MadGraph5_aMC@NLO

off shell process

Asked by Safinaz

Dear MadGraph authors,

I have the following vertex in my model:

sr > ss W-,

where sr and ss are neutral and charged scalars having equal masses m_sr = m_ss. When I generate this process
in MG it says: Zero result detected and the matrix amplitude didn't calculated, this means that MG doesn’t calculate
off shell decay width ..

But when I generate : p p > sr > ss w-, ss > t b~,(w- > l- vl~)

It gives me a result , so how does the program calculate the last process ,
does it calculate sr > ss w- as off shell or what ?

Thanks,
Safinaz

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Olivier Mattelaer
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Best Olivier Mattelaer (olivier-mattelaer) said : 		#1

Dear Safinaz,

>I have the following vertex in my model:
>
>sr > ss W-,
>
>where sr and ss are neutral and charged scalars having equal masses m_sr = m_ss. When I generate this process
>in MG it says: Zero result detected and the matrix amplitude didn't calculated, this means that MG doesn’t calculate
>off shell decay width ..

This is normal. When you compute an amplitude, you use the diffraction theory (i.e. the S-matrix) which assume that initial/final state are asymptomatically free. Therefore such computation make sense only if the initial/final state are on-shell.
This is clearly impossible for
sr > ss W-
and the code didn't compute anything as it should.

> But when I generate : p p > sr > ss w-, ss > t b~,(w- > l- vl~)
> It gives me a result.

In this case the ss (and the w) are allowed to be off-shell and they are no "physical" problem associate to this since the sr/ss/w are not initial/final state. In the matrix-element they are associated to a propagator which is define (and meningfull) both on-shell and off-shell.

Now the syntax that you use assume that ss and w are on-shell but not the sr. This might not be the contribution that you want/need.
See the following tutorial, for more details on the signification of the various syntax:
https://cp3.irmp.ucl.ac.be/projects/madgraph/attachment/wiki/MGTalks/13_06_10_tutomg_tasi.pdf

Cheers,

Olivier

Revision history for this message
Safinaz (sramadan) said : 		#2

Hi Dr. Olivier,

Thanks, I were just need to make sure that the process is calculated as a propagator can be off shell or on shell.
Also thanks for the tutorial I will check it ..

May I have another question please, for example for the previous process
 p p > sr > ss w-, ss > t b~,(w- > l- vl~),

To investigate (sr ) signal, should I went through the complete decay chain of the t-quark, like:

p p > sr > ss w-, w- > l- vl~,(ss > t b~, (t > w+ b, w+ > l+ vl))

Or I can just generate : p p > sr > ss w-, ss > t b~,(w- > l- vl~)

In both cases I run the event file through pythia/pgs .

Regards,
Safinaz

Revision history for this message
Safinaz (sramadan) said : 		#3

Thanks Olivier Mattelaer, that solved my question.

Revision history for this message
Olivier Mattelaer (olivier-mattelaer) said : 		#4

Hi,

> p p > sr > ss w-, w- > l- vl~,(ss > t b~, (t > w+ b, w+ > l+ vl))
>
> Or I can just generate : p p > sr > ss w-, ss > t b~,(w- > l- vl~)
>
> In both cases I run the event file through pythia/pgs .

You can do both, this is up to you. Indeed Pythia will decay the top automatically.
But the Pythia will not include spin-correlation for the top-decay while MG does.
The precision on the cross-section also will not be the same since Pythia will approximate the cross-section via the narrow-width approximation.

So this is usually a trade between precision (which also depend of your observables that you want to investigate) and the speed (MG is slower than pythia).

Cheers,

Olivier

On Jan 3, 2014, at 1:31 PM, Safinaz <email address hidden> wrote:

> Question #241618 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/madgraph5/+question/241618
>
> Safinaz posted a new comment:
>
> Hi Dr. Olivier,
>
> Thanks, I were just need to make sure that the process is calculated as a propagator can be off shell or on shell.
> Also thanks for the tutorial I will check it ..
>
> May I have another question please, for example for the previous process
> p p > sr > ss w-, ss > t b~,(w- > l- vl~),
>
> To investigate (sr ) signal, should I went through the complete decay
> chain of the t-quark, like:
>
> p p > sr > ss w-, w- > l- vl~,(ss > t b~, (t > w+ b, w+ > l+ vl))
>
> Or I can just generate : p p > sr > ss w-, ss > t b~,(w- > l- vl~)
>
> In both cases I run the event file through pythia/pgs .
>
>
> Regards,
> Safinaz
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Revision history for this message
Safinaz (sramadan) said : 		#5

Could writing the complete decay chain be useful in specifying in which decay mode W- boson will decay,
that it can decay into leptons or quarks, so that I will have three different final states in my process ..

Regards,
Safinaz

Revision history for this message
Olivier Mattelaer (olivier-mattelaer) said : 		#6

You can do that:

for example
define wdecay1 = u d s c l-
define wdecay2 = u~ d~ s~ c~ vl~
and then
generate p p > sr > ss w-, ss > t b~, (w- > wdecay1 wdecay2)

if you want to also have the tau decay, then of course you need to add the tau in the definition…

Cheers,

Olivier

On Jan 3, 2014, at 3:01 PM, Safinaz <email address hidden> wrote:

> Question #241618 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/madgraph5/+question/241618
>
> Safinaz posted a new comment:
>
> Could writing the complete decay chain be useful in specifying in which decay mode W- boson will decay,
> that it can decay into leptons or quarks, so that I will have three different final states in my process ..
>
> Regards,
> Safinaz
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Revision history for this message
Safinaz (sramadan) said : 		#7

Hi Dr. Olivier,

Thank you for help.

Regards,
Safinaz