MadGraph5_aMC@NLO

gamma fusion at e+e-

Asked by Roberto Franceschini

I am succesfully generating non-zero cross-sections for a a > f f~ at e+ e- colliders through

     3 = lpp1 ! beam 1 type
     3 = lpp2 ! beam 2 type

However if I try a resonant production a a > resonance the cross-section is always zero.

The resonance is very light compared to the CoM energy, so the only thing I can imagine is the particle being too narrow, but I would have said with the sintax 2>1 above the width should not even be called in the calculation.

1) A massive s-channel particle has a width set to zero. (it is light enough)
   2) The pdf are zero for at least one of the initial state particles
      or you are using maxjetflavor=4 for initial state b:s. (unclear if this applies for 3 = lpp1)
   3) The cuts are too strong. (no cuts on the resonance)
   Please check/correct your param_card and/or your run_card.

Am I asking something MG is not supposed to compute?

Question information

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

Could you send me the banner of the run?

Cheers,

Olivier

Revision history for this message
Roberto Franceschini (franceschini-roberto) said : 		#2

Sure! Thanks for looking into it!

<LesHouchesEvents version="3.0">
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# MadGraph5_aMC@NLO *
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# Going Beyond *
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set group_subprocesses Auto
set ignore_six_quark_processes False
set loop_optimized_output True
set loop_color_flows False
set gauge unitary
set complex_mass_scheme False
set max_npoint_for_channel 0
import model sm
define p = g u c d s u~ c~ d~ s~
define j = g u c d s u~ c~ d~ s~
define l+ = e+ mu+
define l- = e- mu-
define vl = ve vm vt
define vl~ = ve~ vm~ vt~
import model axion_model
generate a a > r0 /z
output Belle2/ee_aa2ALP
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Revision history for this message
Olivier Mattelaer (olivier-mattelaer) said : 		#3

Ah you are using the default dynamical scale which includes a cut-off when such scale is too small.
Additionally for photon from electron/proton the dynamical scale choice is use as the cutoff of the effective Photon approximation.
So I do not think that it makes sense to use the default dynamical scale in that case.

In 2.7.1, we have actually changed the behavior to force you to run in fixed factorization scale for your process.
(and we also force you to not use MZ for the cut-off scale)

Cheers,

Olivier

Revision history for this message
Roberto Franceschini (franceschini-roberto) said : 		#4

Hi Olivier, thanks for this precious input and sorry for the delay in checking it. It took some time to sit down and make all the necessary tests.

In 2.6.0 setting the scale to fixed did not help, however I moved to 2.7.2 and I can see that for fixed scale the cross-section is computed. I do not know if that is what you meant, but the default run_card.dat I got for this process had dynamical scale. Following you advice I set it to fixed and changed the factorization scale to something different from 91 GeV that appears in the default. In this way the cross-section is computed.

I have tried to experiment a bit with the scale and I have seen the following two, maybe three, interesting things:

 - the `scale` setting does not affect the cross-section
- sometimes the code complains that the `dsqrt_q2fact1` or `dsqrt_q2fact2` are too large
- `scalefact` does not seem to have an effect at all on the cross-section

All behaviors (may) make sense, but I'd like to know better the specifics.

- `scale` should be the renormalization scale, so, even for a pure QED process I would have said that I had to see some effect, small, but not zero. Instead I see two identical (all digits!) numbers such as 0.002257 ± 2.5e-07. Should I not count on any running of alpha_EM in MadGraph? Maybe there is another reason why my cross-section does not change with `scale` (e.g. it contains a pure BSM coupling)

- The `dsqrt_q2fact1` seems to give errors every time it is larger than 4 GeV. The error is
         "Command "generate_events -f" interrupted with error:
         InvalidRunCard : Photon from electron are using fixed scale value of muf [dsqrt_q2fact1] as the cut off value of the approximation.
          For EPA this number should be small (for HERA prediction it should be 2 at most)"

Is there some specific manual page for this? I would have said that the factorization scale could be taken larger than 4 GeV, e.g. if I am doing a photon fusion process for a heavy object (not HERA). Is this a technical limitation or am I misreading the physics?

- `scalefact` is probably ignored by the code when run for fixed scale choices, is that the case?

Thanks a lot for helping out!
Roberto

Revision history for this message
Olivier Mattelaer (olivier-mattelaer) said : 		#5

Hi,

> - `scale` should be the renormalization scale, so, even for a pure QED
> process I would have said that I had to see some effect, small, but not
> zero. Instead I see two identical (all digits!) numbers such as
> 0.002257 ± 2.5e-07. Should I not count on any running of alpha_EM in
> MadGraph? Maybe there is another reason why my cross-section does not
> change with `scale` (e.g. it contains a pure BSM coupling)

Indeed, MG5aMC does not have the running of the weak coupling implemented.

