q2max Photon Flux

Dear Mariano,

the formula implemented in MG for the photonflux corresponds to formula (20) in hep-ph/9310350 (just q2min_madgraph =-q2max and q2max_madgraph=-q2min). So q2min_madgraph corresponds to formula (25).
q2max_madgraph is left to the user to set as it depends on the maximum outgoing angle \theta_c of the particle that emitted the photon (electron or proton). At the LHC this is not measured, so in practice,
one normally chooses q2max of the order of a few GeV at least. The dependence is in fact quite weak and the result
don't depend on this value very much (to be checked explicitly).

Hope that helps.

Cheers

Fabio

Hi Fabio, I was also interested in the question asked by this fellow MG user, so I'll keep your answer with other MG documentation for future use. But I'm stuck at a more elementary level. I've been looking for documentation on the use of the option lpp=3 (photon from electron) in the runcard, but haven't found any. When I try the most obvious possibility, just setting, say, 3 = lpp1 in the runcard, I just get a zero result.
Could you please give me a pointer, or hint, on how to actually use this beam type? Thanks a lot in advance.

Hi John,

I did the following commands in 1.5.8 (you can write all those commands in a file and launch it via ./bin/mg5 PATH):
generate a a > e+ e-
output
launch
parton
set lpp1 3
set lpp2 3
done

and this give:
Cross-section : 15.62 +- 0.07106 pb

So this is working fine. The point is to have define a process with photon in the initial state.

Could you reproduce the above run?
What is your process?

Cheers,

Olivier

Hi Olivier, thanks for your prompt reply! The problem was that I was using electron as the initial beam, much as if the emitted photon were a parton of the electron. I can now reproduce your example. Thanks again for your help. John.

Dear Fabio,

Thank you for your answer. When you say "q2max_madgraph is left to the user to set as it depends on the maximum outgoing angle" How can set this parameter in Madgraph5?

Cheers

Mariano

Hi Mariano,

by looking at the code, I found that
q2max_madgraph = dsqrt_q2fact1 **2
or
q2max_madgraph = dsqrt_q2fact2 **2
(depending of the beam)

dsqrt_q2fact1 and dsqrt_q2fact2 are input of the run_card if you are in fix scale:
  91.1880 = dsqrt_q2fact1 ! fixed fact scale for pdf1
  91.1880 = dsqrt_q2fact2 ! fixed fact scale for pdf2
to be in fix scale, you need to set (in the run_card as well):
  T = fixed_ren_scale ! if .true. use fixed ren scale
  T = fixed_fac_scale ! if .true. use fixed fac scale

Cheers,

Olivier

Thank you very much for the reply. I have solved my problem. But I ask one last thing: How I can do to make q2max, in MG5, can depend on the energy fraction x (or y according to the expression (26) in the reference /hep-ph/93103509)

Cheers

Mariano

If you don't set the scale as fixed in the run_card (the default in fact)
then each scale (so sqrt(q2max)) will be defined in the function
SubProcesses/setscales.f

You can edit those function and put the function set_fac_scale(P,q2fact) as you like.
(you have a couple of example inside the file)

Cheers,

Olivier

Thank you very much for your answer.
I found the file setscales.f, but how I can introduce a dependency on x in q2max, particularly in q2fact (2)?
Should I put q2fact (2) = f (x) directly?
How I can refer to the same variable x that is in PhotonFlux.f?

Cheers,

Mariano

Hi Mariano,

> Should I put q2fact (2) = f (x) directly?

yes.

> How I can refer to the same variable x that is in PhotonFlux.f?

I think the easier (i.e. minimal modification of the code) is to redefine all variable that you need inside the appropriate function.
Now, you can of course define a new function, and/or a global variable.

Cheers,

Olivier

Sorry to be so insistent. You have already been able to answer, very well by the way, about modifying q2max.
This time the question is where are the values ​​of the fraction of the photon energy, denoted by x, associated with the PDF and that is in PhotonFlux.f, where the function is defined epa_electron (x, q2max).
Using the command COMMON, I tried to put this common x variable between PhotonFlux.f and setscales.f

Comparing MG5 of a routine that performs the same calculation with all cuts and already confirmed previously, I can not get the same results. To be more precise, if I put x as a common variable and define, in setscales.f, q2max = cx, where c is a known number, everything works great, but if I put q2max = c (1-x) begin to appear all kinds of errors .

Then I have some specific questions:

How I can read or pooling the variable x?

How do the same but with the energy of the beam 1 or 2 (variable found in the input of the name run_card.dat ebeam1 or ebeam2)?

