# 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*

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?

Thanks in advance

## Question information

- Language:
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- Status:
- Answered

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- Last query:
- 2019-11-11

- Last reply:
- 2019-11-11

> 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:

> https:/

>

> 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*

>

> 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?

>

> Thanks in advance

>

> --

> You received this question notification because you are an answer

> contact for MadGraph5_aMC@NLO.

A.Hayreter (alperhayreter) said : | #2 |

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

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:

> https:/

>

> Status: Answered => Open

>

> 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?

>

> --

> You received this question notification because you are an answer

> contact for MadGraph5_aMC@NLO.

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