does sequence of process matter?

Hi,

No the order should not matter.
It is just that we think to forbid your syntax in one order and do not think to forbid such syntax in the second order. I have actually no clue what would be computed in the context of your second syntax.

To do what you want, you should use the syntax

> generate p p > l+ l- [QCD] @0
> add process p p > l+ l- j [LOonly=QCD] @1

You should also activate FxFx merging when doing that.
I will assign Rikkert to this question just in case, since he is much more expert than me with FxFx merging.

Cheers,

Olivier

> On 23 Oct 2018, at 16:07, Meng Lu <email address hidden> wrote:
>
> New question #675488 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/675488
>
> Dear MG experts,
>
> i'm wondering if MG can produce this kind of process, to do a comparison on the xsection with sherpa,
>
> p p > l+ l- 0jet @NLO
> p p > l+ l- 1jet @LO
>
> i tried with following commands and it threw exception:
> generate p p > l+ l- [QCD] @0
> add process p p > l+ l- j @1
>
> then i tried with:
> generate p p > l+ l- j @0
> add process p p > l+ l- [QCD] @1
>
> but i do not know if these commands make sense or not.
>
> Best,
> Meng
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Dear Meng,

Indeed, as Olivier is writing, you should do

> generate p p > l+ l- [QCD] @0
> add process p p > l+ l- j [LOonly=QCD] @1

and active the FxFx merging as described here: http://amcatnlo.web.cern.ch/amcatnlo/FxFx_merging.htm

However, what request does not make 100% sense: the real emission contribution to p p > l+ l- [QCD] already include the tree-level p p > l+ l- j processes. Hence, you are not adding more information when doing the merging this way. In fact, you are deteriorating the accuracy of the prediction, since when you do the merging --as you want to do-- you have to introduce a non-physical merging scale to combine the processes.

Best regards,
Rikkert

Dear Olivier and Rikkert,

thanks for your answers, i can successfully do the process with:
> generate p p > l+ l- [QCD] @0
> add process p p > l+ l- j [LOonly=QCD] @1

but when i tried to generate some events with script generate_events, it returned errors like following in the log file:

Process generated with [LOonly=QCD]. Setting abrv to "born".
 FxFx merging not possible with [LOonly=QCD] processes,

i set:
'3 = ickkw
False = fixed_ren_scale
False = fixed_fac_scale'

in the run card, what should i do to fix this error?

Cheers,
Meng

Dear Meng,

Which version of MG5_aMC are you using?
If you are not using one of the latest versions, please update your code and try again.

Best,
Rikkert

Dear Rikkert,

thanks, i'm using MG2.4, i'll try with MG2.6.

Best,
Meng

Dear Rikkert and Olivier,

the problem was solved by using the MG2.6.3.2, thanks very much for your help.

Cheers,
Meng