MadGraph5_aMC@NLO

Higgs into 2 photons: decay processes cannot be perturbed

Asked by Fabio

Dear MG5 experts,

Happy new year.

I would like to ask please what is missing or I should be added to fix the error message "str : Decay processes cannot be perturbed" in the generation below where I would like to have the Higgs boson decaying into two photons: p p > z h, h > a a aS=0 aEW=3 [QED]

The full log of the running code is below:

[falves@lxplus746 MG5_aMC_v3_3_1]$ ./bin/mg5_aMC
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* W E L C O M E to *
* M A D G R A P H 5 _ a M C @ N L O *
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* * * * * 5 * * * * *
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* VERSION 3.3.1 2021-12-04 *
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* The MadGraph5_aMC@NLO Development Team - Find us at *
* https://server06.fynu.ucl.ac.be/projects/madgraph *
* and *
* http://amcatnlo.web.cern.ch/amcatnlo/ *
* *
* Type 'help' for in-line help. *
* Type 'tutorial' to learn how MG5 works *
* Type 'tutorial aMCatNLO' to learn how aMC@NLO works *
* Type 'tutorial MadLoop' to learn how MadLoop works *
* *
************************************************************
load MG5 configuration from input/mg5_configuration.txt
set fastjet to fastjet-config
set ninja to /afs/cern.ch/work/f/falves/kEW/madgraphewk/MG5_aMC_v3_3_1/HEPTools/lib
set collier to /afs/cern.ch/work/f/falves/kEW/madgraphewk/MG5_aMC_v3_3_1/HEPTools/lib
set lhapdf to lhapdf-config
set lhapdf to /afs/cern.ch/work/f/falves/kEW/madgraphewk/MG5_aMC_v3_3_1/HEPTools/lhapdf6_py3/bin/lhapdf-config
Using default eps viewer "evince". Set another one in ./input/mg5_configuration.txt
Using default web browser "firefox". Set another one in ./input/mg5_configuration.txt
Loading default model: sm
INFO: Restrict model sm with file models/sm/restrict_default.dat .
INFO: Run "set stdout_level DEBUG" before import for more information.
INFO: Change particles name to pass to MG5 convention
Defined multiparticle p = g u c d s u~ c~ d~ s~
Defined multiparticle j = g u c d s u~ c~ d~ s~
Defined multiparticle l+ = e+ mu+
Defined multiparticle l- = e- mu-
Defined multiparticle vl = ve vm vt
Defined multiparticle vl~ = ve~ vm~ vt~
Defined multiparticle all = g u c d s u~ c~ d~ s~ a ve vm vt e- mu- ve~ vm~ vt~ e+ mu+ t b t~ b~ z w+ h w- ta- ta+
MG5_aMC>set complex_mass_scheme true
Activate complex mass scheme.
import model sm
INFO: Restrict model sm with file models/sm/restrict_default.dat .
INFO: Run "set stdout_level DEBUG" before import for more information.
INFO: Change particles name to pass to MG5 convention
Kept definitions of multiparticles p / j / l+ / l- / vl / vl~ unchanged
Defined multiparticle all = g u c d s u~ c~ d~ s~ a ve vm vt e- mu- ve~ vm~ vt~ e+ mu+ t b t~ b~ z w+ h w- ta- ta+
MG5_aMC>set auto_convert_model T
save options auto_convert_model
save configuration file to /afs/cern.ch/work/f/falves/kEW/madgraphewk/MG5_aMC_v3_3_1/input/mg5_configuration.txt
MG5_aMC>import model loop_qcd_qed_sm_Gmu
Switching to Feynman gauge because it is the only one supported by the model loop_qcd_qed_sm_Gmu.
INFO: Restrict model loop_qcd_qed_sm_Gmu with file models/loop_qcd_qed_sm_Gmu/restrict_default.dat .
INFO: Run "set stdout_level DEBUG" before import for more information.
INFO: Change particles name to pass to MG5 convention
Pass the definition of 'j' and 'p' to 5 flavour scheme, including the photon.
Kept definitions of multiparticles l+ / l- / vl / vl~ unchanged
Defined multiparticle all = g ghg ghg~ u c d s b u~ c~ d~ s~ b~ a gha gha~ ve vm vt e- mu- ta- ve~ vm~ vt~ e+ mu+ ta+ t t~ z w+ ghz ghwp ghwm h g0 g+ w- ghz~ ghwp~ ghwm~ g-
MG5_aMC>define p = g d d~ u u~ s s~ c c~ b b~ a
Defined multiparticle p = g d u s c b d~ u~ s~ c~ b~ a
MG5_aMC>define j = g d d~ u u~ s s~ c c~ b b~ a
Defined multiparticle j = g d u s c b d~ u~ s~ c~ b~ a
MG5_aMC>generate p p > z h, h > a a aS=0 aEW=3 [QED]
------------------------------------------------------------------------
This computation involves NLO EW corrections.
Please also cite ref. 'arXiv:1804.10017' when using results from this code.
------------------------------------------------------------------------

