# set the value of one parameter to be pi/4

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

## Question information

- Language:
- English Edit question

- Status:
- Solved

- Assignee:
- No assignee Edit question

- Solved by:
- Olivier Mattelaer

- Solved:
- 2020-06-03

- Last query:
- 2020-06-03

- Last reply:
- 2020-06-03

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:/

>

> 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.

Shi-Ping He (shi-pinghe) said : | #2 |

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_

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_

%%%%%%%

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

The syntax

> set theta pi/4

is not supported in our code.

When I tried, I have the following warning/crash:

> pi/4 can not be mapped to a float

if you do see such warning or another ones, then it means that you still have 0 for that parameter in the first run. You can check by checking the value for that parameter in the banner of the run or in the header of the lhef file.

Cheers,

Olivier

> On 3 Jun 2020, at 05:01, Shi-Ping He <email address hidden> wrote:

>

> Question #691093 on MadGraph5_aMC@NLO changed:

> https:/

>

> Status: Answered => Open

>

> Shi-Ping He is still having a problem:

> 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_

>

> 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_

> %%%%%%%

> 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

>

> --

> You received this question notification because you are an answer

> contact for MadGraph5_aMC@NLO.

Shi-Ping He (shi-pinghe) said : | #4 |

Thanks! That helps me.

Shi-Ping He (shi-pinghe) said : | #5 |

Thanks Olivier Mattelaer, that solved my question.