How I can define x as a global variable? Or What is the correct command syntax COMMON?

I thank you for your time and dedication in each of its response

Mariano

Hi Mariano,

> To be more precise, if I put x as a common variable and define, in
> setscales.f, q2max = cx, where c is a known number, everything works
> great, but if I put q2max = c (1-x) begin to appear all kinds of errors

The only reason why the second can cause trouble and not the first one is that
q2max is going to be negative or something like that. Did x is between 0 and 1?

> How I can read or pooling the variable x?

> How do the same but with the energy of the beam 1 or 2 (variable found
> in the input of the name run_card.dat ebeam1 or ebeam2)?

In fact they are already included since all those variables are parts of
run.inc which is included.

The associate variable name:
ebeam energy of the beam
xbk Bjorken fraction

Cheers,

Olivier

On Apr 5, 2013, at 3:11 PM, Mariano Guillermo Caruso <email address hidden> wrote:

> Question #224904 on MadGraph5 changed:
> https://answers.launchpad.net/madgraph5/+question/224904
>
> Status: Answered => Open
>
> Mariano Guillermo Caruso is still having a problem:
> Sorry to be so insistent. You have already been able to answer, very well by the way, about modifying q2max.
> This time the question is where are the values ​​of the fraction of the photon energy, denoted by x, associated with the PDF and that is in PhotonFlux.f, where the function is defined epa_electron (x, q2max).
> Using the command COMMON, I tried to put this common x variable between PhotonFlux.f and setscales.f
>
> Comparing MG5 of a routine that performs the same calculation with all
> cuts and already confirmed previously, I can not get the same results.
> To be more precise, if I put x as a common variable and define, in
> setscales.f, q2max = cx, where c is a known number, everything works
> great, but if I put q2max = c (1-x) begin to appear all kinds of errors
> .
>
> Then I have some specific questions:
>
> How I can read or pooling the variable x?
>
> How do the same but with the energy of the beam 1 or 2 (variable found
> in the input of the name run_card.dat ebeam1 or ebeam2)?
>
> How I can define x as a global variable? Or What is the correct command
> syntax COMMON?
>
>
> I thank you for your time and dedication in each of its response
>
> Mariano
>
> --
> You received this question notification because you are a member of
> MadTeam, which is an answer contact for MadGraph5.

Thank you for your prompt response
But I do not understand how to use the eBeam variables x (fraction of the photon energy, and equal to xbk?)

If you would like to use these variables to define q2fact(2) in setscales.f
Should I put, for example, q2fact(2) = x * ebeam?

Do I have to define x and ebeam within setscales.f?

How do I do this definition?

Something I have not understood is that, as I've mentioned, these variables are included in the subroutine set_ren_scale (P, rscale) (within setscales.f) through the include command 'run.inc'.
It Is also included in the subroutine set_fac_scale (P, q2fact), despite not being present there the include command 'run.inc'?

Regarding the sign of q2max, it is advisable to restrict the values ​​of x so that 0 <x <1?

What if so how do you recommend that restriction?

I'm learning a lot with all your answers, thank you very much.

Mariano

Thanks FabioMaltoni, that solved my question.

Dear authors,

concerning this question I would like to get the values of the maximum outgoing angle \theta_c of the particle that emitted the photon (electron or proton)

Where could I found it in Madgraph?

Thank you, Patricia.

\theta_c is actually an input in the madgraph comptuation.
to sum-up this thread:

1) the formula implemented in MG for the photonflux corresponds to formula (20) in hep-ph/9310350 (just q2min_madgraph =-q2max and q2max_madgraph=-q2min). So q2min_madgraph corresponds to formula (25).
q2max_madgraph is left to the user to set as it depends on the maximum outgoing angle \theta_c of the particle that emitted the photon (electron or proton).

2) in madgraph:
q2max_madgraph = dsqrt_q2fact1 **2
q2max_madgraph = dsqrt_q2fact2 **2

3) so to choose the value of those parameter, you need to modify those two line in the run_card:
  91.1880 = dsqrt_q2fact1 ! fixed fact scale for pdf1
  91.1880 = dsqrt_q2fact2 ! fixed fact scale for pdf2
and to pass in fix scale: (in the run_card as well):
  T = fixed_ren_scale ! if .true. use fixed ren scale
  T = fixed_fac_scale ! if .true. use fixed fac scale

Cheers,

Olivier

Hi Olivier,

I have read hep-ph/9310350 but the implemented formulas at Madgraph (at Photonflux.f) does not consider the theta_c explicitly.