> Is there some specific manual page for this? I would have said that the
> factorization scale could be taken larger than 4 GeV, e.g. if I am doing
> a photon fusion process for a heavy object (not HERA). Is this a
> technical limitation or am I misreading the physics?

This is not really a factorization scale, this is a cutoff for the "lost" electron.
I'm not an expert on that, but the claim of all the people that I have asked is that larger value does not make any sense.
At the same time, it should be very easy to change the limit in the check of the cards if you think that this is really valid. If so please could you please details more such that I can start to arguee for changing that limit.

Now my understanding of that functionality is that you have a log of the type (Qmax/me)
and therefore you do not want to have that log to be too large otherwsie you will screw up the approximation)

> - `scalefact` is probably ignored by the code when run for fixed scale
> choices, is that the case?

Which is fine since you have to use a fixed_scale value in 2.7.2
(so they are no interest in using scalefact here)

Cheers,

Olivier

> On 8 Apr 2020, at 13:37, Roberto Franceschini <email address hidden> wrote:
>
> Question #689175 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/689175
>
> Status: Answered => Open
>
> Roberto Franceschini is still having a problem:
> Hi Olivier, thanks for this precious input and sorry for the delay in
> checking it. It took some time to sit down and make all the necessary
> tests.
>
> In 2.6.0 setting the scale to fixed did not help, however I moved to
> 2.7.2 and I can see that for fixed scale the cross-section is computed.
> I do not know if that is what you meant, but the default run_card.dat I
> got for this process had dynamical scale. Following you advice I set it
> to fixed and changed the factorization scale to something different from
> 91 GeV that appears in the default. In this way the cross-section is
> computed.
>
> I have tried to experiment a bit with the scale and I have seen the
> following two, maybe three, interesting things:
>
> - the `scale` setting does not affect the cross-section
> - sometimes the code complains that the `dsqrt_q2fact1` or `dsqrt_q2fact2` are too large
> - `scalefact` does not seem to have an effect at all on the cross-section
>
> All behaviors (may) make sense, but I'd like to know better the
> specifics.
>
> - `scale` should be the renormalization scale, so, even for a pure QED
> process I would have said that I had to see some effect, small, but not
> zero. Instead I see two identical (all digits!) numbers such as
> 0.002257 ± 2.5e-07. Should I not count on any running of alpha_EM in
> MadGraph? Maybe there is another reason why my cross-section does not
> change with `scale` (e.g. it contains a pure BSM coupling)
>
> - The `dsqrt_q2fact1` seems to give errors every time it is larger than 4 GeV. The error is
> "Command "generate_events -f" interrupted with error:
> InvalidRunCard : Photon from electron are using fixed scale value of muf [dsqrt_q2fact1] as the cut off value of the approximation.
> For EPA this number should be small (for HERA prediction it should be 2 at most)"
>
> Is there some specific manual page for this? I would have said that the
> factorization scale could be taken larger than 4 GeV, e.g. if I am doing
> a photon fusion process for a heavy object (not HERA). Is this a
> technical limitation or am I misreading the physics?
>
> - `scalefact` is probably ignored by the code when run for fixed scale
> choices, is that the case?
>
> Thanks a lot for helping out!
> Roberto
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Revision history for this message
Roberto Franceschini (franceschini-roberto) said : 		#6

Hi there! Thanks for this input. I think the only cumbersome part is
related to this factorization scale. It would be nice to have it
documented. Is there any detailed documentation?

I think you see my point that a factorization scale, as we do in LHC cases,
may need to be take large because the process involves some large scales.
Of course we do evolve PDFs to this scale for each process, but this is a
consequence of how we organize our computations. For the same reason I
think an EPA cross-section may contain other logs where the IR cutoff is
not m_e but is is process dependent.

I think this is best discussed over some physics manual for the implemented
EPA. Is there such thing?

