Complex Mass Scheme gives significantly different result

Asked by Christian Weiss

Hi,

treating the process e+ e- -> b b~ w+ w- with and without using the complex mass scheme, I get the following leading-order cross sections

w.o. CMS: 2.959e-01 +- 1.8e-03 pb
with CMS: 7.814e-01 +- 4.2e-02 pb

I use the same param_card and run_card for both processes.
The complex mass scheme has been activated by setting "set complex_mass_scheme" before generating the process.
I see that you automatically switch to the EW Scheme when the CMS is activated, so the input value of alpha_e might not match the actual one. However, I would assume that Madgraph just ignores that input.

Greetings,
Christian

Question information

Language:
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Status:
Answered
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MadGraph5_aMC@NLO Edit question
Assignee:
marco zaro Edit question
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Christian Weiss (christian-weiss-g) said :
#1

One additional information: I generated the process using the [QCD]-option, so that also NLO calculations are possible. At NLO, I get the same excess.

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Olivier Mattelaer (olivier-mattelaer) said :
#2

Dear Christian,

This is not consistent to have W in the final state with the complex mass scheme on ON.
So you should decay your W as well.

Cheers,

Olivier
> On May 30, 2016, at 18:27, Christian Weiss <email address hidden> wrote:
>
> Question #294623 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/294623
>
> Christian Weiss posted a new comment:
> One additional information: I generated the process using the
> [QCD]-option, so that also NLO calculations are possible. At NLO, I get
> the same excess.
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

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Christian Weiss (christian-weiss-g) said :
#3

Hi Olivier,

I am a bit confused. In this post

https://answers.launchpad.net/mg5amcnlo/+question/257096

it is stated that in order to compute the cross section at NLO, the complex mass scheme is required. Indeed, if I try to integrate without the CMS, I get the same error message as in the post above.

Greetings,
Christian

Revision history for this message
marco zaro (marco-zaro) said :
#4

Hi Christian,
if you do not set the complex mass scheme, you will use the fixed-width scheme for internal particles.
If you use the CMS, at NLO QCD you can still keep the W stable setting its width (and also the width of the Z / H) to zero, keeping only the top width !=0 in the param card.

Note that setting the CMS, the scheme for EW parameters changes using the W mass as an input parameter.

Does this answer?
Cheers,

Marco

Revision history for this message
Christian Weiss (christian-weiss-g) said :
#5

Hi Marco,

I get the point, thank you. However, setting the gauge boson widths to zero does not actually correspond to the physics I want to describe. So I will go with Olivier's advice and generate the full off-shell process (which brings me to another problem concerning cuts I will post in another question).

Greetings,
Christian

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