looks as expected with positive and negative differential cross-sections in different invariant mass bins. The trouble is that Pythia8 produces an empty pythia8_events.hepmc file with no events/data. I think this is because of the wrong color flow assignment in the unweighted_events.lhe file i.e. all the entries in the columns ICOLUP(1,I) and ICOLUP(2,I) are zeroes. Is this a bug? or is this expected for the interference terms?

The reason I had thought this might have worked is that the "help generate" message gives the example of "generate p p > j j QED^2==2 QCD^2==2" to select the QED-QCD interference terms only. For this process, I do get pythia8_events.hepmc and .lhco files with events?

So, to get detector level events in my case (i.e. interference between g g > X > g g (resonance) and g g > g g (pure QCD) ), I wrote a shell script which changes the zeroes in the color flow entries in the unweighted_events.lhe file to match the color flow entries of resonance (i.e. g g > X > g g). I then split the unweighted_events.lhe into two files such that one file only has the events with positive weights and the other with negative weights only. If I then pass both .lhe files independently through Pythia8 and Delphes, I get pythia8_events.hepmc and .lhco files with events.

Finally, is there a fix for the wrong color flow assignment for the interference terms in the unweighted_events.lhe? if not, can I keep using the above procedure of changing color flow entries "by hand"?

Please let me know if anything is needed from my end.

Thank you very much!

Cheers,
Prudhvi

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You received this question notification because you are an answer
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Hello Olivier,

Thanks. When you say that you have no idea what is the correct leading
color for this specific case, do you mean that it is impossible to say
what the color flow is even in principle, or just that in my particular
case (with QCD interfering with a new color-singlet particle X that
couples to two gluons) it isn't clear?

The problem from my perspective is that in the .lhe event file, the
gluons are being assigned 0 color, which is resulting in Pythia not
accepting the events (and generating no output). So, to pass them
through Pythia, I think it must be a requirement to assign colors to the
gluons, otherwise Pythia will always return an error. You mentioned
that you think my color choice is incorrect, and you don't know how to
check if it is or not. However, when I look at each event, it indeed
seems to me that I can assign the colors for each gluon in a sensible
way (or at least, a consistent way) just based on knowing the
parent/daughter parton relationships, and that this assignment is unique
(up to relabelings). Is that not correct?

One of the reasons I am puzzled is that in the somewhat similar QCD/QED
interference in the Standard Model, the .lhe file does give color flow
assignments that seem to work and are accepted by Pythia, For example,
below is one of the events from unweighted_events.lhe for the case of
p p > j j QED^2==2 QCD^2==2 .

<event>
 4 1 -2.0135798e+05 3.88930200e+01 7.54677100e-03 1.51651900e-01
       -2 -1 0 0 0 501 -0.0000000000e+00 +0.0000000000e+00 +3.9928176138e+00 3.9928176138e+00 0.0000000000e+00 0.0000e+00 1.0000e+00
        2 -1 0 0 501 0 +0.0000000000e+00 -0.0000000000e+00 -9.5416465697e+02 9.5416465697e+02 0.0000000000e+00 0.0000e+00 -1.0000e+00
        2 1 1 2 502 0 -1.7423717429e+01 -3.4771841059e+01 -8.4709119252e+02 8.4798358198e+02 0.0000000000e+00 0.0000e+00 -1.0000e+00
       -2 1 1 2 0 502 +1.7423717429e+01 +3.4771841059e+01 -1.0308064684e+02 1.1017389261e+02 0.0000000000e+00 0.0000e+00 1.0000e+00
</event>

If there is a sensible color flow assigned for the above case, why can't
there be a sensible color flow for my case i.e. g g > g g QCD^2==2 (in
my model) ? Here is one of the event from unweighted_events.lhe which is then rejected by Pythia because the gluons have been assigned 0 color:

<event>
 4 1 +1.7475443e+02 7.55404200e+01 7.81860800e-03 1.34220600e-01
       21 -1 0 0 0 0 +0.0000000000e+00 +0.0000000000e+00 +6.1343470561e+02 6.1343470561e+02 0.0000000000e+00 0.0000e+00 1.0000e+00
       21 -1 0 0 0 0 -0.0000000000e+00 -0.0000000000e+00 -1.5582142725e+02 1.5582142725e+02 0.0000000000e+00 0.0000e+00 1.0000e+00
       21 1 1 2 0 0 -7.2686203896e+01 -2.0568688575e+01 -1.4416391044e+02 1.6275622342e+02 0.0000000000e+00 0.0000e+00 1.0000e+00
       21 1 1 2 0 0 +7.2686203896e+01 +2.0568688575e+01 +6.0177718881e+02 6.0649990944e+02 0.0000000000e+00 0.0000e+00 1.0000e+00
</event>

