does sequence of process matter?

Asked by Meng Lu on 2018-10-23

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

Question information

Language:
English Edit question
Status:
Solved
For:
MadGraph5_aMC@NLO Edit question
Assignee:
Rikkert Frederix Edit question
Solved by:
Meng Lu
Solved:
2018-10-31
Last query:
2018-10-31
Last reply:
2018-10-24

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.

Rikkert Frederix (frederix) said : #2

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

Meng Lu (meng-lu) said : #3

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

Rikkert Frederix (frederix) said : #4

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

Meng Lu (meng-lu) said : #5

Dear Rikkert,

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

Best,
Meng

Meng Lu (meng-lu) said : #6

Dear Rikkert and Olivier,

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

Cheers,
Meng