MadGraph5_aMC@NLO

Photon isolation cone requirement R0gamma

Asked by Julia Djuvsland

Dear MadGraph Team,

I have a question about the photon isoalation cuts:

Looking at the plots MadGraph produces for me, I conclude, that the photon isolation cone
requirement is also applied between photons and leptons, where in the reference you mention
for this cut (hep-ph/9801442) it seems to be only applied between photons and gluons/quarks.

I am asking since I am comparing my results from MadGraph with computations from VBFNLO
and don't get agreement. It seems to me that in VBFNLO the cut is only applied between quarks/gluons
and the photon.

Can you confirm my suspicions and maybe explain why it is implemented the way it is?
Do you have any clever idea on how to get consistent results between the two generators?

Thanks,

Julia

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Stefano Frixione
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Stefano Frixione (stefano-frixione) said : 		#1

Hi,
you can choose to isolate photon from EM energy in the same way as from hadronic energy.
This is controlled by
   .true. = isoEM
in run_card.dat. Set that variable to .false. if you wish to isolate the photon only from
hadronic energy.
Regards.

Revision history for this message
Julia Djuvsland (julia-djuvsland) said : 		#2

Hi Stefano,

thanks a lot for your reply. I understand, I can compare the results of the two generators by turning the isoEM off.
I find an agreement of ~ 4% at LO.

I am looking at WWy leptonic final states. Which photon isolation would you find suited for this case? (With or without EM energy isolation?)

Best,

Julia

Revision history for this message
Best Stefano Frixione (stefano-frixione) said : 		#3

Hello,
I'm afraid 4% is far from being satisfactory. You should try and choose all input
parameters to be identical; when that's the case, the agreement at the LO should
be at the level of the integration error (ie make sure you integrate with a sufficient
number of points to get well below 1%).
The choice of isolation is up to you; if one doesn't push things too far (ie too small
cones, too strict isolations), there's nothing striking about the process you're
interested into.
Regards.

Revision history for this message
Julia Djuvsland (julia-djuvsland) said : 		#4

Thanks a lot!

Revision history for this message
Shu Li (llss233) said : 		#5

Dear Julia and Stefano,

Sorry for jumping into this old thread.
I have got a question concerning the MadGraph photon isolation as long as I am trying to make it consistent with Sherpa.

While I am using MG5_aMC@NLO 2.3.3, I have got out-of-box the truth iso as:
#***********************************************************************
# Photon-isolation cuts, according to hep-ph/9801442 *
# When ptgmin=0, all the other parameters are ignored *
# When ptgmin>0, pta and draj are not going to be used *
#***********************************************************************
 0.0 = ptgmin ! Min photon transverse momentum
 0.4 = R0gamma ! Radius of isolation code
 1.0 = xn ! n parameter of eq.(3.4) in hep-ph/9801442
 1.0 = epsgamma ! epsilon_gamma parameter of eq.(3.4) in hep-ph/9801442
 True = isoEM ! isolate photons from EM energy (photons and leptons)

while in Sherpa it is configured as:
 IsolationCut 22 0.3 2 0.025;

Would it then have to be:
R0gamma=0.3, epsgamma=0.025, xn=2 and isoEM=false
in MG5 instead, so that it could be then consistent with Sherpa?

Many thanks.

best,
Shu

Revision history for this message
Olivier Mattelaer (olivier-mattelaer) said : 		#6

Hi,

I do not know what definition Sherpa is using.
In oour case this is the frixione isolation as explained in hep-ph/9801442

Cheers,

Olivier

> On 12 Apr 2017, at 23:17, Shu Li <email address hidden> wrote:
>
> Question #258783 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/258783
>
> Shu Li posted a new comment:
> Dear Julia and Stefano,
>
> Sorry for jumping into this old thread.
> I have got a question concerning the MadGraph photon isolation as long as I am trying to make it consistent with Sherpa.
>
> While I am using MG5_aMC@NLO 2.3.3, I have got out-of-box the truth iso as:
> #***********************************************************************
> # Photon-isolation cuts, according to hep-ph/9801442 *
> # When ptgmin=0, all the other parameters are ignored *
> # When ptgmin>0, pta and draj are not going to be used *
> #***********************************************************************
> 0.0 = ptgmin ! Min photon transverse momentum
> 0.4 = R0gamma ! Radius of isolation code
> 1.0 = xn ! n parameter of eq.(3.4) in hep-ph/9801442
> 1.0 = epsgamma ! epsilon_gamma parameter of eq.(3.4) in hep-ph/9801442
> True = isoEM ! isolate photons from EM energy (photons and leptons)
>
> while in Sherpa it is configured as:
> IsolationCut 22 0.3 2 0.025;
>
>
> Would it then have to be:
> R0gamma=0.3, epsgamma=0.025, xn=2 and isoEM=false
> in MG5 instead, so that it could be then consistent with Sherpa?
>
> Many thanks.
>
> best,
> Shu
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Revision history for this message
Shu Li (llss233) said : 		#7

Hi Olivier,

Thanks a lot for the quick response.
Yes, Sherpa likes Frixione isolation ,too.

I checked Sherpa manu:
https://sherpa.hepforge.org/doc/SHERPA-MC-2.2.0.html
and it reads:
Cuts on photons
IsolationCut 22 <dR> <exponent> <epsilon>

So I assume <dR> is exactly R0gamma in MG5, <exponent> is xn in MG5 and is <epsilon> is epsgamma in MG5?

Many thanks.

best,
Shu

Revision history for this message
Olivier Mattelaer (olivier-mattelaer) said : 		#8

Hi Shu,

That sounds a reasonable assumption then.
If you have some doubt I would suggest to ask Sherpa author

Cheers,

Olivier

> On 13 Apr 2017, at 01:47, Shu Li <email address hidden> wrote:
>
> Question #258783 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/258783
>
> Shu Li posted a new comment:
> Hi Olivier,
>
> Thanks a lot for the quick response.
> Yes, Sherpa likes Frixione isolation ,too.
>
> I checked Sherpa manu:
> https://sherpa.hepforge.org/doc/SHERPA-MC-2.2.0.html
> and it reads:
> Cuts on photons
> IsolationCut 22 <dR> <exponent> <epsilon>
>
> So I assume <dR> is exactly R0gamma in MG5, <exponent> is xn in MG5 and
> is <epsilon> is epsgamma in MG5?
>
> Many thanks.
>
> best,
> Shu
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.