MadGraph5_aMC@NLO

query regarding offshell and onshell decay

Asked by nabanita ganguly

Hi,

I am calculating the process p p > H+ A0 in Inert Doublet Model where A0 is the stable one. The only available decay mode for H+ is
   H+ > W+ A0 or H+ > j j A0 / l vl A0 depending on it's mass . I am looking for dijet+etmis final states.

I am calculating the process in two ways :

1. p p > H+ A0 , H+ > j j A0
2. p p > W+ > j j A0 A0 / g a h (and setting W+W-A0A0 coupling zero) (This is to ensure that I only have H+ mediated diagram leading to dijet + etmis final state)

My BMP is such that m(H+) < m(W+)+m(A0) . So H+ can only decay to virtual W+.

xsectn calculated in two methods are totally different . For case1 109.1fb (which is just (production xsectn of pp > H+ A0)times BR(H+ > j j A0)) . for case 2 ,however it is 0.252fb.

My question is how does MG5 calculate the process in case 2 ?? Why is the difference in xsectn coming?

However,as W+ becomes onshell, xsection calculated in both the methods converge.

Best,
Nabanita

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Olivier Mattelaer (olivier-mattelaer) said : 		#1

Olivier Mattelaer suggests this article as an answer to your question:
FAQ #2778: “Different syntax for the same diagram leads to different cross-sections: Why?”.

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nabanita ganguly (nabanita-rimpi) said : 		#2

Hi,

Thank u for the answer. One thing still needs clarification. What about syntax b ? Does madgraph always calculate it assuming w+ to be offshell?

In my case , how does mg5 calculate the process in case of 2 i.e. does it assume H+ to be offshell always which appears as a mediator in this case?

Best,
Nabanita

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Olivier Mattelaer (olivier-mattelaer) said : 		#3

Hi,

For your syntax 2,

1) The > w+ > syntax force to have at least one W+ in S-channel, this includes both on-shell and off-shell contribution.
Note that this does not force to have a 2>1 processes.

2) They are no restriction on H+, we will compute both the onshell and the offshell contribution. you also do not require the presence of the H+ and therefore we also include diagrams without any H+ in the process (if such diagram exists)

Cheers,

Olivier

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nabanita ganguly (nabanita-rimpi) said : 		#4

Hi,

Thank u for the quick response. I have chosen the process in such a way (in case of 2) that I have only H+ mediated diagram:

     p p > W+ > j j A0 A0 / g a h (and setting W+W-A0A0 coupling zero)

Another way that gives the same diagram is p p > H+ A0 , H+ > j j A0
 They give very different xsectns ( mH+ = 80 , mA0 = 65)

Now is this happening due to the fact that in the later case H+ can only be onshell where for the former it can be both onshell and offshell depending on it's mass ? Is there any way to make H+ always onshell in the former?

Regards,
Nabanita

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Olivier Mattelaer (olivier-mattelaer) said : 		#5

> Is there any way to make H+ always onshell in
> the former?

You can use the first syntax for that.
If you want to keep your second choice syntax, you have to implement your own cuts based on the final state particles in order to have the some on shell effect.

Cheers,

Olivier

> On May 25, 2016, at 10:43, nabanita ganguly <email address hidden> wrote:
>
> Question #293962 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/293962
>
> Status: Answered => Open
>
> nabanita ganguly is still having a problem:
> Hi,
>
> Thank u for the quick response. I have chosen the process in such a way
> (in case of 2) that I have only H+ mediated diagram:
>
> p p > W+ > j j A0 A0 / g a h (and setting W+W-A0A0 coupling zero)
>
> Another way that gives the same diagram is p p > H+ A0 , H+ > j j A0
> They give very different xsectns ( mH+ = 80 , mA0 = 65)
>
> Now is this happening due to the fact that in the later case H+ can only
> be onshell where for the former it can be both onshell and offshell
> depending on it's mass ? Is there any way to make H+ always onshell in
> the former?
>
>
> Regards,
> Nabanita
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

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