# 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

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Solved by:
Olivier Mattelaer
Solved:
2019-10-04
Last query:
2019-10-04
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)

Cheers,

Olivier

> On 3 Oct 2019, at 01:27, Fang Xu <email address hidden> wrote:
>
> New question #684608 on MadGraph5_aMC@NLO:
>
> 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...
>
> --