# ab>c,c>de Does c have to be on-shell?

Asked by Fang Xu on 2019-10-02

Hi, I am a new user and considering some process like a b > c d, d >e f, where I should scan the mass M_d. And this will let d to be on-shell or off-shell.
But I find that when Ecm < M_d+M_c, if I use a b > c d, d > e f. I can still get some positive cross section which is weird I think.
(1) Does this mean that when I am writing x y > z, z>..... z is not forced on-shell?
(2) What is the difference if I use a b > d > c e f. instead of a b > c d, d > e f? I checked the cross section but they give me different value. At first I think thats just because "a b > c d, d > e f" means d is always on-shell and "a b > d > c e f" means d is always off-shell. But it seems this understanding is not right...

## Question information

Language:
English Edit question
Status:
Solved
For:
MadGraph5_aMC@NLO Edit question
Assignee:
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Solved by:
Olivier Mattelaer
Solved:
2019-10-04
Last query:
2019-10-04
Last reply:
2019-10-03
 Olivier Mattelaer (olivier-mattelaer) said on 2019-10-03: #1

HI

Please follow/read the following tutorial. This is one of the first point/exercise (solution are at the end of the tuto)

https://cp3.irmp.ucl.ac.be/projects/madgraph/raw-attachment/wiki/Shanghai2015/Tutorial_shangai_basic_mg5.pdf

Cheers,

Olivier

> On 3 Oct 2019, at 01:27, Fang Xu <email address hidden> wrote:
>
> New question #684608 on MadGraph5_aMC@NLO:
> https://eur03.safelinks.protection.outlook.com/?url=https%3A%2F%2Fanswers.launchpad.net%2Fmg5amcnlo%2F%2Bquestion%2F684608&amp;data=02%7C01%7C%7C76555fb4cc3f4e95cd1608d747900819%7C7ab090d4fa2e4ecfbc7c4127b4d582ec%7C0%7C0%7C637056556737592292&amp;sdata=N4exPyHbIq9CUWifRdQDkv747aTNtlkJP9cixeyxO1k%3D&amp;reserved=0
>
> Hi, I am a new user and considering some process like a b > c d, d >e f, where I should scan the mass M_d. And this will let d to be on-shell or off-shell.
> But I find that when Ecm < M_d+M_c, if I use a b > c d, d > e f. I can still get some positive cross section which is weird I think.
> (1) Does this mean that when I am writing x y > z, z>..... z is not forced on-shell?
> (2) What is the difference if I use a b > d > c e f. instead of a b > c d, d > e f? I checked the cross section but they give me different value. At first I think thats just because "a b > c d, d > e f" means d is always on-shell and "a b > d > c e f" means d is always off-shell. But it seems this understanding is not right...
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

 Fang Xu (xufang) said on 2019-10-04: #2

Thanks Olivier Mattelaer, that solved my question.

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