# alphas is not applied

In the Mg5_aNC, I am trying to generate b b~ > t t~ or g b > t w-.

By the way, even though I change the value of alpha_s in the param card, total cross section is not changed.

what's wrong?? alpha_s is connected with QCD coupling.

In the case of g b > t w-, the result give samely 27.8, even though I set alpha_s=11.8....

I don't know why alpha_s is not applied to MG5 simulation...

please help!!!

## Question information

- Language:
- English Edit question

- Status:
- Solved

- Assignee:
- No assignee Edit question

- Solved by:
- Olivier Mattelaer

- Solved:
- 2019-09-24

- Last query:
- 2019-09-24

- Last reply:
- 2019-09-24

Hi,

This is expected if you use a PDF for the proton.

Since the PDF set is fit for a given value of a_s(MZ) it is important that you use the same value for your computation. Consequently, we automatically use the value of a_S(MZ) to the one fitted by the PDF set when such PDF are used.

Cheers,

Olivier

> On 23 Sep 2019, at 12:53, Jae Hoon Jeong <email address hidden> wrote:

>

> Question #684139 on MadGraph5_aMC@NLO changed:

> https:/

>

> Description changed to:

> In the Mg5_aNC, I am trying to generate b b~ > t t~ or g b > t w-.

> By the way, even though I change the value of alpha_s in the param card, total cross section is not changed.

> what's wrong?? alpha_s is connected with QCD coupling.

> In the case of g b > t w-, the result give samely 27.8, even though I set alpha_s=11.8....

> I don't know why alpha_s is not applied to MG5 simulation...

> please help!!!

>

> --

> You received this question notification because you are an answer

> contact for MadGraph5_aMC@NLO.

Thank you so much!

By the way, I now use 230000 PDF. In the info file, alpha_s is 0.119002 and top mass is 175.

In my case (g b > t w-), I want to match analytic result and simulation result. Now my analytic result is about 180 pb.

Now I didn't consider color factor in analytic computation. Then, 27.8/180 is almost 2/13. this value doesn't look from color factor.

Then, should I use top quark mass as 175 GeV?

Additionally, I set fixed factorization scale as 173GeV in MG5_aMC.

And In order to get the result of analytic form, I use Maneparse.

What do you think is the problem?

For me it seems that you have made a mistake somewhere.

Cheers,

Olivier

Dear Olivier

Finally, I get same cross section!!! If I consider color factor and average, 1/6 x 180 =30 pb.

but, Mg5 result was 27.8. So I thought there is a difference of SU(3) coupling constant alphas between Mg5 and my computation.

My set was like this.

lhapdf = pdlabel ! PDF set

230000 = lhaid ! if pdlabel=lhapdf, this is the lhapdf number

# To see heavy ion options: type "update ion_pdf"

#******

# Renormalization and factorization scales *

#******

true = fixed_ren_scale ! if .true. use fixed ren scale

true = fixed_fac_scale ! if .true. use fixed fac scale

173 = scale ! fixed ren scale

173 = dsqrt_q2fact1 ! fixed fact scale for pdf1

173 = dsqrt_q2fact2 ! fixed fact scale for pdf2

-1 = dynamical_

1.0 = scalefact ! scale factor for event-by-event scales

I guessed renormalization scale affect to alphas(M). And I guess M = renormalization scale.

When I change as

lhapdf = pdlabel ! PDF set

230000 = lhaid ! if pdlabel=lhapdf, this is the lhapdf number

# To see heavy ion options: type "update ion_pdf"

#******

# Renormalization and factorization scales *

#******

true = fixed_ren_scale ! if .true. use fixed ren scale

true = fixed_fac_scale ! if .true. use fixed fac scale

91.188 = scale ! fixed ren scale

173 = dsqrt_q2fact1 ! fixed fact scale for pdf1

173 = dsqrt_q2fact2 ! fixed fact scale for pdf2

-1 = dynamical_

1.0 = scalefact ! scale factor for event-by-event scales

and I put the value of alphas as 0.119, I finally same value from simulation and my computation both as 28.13 !!!!!

And Using the relation of alphas from Max Lautsch's "Running Couplings in the Standard Model and their

Implications to possible Physics beyond the Standard

Model", I change alphas(173)=0.109. Then, samely I get the 27.8 from computation and simulation!!

So, My final question is that the option fixed ren scale means alphas(

Is it correct?

Since I don't know how I remove my last question. I write down again revised writing.

Dear Olivier

Finally, I get same cross section!!! If I consider color factor and average, 1/6 x 180 =30 pb.

but, Mg5 result was 27.8. So I thought there is a difference of SU(3) coupling constant alphas between Mg5 and my computation.