Since Pythia won't take such events, I then wrote a script to change the color assignments of the gluons "by hand", so that, for example, the above event becomes:

<event>
 4 1 +1.7475443e+02 7.55404200e+01 7.81860800e-03 1.34220600e-01
       21 -1 0 0 502 501 +0.0000000000e+00 +0.0000000000e+00 +6.1343470561e+02 6.1343470561e+02 0.0000000000e+00 0.0000e+00 1.0000e+00
       21 -1 0 0 501 502 -0.0000000000e+00 -0.0000000000e+00 -1.5582142725e+02 1.5582142725e+02 0.0000000000e+00 0.0000e+00 1.0000e+00
       21 1 1 2 503 504 -7.2686203896e+01 -2.0568688575e+01 -1.4416391044e+02 1.6275622342e+02 0.0000000000e+00 0.0000e+00 1.0000e+00
       21 1 1 2 504 503 +7.2686203896e+01 +2.0568688575e+01 +6.0177718881e+02 6.0649990944e+02 0.0000000000e+00 0.0000e+00 1.0000e+00
</event>

Pythia does accept the modified events and gives what appear to me to be
sensible results in the end. Is there a flaw in this procedure, and if so is there
a better way to proceed?

Also, I am attaching my madgraph model file here https://www.dropbox.com/sh/67y6sytv2is35vw/AAC-rCWNATCKx0pEbajcNHdXa?dl=0 , in case if it is required.

Thanks much!

Cheers,
Prudhvi

Hi,

Your assignment is certainly not unique. I can do the following one:
(one standard QCD one)
501 502
502 503
503 504
504 501
or the following one (singlet in T-channel)
501 502
503 504
502 501
504 503
and you should have plenty of others.

The problem with gluon is that you have sub loeading color contribution but the information to write in the events should be leading color so the second color written above also occur in the SM but should never be written because this is a sub-leading color.

For me the strategy for assigning the color for interference should be the same as if we do the full computation since the idea is that you separate the pieces and if you put them back togheter you should get the same result. In your particular case that strategy is inconsistent with the picking of the leading color diagram based on the diagram enhancement method that we used.
So we react as we should (i.e. not assign any color in order to forbid the use of wrong color assignment and therefore made any tool using such information to crash).

Now this is up to you to fully re-design a strategy for the color assignement. But to be consistent you might also need to change it for the non interference part of the computation in order to have it coherent when you sum back the pieces together.

