# Value of W mass used by MadGraph

Hello,

I have a couple questions regarding the parameters appearing in the param_card.dat.

First of all, I'm unclear about the value of the W mass appearing in the card and the one that is actually used by MadGraph. What I understand from the comments in the param card and from FAQ #2885 is that the number appearing here is not the value used by MadGraph. However when I generate p p > w+ w- events and check the W masses I find that they're all produced with the mass appearing in the param card. I am therefore confused what is meant then by FAQ #2885 "We never use the input of those [dependent] parameter for the computation of the matrix-element/cross-section/generation of events."

Second, I understand that the value of the W mass appearing in the card is supposedly calculated by the formula appearing next to the value. What I have found is that given the values of aEWM1, Gf, and MZ in the card, using the formula for the W mass doesn't give the value in the card. Is there anything else to the value in the card? For example, from the default MadGraph SM, I have the following values in the card:

Block mass
5 4.700000e+00 # MB
6 1.730000e+02 # MT
15 1.777000e+00 # MTA
23 9.118800e+01 # MZ
25 1.250000e+02 # MH
## Dependent parameters, given by model restrictions.
## Those values should be edited following the
## analytical expression. MG5 ignores those values
## but they are important for interfacing the output of MG5
## to external program such as Pythia.
1 0.000000 # d : 0.0
2 0.000000 # u : 0.0
3 0.000000 # s : 0.0
4 0.000000 # c : 0.0
11 0.000000 # e- : 0.0
12 0.000000 # ve : 0.0
13 0.000000 # mu- : 0.0
14 0.000000 # vm : 0.0
16 0.000000 # vt : 0.0
21 0.000000 # g : 0.0
22 0.000000 # a : 0.0
24 80.419002 # w+ : cmath.sqrt(MZ__exp__2/2. + cmath.sqrt(MZ__exp__4/4. - (aEW*cmath.pi*MZ__exp__2)/(Gf*sqrt__2)))

###################################
## INFORMATION FOR SMINPUTS
###################################
Block sminputs
1 1.325070e+02 # aEWM1
2 1.166390e-05 # Gf
3 1.180000e-01 # aS

but using these values I calculate m_W = 84.5116 using the formula given.

Finally, I made a model that is the default FeynRules SM with an added scalar particle. In the .fr file the value for aEWM1 specified is 137, but after generating the UFO file and checking the parameters.py file I notice the value has changed to 127. Ultimately this isn't a huge problem because if necessary I can just change the value in the parameters.py file, but I'm wondering if this behavior is done on purpose or if it's something I am doing when generating the UFO file that should be corrected?

Jake

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2017-12-06
2017-12-06
 Olivier Mattelaer (olivier-mattelaer) said on 2017-12-06: #1

Hi,

> Second, I understand that the value of the W mass appearing in the card is supposedly calculated by the formula appearing next to the value. What I have found is that given the values of aEWM1, Gf, and MZ in the card, using the formula for the W mass doesn't give the value in the card. Is there anything else to the value in the card? For example, from the default MadGraph SM, I have the following values in the card:

I have rewritten the expression such that it can be evaluated in a python script

aew=(1.325070e+02 )**-1
Gf=1.166390e-05
MW=cmath.sqrt(9.118800e+01**2/2 + cmath.sqrt(9.118800e+01**4/4 - (aew * cmath.pi*9.118800e+01**2)/(Gf*cmath.sqrt(2))))

and it returned:
(80.41900244575616+0j)

I guess that you made a typo when computing your values and that the rest of the question is then irrelevant.

Cheers,

Olivier

> On Dec 6, 2017, at 17:57, Jake <email address hidden> wrote:
>
> New question #661454 on MadGraph5_aMC@NLO:
>
> Hello,
>
> I have a couple questions regarding the parameters appearing in the param_card.dat.
>
> First of all, I'm unclear about the value of the W mass appearing in the card and the one that is actually used by MadGraph. What I understand from the comments in the param card and from FAQ #2885 is that the number appearing here is not the value used by MadGraph. However when I generate p p > w+ w- events and check the W masses I find that they're all produced with the mass appearing in the param card. I am therefore confused what is meant then by FAQ #2885 "We never use the input of those [dependent] parameter for the computation of the matrix-element/cross-section/generation of events."
>
> Second, I understand that the value of the W mass appearing in the card is supposedly calculated by the formula appearing next to the value. What I have found is that given the values of aEWM1, Gf, and MZ in the card, using the formula for the W mass doesn't give the value in the card. Is there anything else to the value in the card? For example, from the default MadGraph SM, I have the following values in the card:
>
> Block mass
> 5 4.700000e+00 # MB
> 6 1.730000e+02 # MT
> 15 1.777000e+00 # MTA
> 23 9.118800e+01 # MZ
> 25 1.250000e+02 # MH
> ## Dependent parameters, given by model restrictions.
> ## Those values should be edited following the
> ## analytical expression. MG5 ignores those values
> ## but they are important for interfacing the output of MG5
> ## to external program such as Pythia.
> 1 0.000000 # d : 0.0
> 2 0.000000 # u : 0.0
> 3 0.000000 # s : 0.0
> 4 0.000000 # c : 0.0
> 11 0.000000 # e- : 0.0
> 12 0.000000 # ve : 0.0
> 13 0.000000 # mu- : 0.0
> 14 0.000000 # vm : 0.0
> 16 0.000000 # vt : 0.0
> 21 0.000000 # g : 0.0
> 22 0.000000 # a : 0.0
> 24 80.419002 # w+ : cmath.sqrt(MZ__exp__2/2. + cmath.sqrt(MZ__exp__4/4. - (aEW*cmath.pi*MZ__exp__2)/(Gf*sqrt__2)))
>
> ###################################
> ## INFORMATION FOR SMINPUTS
> ###################################
> Block sminputs
> 1 1.325070e+02 # aEWM1
> 2 1.166390e-05 # Gf
> 3 1.180000e-01 # aS
>
> but using these values I calculate m_W = 84.5116 using the formula given.
>
> Finally, I made a model that is the default FeynRules SM with an added scalar particle. In the .fr file the value for aEWM1 specified is 137, but after generating the UFO file and checking the parameters.py file I notice the value has changed to 127. Ultimately this isn't a huge problem because if necessary I can just change the value in the parameters.py file, but I'm wondering if this behavior is done on purpose or if it's something I am doing when generating the UFO file that should be corrected?
>
> Thank you for your time,
> Jake
>
> --

 Jake (hasek05-06) said on 2017-12-06: #2

Hi Olivier,

Indeed I found a typo and the W mass is as printed in the param card. Thanks.

The final part of my question remains, however. I don't see why a discrepancy arises between the UFO file parameter value and the FeynRule file parameter value for aEWM1.

Cheers,
Kays

 Olivier Mattelaer (olivier-mattelaer) said on 2017-12-06: #3

Hi,

This I have no clue since I’m not a FeynRules developper (not even a user of that code actually)
You should contact the FeynRules authors by email to have a clue on this.

Sorry,

Olivier

> On Dec 6, 2017, at 19:32, Jake <email address hidden> wrote:
>
> Question #661454 on MadGraph5_aMC@NLO changed:
>
>
> Jake is still having a problem:
> Hi Olivier,
>
> Indeed I found a typo and the W mass is as printed in the param card.
> Thanks.
>
> The final part of my question remains, however. I don't see why a
> discrepancy arises between the UFO file parameter value and the FeynRule
> file parameter value for aEWM1.
>
> Cheers,
> Kays
>
> --