My set was like this.

lhapdf = pdlabel ! PDF set

230000 = lhaid ! if pdlabel=lhapdf, this is the lhapdf number

# To see heavy ion options: type "update ion_pdf"

#******

# Renormalization and factorization scales *

#******

true = fixed_ren_scale ! if .true. use fixed ren scale

true = fixed_fac_scale ! if .true. use fixed fac scale

173 = scale ! fixed ren scale

173 = dsqrt_q2fact1 ! fixed fact scale for pdf1

173 = dsqrt_q2fact2 ! fixed fact scale for pdf2

-1 = dynamical_

1.0 = scalefact ! scale factor for event-by-event scales

I guessed renormalization scale affect to alphas(M). And I guess M = renormalization scale.

When I change as

lhapdf = pdlabel ! PDF set

230000 = lhaid ! if pdlabel=lhapdf, this is the lhapdf number

# To see heavy ion options: type "update ion_pdf"

#******

# Renormalization and factorization scales *

#******

true = fixed_ren_scale ! if .true. use fixed ren scale

true = fixed_fac_scale ! if .true. use fixed fac scale

91.188 = scale ! fixed ren scale

173 = dsqrt_q2fact1 ! fixed fact scale for pdf1

173 = dsqrt_q2fact2 ! fixed fact scale for pdf2

-1 = dynamical_

1.0 = scalefact ! scale factor for event-by-event scales

and I put the value of alphas as 0.119 to my computation, I finally same value from simulation and my computation both as 30.5 !!!!!

And Using the relation of alphas from Max Lautsch's "Running Couplings in the Standard Model and their

Implications to possible Physics beyond the Standard

Model", I change alphas(173)=0.109. Then, samely I get the 27.8 from computation and simulation!!

So, My final question is that the option fixed ren scale means alphas(

Is it correct?

Yes it is

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_______

From: <email address hidden> <email address hidden> on behalf of Jae Hoon Jeong <email address hidden>

Sent: Tuesday, September 24, 2019 4:57:33 AM

To: Olivier Mattelaer <email address hidden>

Subject: Re: [Question #684139]: alphas is not applied

Question #684139 on MadGraph5_aMC@NLO changed:

https:/

Jae Hoon Jeong posted a new comment:

Since I don't know how I remove my last question. I write down again

revised writing.

Dear Olivier

Finally, I get same cross section!!! If I consider color factor and

average, 1/6 x 180 =30 pb.

but, Mg5 result was 27.8. So I thought there is a difference of SU(3)

coupling constant alphas between Mg5 and my computation.

My set was like this.

lhapdf = pdlabel ! PDF set

230000 = lhaid ! if pdlabel=lhapdf, this is the lhapdf number

# To see heavy ion options: type "update ion_pdf"

#******

# Renormalization and factorization scales *

#******

true = fixed_ren_scale ! if .true. use fixed ren scale

true = fixed_fac_scale ! if .true. use fixed fac scale

173 = scale ! fixed ren scale

173 = dsqrt_q2fact1 ! fixed fact scale for pdf1

173 = dsqrt_q2fact2 ! fixed fact scale for pdf2

-1 = dynamical_

1.0 = scalefact ! scale factor for event-by-event scales

I guessed renormalization scale affect to alphas(M). And I guess M = renormalization scale.

When I change as

lhapdf = pdlabel ! PDF set

230000 = lhaid ! if pdlabel=lhapdf, this is the lhapdf number

# To see heavy ion options: type "update ion_pdf"

#******

# Renormalization and factorization scales *

#******

true = fixed_ren_scale ! if .true. use fixed ren scale

true = fixed_fac_scale ! if .true. use fixed fac scale

91.188 = scale ! fixed ren scale

173 = dsqrt_q2fact1 ! fixed fact scale for pdf1

173 = dsqrt_q2fact2 ! fixed fact scale for pdf2

-1 = dynamical_

1.0 = scalefact ! scale factor for event-by-event scales

and I put the value of alphas as 0.119 to my computation, I finally same

value from simulation and my computation both as 30.5 !!!!!

And Using the relation of alphas from Max Lautsch's "Running Couplings in the Standard Model and their

Implications to possible Physics beyond the Standard

Model", I change alphas(173)=0.109. Then, samely I get the 27.8 from computation and simulation!!

So, My final question is that the option fixed ren scale means

alphas(

Is it correct?

--

You received this question notification because you are an answer

contact for MadGraph5_aMC@NLO.

Thanks Olivier Mattelaer, that solved my question.