Cheers,

Olivier

> On 26 Sep 2019, at 22:57, Prudhvi Nikhil Bhattiprolu <email address hidden> wrote:
>
> Question #684170 on MadGraph5_aMC@NLO changed:
> https://eur03.safelinks.protection.outlook.com/?url=https%3A%2F%2Fanswers.launchpad.net%2Fmg5amcnlo%2F%2Bquestion%2F684170&amp;data=02%7C01%7Colivier.mattelaer%40uclouvain.be%7Cd78af3da7ac24c78ecd808d742c42a58%7C7ab090d4fa2e4ecfbc7c4127b4d582ec%7C0%7C0%7C637051283360617127&amp;sdata=5%2FwV6goJpa3xddmHoTFcndAcS6Hu0ULdVmHWy91j%2BIc%3D&amp;reserved=0
>
> Status: Answered => Open
>
> Prudhvi Nikhil Bhattiprolu is still having a problem:
> Hello Olivier,
>
> Thanks. When you say that you have no idea what is the correct leading
> color for this specific case, do you mean that it is impossible to say
> what the color flow is even in principle, or just that in my particular
> case (with QCD interfering with a new color-singlet particle X that
> couples to two gluons) it isn't clear?
>
> The problem from my perspective is that in the .lhe event file, the
> gluons are being assigned 0 color, which is resulting in Pythia not
> accepting the events (and generating no output). So, to pass them
> through Pythia, I think it must be a requirement to assign colors to the
> gluons, otherwise Pythia will always return an error. You mentioned
> that you think my color choice is incorrect, and you don't know how to
> check if it is or not. However, when I look at each event, it indeed
> seems to me that I can assign the colors for each gluon in a sensible
> way (or at least, a consistent way) just based on knowing the
> parent/daughter parton relationships, and that this assignment is unique
> (up to relabelings). Is that not correct?
>
> One of the reasons I am puzzled is that in the somewhat similar QCD/QED
> interference in the Standard Model, the .lhe file does give color flow
> assignments that seem to work and are accepted by Pythia, For example,
> below is one of the events from unweighted_events.lhe for the case of
> p p > j j QED^2==2 QCD^2==2 .
>
> <event>
> 4 1 -2.0135798e+05 3.88930200e+01 7.54677100e-03 1.51651900e-01
> -2 -1 0 0 0 501 -0.0000000000e+00 +0.0000000000e+00 +3.9928176138e+00 3.9928176138e+00 0.0000000000e+00 0.0000e+00 1.0000e+00
> 2 -1 0 0 501 0 +0.0000000000e+00 -0.0000000000e+00 -9.5416465697e+02 9.5416465697e+02 0.0000000000e+00 0.0000e+00 -1.0000e+00
> 2 1 1 2 502 0 -1.7423717429e+01 -3.4771841059e+01 -8.4709119252e+02 8.4798358198e+02 0.0000000000e+00 0.0000e+00 -1.0000e+00
> -2 1 1 2 0 502 +1.7423717429e+01 +3.4771841059e+01 -1.0308064684e+02 1.1017389261e+02 0.0000000000e+00 0.0000e+00 1.0000e+00
> </event>
>
> If there is a sensible color flow assigned for the above case, why can't
> there be a sensible color flow for my case i.e. g g > g g QCD^2==2 (in
> my model) ? Here is one of the event from unweighted_events.lhe which is then rejected by Pythia because the gluons have been assigned 0 color:
>
> <event>
> 4 1 +1.7475443e+02 7.55404200e+01 7.81860800e-03 1.34220600e-01
> 21 -1 0 0 0 0 +0.0000000000e+00 +0.0000000000e+00 +6.1343470561e+02 6.1343470561e+02 0.0000000000e+00 0.0000e+00 1.0000e+00
> 21 -1 0 0 0 0 -0.0000000000e+00 -0.0000000000e+00 -1.5582142725e+02 1.5582142725e+02 0.0000000000e+00 0.0000e+00 1.0000e+00
> 21 1 1 2 0 0 -7.2686203896e+01 -2.0568688575e+01 -1.4416391044e+02 1.6275622342e+02 0.0000000000e+00 0.0000e+00 1.0000e+00
> 21 1 1 2 0 0 +7.2686203896e+01 +2.0568688575e+01 +6.0177718881e+02 6.0649990944e+02 0.0000000000e+00 0.0000e+00 1.0000e+00
> </event>
>
> Since Pythia won't take such events, I then wrote a script to change the
> color assignments of the gluons "by hand", so that, for example, the
> above event becomes:
>
> <event>
> 4 1 +1.7475443e+02 7.55404200e+01 7.81860800e-03 1.34220600e-01
> 21 -1 0 0 502 501 +0.0000000000e+00 +0.0000000000e+00 +6.1343470561e+02 6.1343470561e+02 0.0000000000e+00 0.0000e+00 1.0000e+00
> 21 -1 0 0 501 502 -0.0000000000e+00 -0.0000000000e+00 -1.5582142725e+02 1.5582142725e+02 0.0000000000e+00 0.0000e+00 1.0000e+00
> 21 1 1 2 503 504 -7.2686203896e+01 -2.0568688575e+01 -1.4416391044e+02 1.6275622342e+02 0.0000000000e+00 0.0000e+00 1.0000e+00
> 21 1 1 2 504 503 +7.2686203896e+01 +2.0568688575e+01 +6.0177718881e+02 6.0649990944e+02 0.0000000000e+00 0.0000e+00 1.0000e+00
> </event>
>
>
> Pythia does accept the modified events and gives what appear to me to be
> sensible results in the end. Is there a flaw in this procedure, and if so is there
> a better way to proceed?
>
> Also, I am attaching my madgraph model file here
> https://eur03.safelinks.protection.outlook.com/?url=https%3A%2F%2Fwww.dropbox.com%2Fsh%2F67y6sytv2is35vw%2FAAC-&amp;data=02%7C01%7Colivier.mattelaer%40uclouvain.be%7Cd78af3da7ac24c78ecd808d742c42a58%7C7ab090d4fa2e4ecfbc7c4127b4d582ec%7C0%7C0%7C637051283360617127&amp;sdata=gzQC8Ic07aWT7fojSMWU7p8zTNLUdhcoMPV6KDHnDRk%3D&amp;reserved=0
> rCWNATCKx0pEbajcNHdXa?dl=0 , in case if it is required.
>
> Thanks much!
>
> Cheers,
> Prudhvi
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Thanks Olivier Mattelaer, that solved my question.