Thanks,
Roberto

> This is not really a factorization scale, this is a cutoff for the "lost"
electron.
> I'm not an expert on that, but the claim of all the people that I have
asked is that larger value does not make any sense.
> At the same time, it should be very easy to change the limit in the check
of the cards if you think that this is really valid. If so please could you
please details more such that I can start to arguee for changing > that
limit.
> Now my understanding of that functionality is that you have a log of the
type (Qmax/me)
> and therefore you do not want to have that log to be too large otherwsie
you will screw up the approximation)

On Wed, Apr 8, 2020 at 8:57 PM Olivier Mattelaer <
<email address hidden>> wrote:

> Your question #689175 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/689175
>
> Status: Open => Answered
>
> Olivier Mattelaer proposed the following answer:
> Hi,
>
> > - `scale` should be the renormalization scale, so, even for a pure QED
> > process I would have said that I had to see some effect, small, but not
> > zero. Instead I see two identical (all digits!) numbers such as
> > 0.002257 ± 2.5e-07. Should I not count on any running of alpha_EM in
> > MadGraph? Maybe there is another reason why my cross-section does not
> > change with `scale` (e.g. it contains a pure BSM coupling)
>
> Indeed, MG5aMC does not have the running of the weak coupling
> implemented.
>
> > Is there some specific manual page for this? I would have said that the
> > factorization scale could be taken larger than 4 GeV, e.g. if I am doing
> > a photon fusion process for a heavy object (not HERA). Is this a
> > technical limitation or am I misreading the physics?
>
>
> This is not really a factorization scale, this is a cutoff for the "lost"
> electron.
> I'm not an expert on that, but the claim of all the people that I have
> asked is that larger value does not make any sense.
> At the same time, it should be very easy to change the limit in the check
> of the cards if you think that this is really valid. If so please could you
> please details more such that I can start to arguee for changing that limit.
>
> Now my understanding of that functionality is that you have a log of the
> type (Qmax/me)
> and therefore you do not want to have that log to be too large otherwsie
> you will screw up the approximation)
>
> > - `scalefact` is probably ignored by the code when run for fixed scale
> > choices, is that the case?
>
> Which is fine since you have to use a fixed_scale value in 2.7.2
> (so they are no interest in using scalefact here)
>
> Cheers,
>
> Olivier
>
>
> > On 8 Apr 2020, at 13:37, Roberto Franceschini <
> <email address hidden>> wrote:
> >
> > Question #689175 on MadGraph5_aMC@NLO changed:
> > https://answers.launchpad.net/mg5amcnlo/+question/689175
> >
> > Status: Answered => Open
> >
> > Roberto Franceschini is still having a problem:
> > Hi Olivier, thanks for this precious input and sorry for the delay in
> > checking it. It took some time to sit down and make all the necessary
> > tests.
> >
> > In 2.6.0 setting the scale to fixed did not help, however I moved to
> > 2.7.2 and I can see that for fixed scale the cross-section is computed.
> > I do not know if that is what you meant, but the default run_card.dat I
> > got for this process had dynamical scale. Following you advice I set it
> > to fixed and changed the factorization scale to something different from
> > 91 GeV that appears in the default. In this way the cross-section is
> > computed.
> >
> > I have tried to experiment a bit with the scale and I have seen the
> > following two, maybe three, interesting things:
> >
> > - the `scale` setting does not affect the cross-section
> > - sometimes the code complains that the `dsqrt_q2fact1` or
> `dsqrt_q2fact2` are too large
> > - `scalefact` does not seem to have an effect at all on the
> cross-section
> >
> > All behaviors (may) make sense, but I'd like to know better the
> > specifics.
> >
> > - `scale` should be the renormalization scale, so, even for a pure QED
> > process I would have said that I had to see some effect, small, but not
> > zero. Instead I see two identical (all digits!) numbers such as
> > 0.002257 ± 2.5e-07. Should I not count on any running of alpha_EM in
> > MadGraph? Maybe there is another reason why my cross-section does not
> > change with `scale` (e.g. it contains a pure BSM coupling)
> >
> > - The `dsqrt_q2fact1` seems to give errors every time it is larger than
> 4 GeV. The error is
> > "Command "generate_events -f" interrupted with error:
> > InvalidRunCard : Photon from electron are using fixed scale
> value of muf [dsqrt_q2fact1] as the cut off value of the approximation.
> > For EPA this number should be small (for HERA prediction it
> should be 2 at most)"
> >
> > Is there some specific manual page for this? I would have said that the
> > factorization scale could be taken larger than 4 GeV, e.g. if I am doing
> > a photon fusion process for a heavy object (not HERA). Is this a
> > technical limitation or am I misreading the physics?
> >
> > - `scalefact` is probably ignored by the code when run for fixed scale
> > choices, is that the case?
> >
> > Thanks a lot for helping out!
> > Roberto
> >
> > --
> > You received this question notification because you are an answer
> > contact for MadGraph5_aMC@NLO.
>
> --
> If this answers your question, please go to the following page to let us
> know that it is solved:
>
> https://answers.launchpad.net/mg5amcnlo/+question/689175/+confirm?answer_id=4
>
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> following page to enter your feedback:
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>
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>

Revision history for this message
Launchpad Janitor (janitor) said : 		#7

This question was expired because it remained in the 'Open' state without activity for the last 15 days.