I would like to list theta_c values according my choice of q2max.

So, I would like to know where it was defined. Would you have this information?

Cheers Patricia

Hi Patricia,

I think equation 30 of the paper is what you are looking for.

Cheers,

Olivier
On May 13, 2014, at 3:21 PM, Patricia Rebello Teles <email address hidden> wrote:

> Question #224904 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/224904
>
> Patricia Rebello Teles posted a new comment:
> Hi Olivier,
>
> I have read hep-ph/9310350 but the implemented formulas at Madgraph (at
> Photonflux.f) does not consider the theta_c explicitly.
>
> I would like to list theta_c values according my choice of q2max.
>
> So, I would like to know where it was defined. Would you have this
> information?
>
> Cheers Patricia
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Hi Olivier

in fact I am looking for the second term of Eq. 26 because in PhotonFlux.f I see just

q2min= xin*xin*x*x/(1-x)

You see, the theta_c does not appear as an input in anywhere.

Does Madgraph have Eq 30 implemented somewhere?

Cheers Patricia

Hi Patricia,

Madgraph neither use and/or compute the value of theta_c.
So no Eq(30) is not part of the code.

Cheers,

Olivier

On May 13, 2014, at 4:02 PM, Patricia Rebello Teles <email address hidden> wrote:

> Question #224904 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/224904
>
> Patricia Rebello Teles posted a new comment:
> Hi Olivier
>
> in fact I am looking for the second term of Eq. 26 because in
> PhotonFlux.f I see just
>
> q2min= xin*xin*x*x/(1-x)
>
> You see, the theta_c does not appear as an input in anywhere.
>
> Does Madgraph have Eq 30 implemented somewhere?
>
> Cheers Patricia
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Hi Olivier, I guess it would not be hard to insert the second term of Eq. 26 and then solve it for theta_c.

Just let me know, if possible, how should I write "E" according Madgraph syntax since my process is gamma gamma to h using PDF=3

According the paper, the four-momentum p(E,0,0,E*beta) belongs to the scattered electron.

You see, I would need this information and also S_min, which is the minimum
value for the invariant mass squared of the produced hadronic system, to build Eq 30 as well.

Cheers Patricia.

Dear Patricia,

Why do you want to compute this value inside the MadGraph file?
Would not be easier to compute this value on a piece of paper?
I don’t see the point to struggle with MadGraph convention in this case.

Cheers,

Olivier
On May 13, 2014, at 5:06 PM, Patricia Rebello Teles <email address hidden> wrote:

> Question #224904 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/224904
>
> Patricia Rebello Teles posted a new comment:
> Hi Olivier, I guess it would not be hard to insert the second term of
> Eq. 26 and then solve it for theta_c.
>
> Just let me know, if possible, how should I write "E" according Madgraph
> syntax since my process is gamma gamma to h using PDF=3
>
> According the paper, the four-momentum p(E,0,0,E*beta) belongs to the
> scattered electron.
>
> You see, I would need this information and also S_min, which is the minimum
> value for the invariant mass squared of the produced hadronic system, to build Eq 30 as well.
>
> Cheers Patricia.
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Hi Olivier,

I have adopted not fixed q2max_madgraph to perform an experimental analysis.

I would like to scan the theta_c since, depending on its value, as well as the electron energy, because these values can show whether the electron is tagged or not.

For LHC this information is not important but for e+e- colliders it is completely the opposite, even the theta_c and energy being very small. So, it is important to recover the information of the electrons.

Cheers Patricia.

Dear Patricia,

Ok. But I can not really help you since I’m not an expert of that type of physics neither that I wrote that part of the code.
So I’m actually as novice as you on that matter.
Now equation 13 is for the initial electron.
In the case of MG , beta=1 since the mass of the electron is neglected (at least it is by default)
so E is simply the beam energy and is fixed.
The value is directly given by the run_card.

Cheers,

Olivier

On May 14, 2014, at 5:02 PM, Patricia Rebello Teles <email address hidden> wrote:

> Question #224904 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/224904
>
> Patricia Rebello Teles posted a new comment:
> Hi Olivier,
>
> I have adopted not fixed q2max_madgraph to perform an experimental
> analysis.
>
> I would like to scan the theta_c since, depending on its value, as well
> as the electron energy, because these values can show whether the
> electron is tagged or not.
>
> For LHC this information is not important but for e+e- colliders it is
> completely the opposite, even the theta_c and energy being very small.
> So, it is important to recover the information of the electrons.
>
> Cheers Patricia.
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.