change syntax aEW=3 to QED^2=6 to correspond to UFO model convention
Interpreting 'QED^2=6' as 'QED^2<=6'
change syntax aS=0 to QCD^2=0 to correspond to UFO model convention
Interpreting 'QCD^2=0' as 'QCD^2<=0'
Error detected in "generate p p > z h, h > a a aS=0 aEW=3 [QED]"
write debug file MG5_debug
If you need help with this issue please contact us on https://answers.launchpad.net/mg5amcnlo
str : Decay processes cannot be perturbed
MG5_aMC>

Thanks a lot in advance,
Fábio.

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Revision history for this message
Olivier Mattelaer (olivier-mattelaer) said : 		#1

NLO/loop syntax (so the use of []) is not compatible with the decay chain syntax (so the use of the comma)

My advice here is to use the higgs effective field theory (which approximates the loop by a tree level interaction:
import model heft
generate p p > z h, h > a a

For (onshell) Higgs decay, the HEFT model is accurate enough and would much faster (and significantly) than the syntax that you tried (if it was supported).

Cheers,

Olivier

Revision history for this message
Fabio (flalves-10) said : 		#2

Dear Olivier,
thanks a lot for the quick response.
But could you please give us some clarifications in the following if possible:

We are now using this syntax (p p > z h, h > a a aS=0 aEW=3 [QED]) because we want to calculate the KEW of ZH production mode and H->yy decay channel, the KEW is the k-factor represents the full set of NLO EW corrections.

From your kind advice it seems that we could not include the decay chain syntax together with the NLO/loop.
May I ask whether the KEW is theoretically dependent on the decay channel?

If it is not, could we just use the inclusive Higgs and don’t need to consider the H->yy decay channel?
If we still need to consider the H->yy decay channel, may I ask do you have some suggestions for us to achieve our goal? (I don’t know whether we could do the parton-shower later and only keep the H->yy decay mode?)

Thanks a lot in advance,
Fábio.

Revision history for this message
Olivier Mattelaer (olivier-mattelaer) said : 		#3

Hi,

Well I guess it depends of what you are looking for then and which precision you are targetting (and obviously what is your limit on precision from other sources (like QCD, PDF,...)

H > a a is already a loop diagram so if you want EW correction on top, this means a two loop computation which is something that is outside of our reach. Maybe some HEFT model might still be an option but I doubt such model exists for EW correction and if this would be theorically sounds since you will miss/ignore some of the loop for sure.

So I do no see how to do such computation with the Higgs decaying.

> May I ask whether the KEW is theoretically dependent on the decay channel?

It has obviously some dependence (now the question is if such dependence is higher or lower than your target accuracy).
The first question to me is wether the narrow-width approximation will still be accurate enough at that level of precision.

> f we still need to consider the H->yy decay channel, may I ask do you have some suggestions for us to achieve our goal? (I don’t know whether we could do the parton-shower later and only keep the H->yy decay mode?)

EW computation are fixed-order only so you can not run parton-shower on them.

Cheers,

Olivier

> On 10 Jan 2024, at 10:40, Fabio <email address hidden> wrote:
>
> Question #708946 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/708946
>
> Status: Answered => Open
>
> Fabio is still having a problem:
> Dear Olivier,
> thanks a lot for the quick response.
> But could you please give us some clarifications in the following if possible:
>
> We are now using this syntax (p p > z h, h > a a aS=0 aEW=3 [QED])
> because we want to calculate the KEW of ZH production mode and H->yy
> decay channel, the KEW is the k-factor represents the full set of NLO EW
> corrections.
>
> From your kind advice it seems that we could not include the decay chain syntax together with the NLO/loop.
> May I ask whether the KEW is theoretically dependent on the decay channel?
>
> If it is not, could we just use the inclusive Higgs and don’t need to consider the H->yy decay channel?
> If we still need to consider the H->yy decay channel, may I ask do you have some suggestions for us to achieve our goal? (I don’t know whether we could do the parton-shower later and only keep the H->yy decay mode?)
>
> Thanks a lot in advance,
> Fábio.
>
> --
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