Missing 0 jet sample

Hi,

I'm not an expert of the CKKW-L merging since all the effect of matching/merging are handle within pythia8 and not within our code.

On our side we just have generation that does not do anything for CKKW-L.
( You just have the cuts obviously like ptlund that you typically setup in order to have a correct CKKW-L merging and such that you do not discard too much event.)

So I do not understand why you do not have the same result with or without pythia.
Obviously any result before running the parton-shower are not physical (since this is Pythia that removes the double counting). But since the number that you quote are before parton-shower those are not impacted by the presence/absence of the parton-shower.
Or do you mean that you do not setup cut like ptlund?

I would suggest to check the run_card.dat (and param_card.dat) between those two runs and check that they are identical.

Cheers,

Olivier

> On 7 Mar 2023, at 16:10, Alex Feike <email address hidden> wrote:
>
> New question #705750 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/705750
>
> Hello,
>
> I am currently trying to generate squark - anti squark pairs with up to 2 jets and merge the jets with CKKW-L. Weirdly the 0 jet sample doesn't contribute at all to the overall cross section.
> However, if I switch the maximum multiplicity to just 1 additional jet or don't try to merge the jets I get nonzero cross sections. If I switch the maximum multiplicity to 3 additional jets, then some of the 2 jet diagrams yield a zero cross section. You'll find an overview of the tables of these processes here: https://uni-muenster.sciebo.de/s/vMxl8bitgDI0ml4
>
> The script I use is given below. I change this script just by adding or deleting the process with the highest number of jets or by setting shower = OFF.
>
> Do you have any idea why this is happening?
>
> Thanks and best,
> Alex
>
>
> import model MSSM_SLHA2-full
> define ups = ur ur~
> generate p p > ur ur~ $ups @1
> add process p p > ur ur~ j $ups @2
> add process p p > ur ur~ j j $ups @3
> output
> launch
> shower = Pythia8
> analysis = OFF
> madspin = ON
> set pdlabel lhapdf
> set lhaid 27000
> set ebeam1 6800.0
> set ebeam2 6800.0
> set nevents 50000
> set use_syst False
> set mass mgo 30000
> set mass msd1 30000
> set mass msd2 30000
> set mass msd3 30000
> set mass msd4 30000
> set mass msd5 30000
> set mass msd6 30000
> set mass msu1 30000
> set mass msu2 30000
> set mass msu3 30000
> set mass msu4 1000
> set mass msu5 30000
> set mass msu6 30000
> set mass mneu1 200
> set mass mneu2 30000
> set mass mneu3 30000
> set mass mneu4 30000
> set mass mch1 30000
> set mass mch2 30000
> decay ur > n1 u
> decay ur~ > n1 u~
> set decay wsu4 auto
> set ickkw=0
> set xqcut = 0
> set ptlund = 150
> set dparameter 0.4
> set Merging:TMS -1
> set Merging:nJetMax 2
> set ktdurham = -1
> set Merging:mayRemoveDecayProducts = on
> set Merging:doPTLundMerging=on
> set Merging:Process=pp>ur,ur~
>
>
> --
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> contact for MadGraph5_aMC@NLO.

Hi Olivier,

I checked the run card and you were right, there is a difference. It works when I use ickkw=1 instead of ptlund. Do you have any idea why this works when ptlund doesn't work. I checked for different values of ptlund and always get zero for the 0jet sample. Using the ktdurham cut instead of ptlund yields the same problem of giving zero cross sections.

Cheers,
Alex

Hi,

I'm afraid that I can not really help to understand the behavior of the ptlund cut.
maybe the issue is associated to the following:
pdgs_for_merging_cut
Where you might need to remove the ur ur~.

I would suggest to read the pythia8 section on CKKW-L to check which cuts are needed for that algorithm.

Cheers,

Olivier

Hi,
just for future reference: when I use the decay chain syntax to decay the squarks everything works.

Cheers
Alex