# discrepancy in production and decay cross sections

Dear MG team,

I am having a rather weird situation. I use a model file that I generated from Feynrules, in which I have two new particles (a W' and a right handed neutrino NR). The Lagrangian is quite simple;

gg*(0.0424*cbar.Ga[mu].ProjP.b+tbar.Ga[mu].ProjP.b+Nrbar.Ga[mu].ProjP.ta)*Wp[mu]

I scan WP mass in two different case which they supposed to be related with a BR. So here it is,

A) p p > WP+
B) p p > WP+ > ta+ NR

I assume that cross-section(B) = cross-section(A) x BR(ta+ NR)

This assumption looks fine with a mass of 500 but completely screws for 5000. In case of MWP=5000 I got cross-section(B) almost 100 times greater than cross-section(A) !!!

I use AUTO width calculation for WP, and for MWP=5000 MG calculates WWP~1650 and BR(ta+ NR)~0.25.

Do I miss anything important?

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2019-11-11
2019-11-11
 Olivier Mattelaer (olivier-mattelaer) said on 2019-11-11: #1

> I assume that cross-section(B) = cross-section(A) x BR(ta+ NR)

This assume Narrow-Width Approximation.
So are you in that setup? (i.e. what is your width/mass)

Cheers,

Olivier

> On 11 Nov 2019, at 19:57, A.Hayreter <email address hidden> wrote:
>
> New question #685754 on MadGraph5_aMC@NLO:
>
> Dear MG team,
>
> I am having a rather weird situation. I use a model file that I generated from Feynrules, in which I have two new particles (a W' and a right handed neutrino NR). The Lagrangian is quite simple;
>
> gg*(0.0424*cbar.Ga[mu].ProjP.b+tbar.Ga[mu].ProjP.b+Nrbar.Ga[mu].ProjP.ta)*Wp[mu]
>
> I scan WP mass in two different case which they supposed to be related with a BR. So here it is,
>
> A) p p > WP+
> B) p p > WP+ > ta+ NR
>
> I assume that cross-section(B) = cross-section(A) x BR(ta+ NR)
>
> This assumption looks fine with a mass of 500 but completely screws for 5000. In case of MWP=5000 I got cross-section(B) almost 100 times greater than cross-section(A) !!!
>
> I use AUTO width calculation for WP, and for MWP=5000 MG calculates WWP~1650 and BR(ta+ NR)~0.25.
>
> Do I miss anything important?
>
>
> --

 A.Hayreter (alperhayreter) said on 2019-11-11: #2

So what must be that ratio? In my case it looks like 0.33 is it too big?

 Olivier Mattelaer (olivier-mattelaer) said on 2019-11-11: #3

Yeah this much too large to consider the Breit-wigner as a delta function.

Cheers,

Olivier

> On 11 Nov 2019, at 20:37, A.Hayreter <email address hidden> wrote:
>
> Question #685754 on MadGraph5_aMC@NLO changed:
>
>
> A.Hayreter is still having a problem:
> So what must be that ratio? In my case it looks like 0.33 is it too big?
>
> --