set the value of one parameter to be pi/4

Hi,

> In the script, I set theta to be pi/4 and 0.78539816,

Here I'm confused about the word "script".
Do you mean that you hardcoded the value in FeynRules?

> I don't know whether it is the problem of pi or the "/" symbol. Could you give me a hand?

> By the way, can we set the value of one parameter to be other numbers such as sqrt{2} or ln(3)?

This also makes me think that you are not speaking of MG5aMC.
We do not have any symbolic way to set a parameter to pi (at least I'm not aware of those).

Cheers,

Olivier

> On 2 Jun 2020, at 17:45, Shi-Ping He <email address hidden> wrote:
>
> New question #691093 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/691093
>
> Dear author,
>
> I have generated a UFO file from the FeynRules. One of the new external parameters is theta, which is the mixing angle between the t and T quarks. In the script, I set theta to be pi/4 and 0.78539816, respectively. To my surprise, I get two different cross section values after running the script and the difference seems not to be caused by the numerical precision. Are there any subtleties I neglected? I don't know whether it is the problem of pi or the "/" symbol. Could you give me a hand?
>
> By the way, can we set the value of one parameter to be other numbers such as sqrt{2} or ln(3)?
>
> Best,
> Shi-Ping He
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Hi,

Sorry, it seems my statement is some kind of misleading.
The script is a MadGraph program, which is coded as follows:
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
define p = p b b~
import model FCNI
generate p p > H H [QCD] NP<=99 QED <=99
output test

launch test
  set MB 0
  set MTp 400
  set theta pi/4
  set ebeam1 7000
  set ebeam2 7000
  set pdlabel lhapdf
  set lhaid 21000
  set dynamical_scale_choice 3

launch test
  set MB 0
  set MTp 400
  set theta 0.78539816
  set ebeam1 7000
  set ebeam2 7000
  set pdlabel lhapdf
  set lhaid 21000
  set dynamical_scale_choice 3
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
In the .fr file, the initial value of theta is set to be zero.
But here we vary the mixing angle theta and run the program by the command "./mg5_aMC script". The script can run well, but we get two different cross section values.
Then I try this in a command line way: "cd bin" -> "./mg5_aMC" -> "import model FCNI" -> "generate p p > H H [QCD] NP<=99 QED <=99" -> "output 0" -> "launch 0" -> press the "Enter" key > type "1" and change the value of theta to be pi/4. But we get an error message "NameError : name 'pi' is not defined".

Best,
Shi-Ping He

Thanks! That helps me.

Thanks Olivier Mattelaer, that solved my question.