# How to set up the mass of c quark and its decay width?

Asked by Maksym Ovchynnikov on 2018-12-25

In the param_card.dat of default model before the values of some masses and decay widths (for example, those of c quark) is written:

## 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.

It seems that it means that by changing the mass and the decay width of the c quark in param_card I will not really change them for MadGraph simulations. I have few questions.

1) Could you please tell me what is the proper way for changing the values of c mass and decay width? Is there a simple way to do this (say, downloading some specific model, changing some values in a piece of code and so on)?

2) I am interested in the process

p p > c c~, c > e+ ve s

and it seems to me that in this process I can "imitate" the c quark by the top quark with changed mass and decay width in param_card. The process which imitates (1) is

p p > t t~, t > e+ ve b,

where b mass is also set to zero. Is it meaningful? If yes, then is it possible?

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2018-12-25
2018-12-25
 Olivier Mattelaer (olivier-mattelaer) said on 2018-12-25: #1

1) Could you please tell me what is the proper way for changing the
values of c mass and decay width? Is there a simple way to do this (say,
and so on)?

The recommended way is to use FeynRules and to generate your model from the Lagrangian expression that you want to use. (you can obviously start from their default sm model)

2) I am interested in the process

p p > c c~, c > e+ ve s

and it seems to me that in this process I can "imitate" the c quark by
the top quark with changed mass and decay width in param_card. Is it
meaningful?

Your PDF (and therefore value of alpha_s), ... will not be correct (not correct number of flavor).
But it might make sense if you use the correct 3 flavor PDF.
In top of that, you have all the question if such process/syntax are meaningfull in perturbation theory/narrow width approximation,..

Cheers,

Olivier

On 25 Dec 2018, at 20:27, Maksym Ovchynnikov <<email address hidden><mailto:<email address hidden>>> wrote:

Description changed to:
In the param_card.dat of default model before the values of some masses
and decay widths (for example, those of c quark) is written:

## 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.

It seems that it means that by changing the mass and the decay width of
the c quark in param_card I will not really change them for MadGraph
simulations. I have few questions.

1) Could you please tell me what is the proper way for changing the
values of c mass and decay width? Is there a simple way to do this (say,
and so on)?

2) I am interested in the process

p p > c c~, c > e+ ve s

and it seems to me that in this process I can "imitate" the c quark by
the top quark with changed mass and decay width in param_card. Is it
meaningful?

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