# using shower calculate next leading order

Hi
I am a beginner. I am learning MadGraph5_aMC@NLO. I have a question about parton showering: For example, I want to calculate the cross section of p p > e ve~ c [NLO] , and investigate the effect of parton shower in this process at NLO . So first, I generate this process in this way:
define p= p /c c~
define j=p
generate p p > e ve~ c [NLO]
...
Then after launch, I launch the process again but this time by choosing pythia=ON (I think that it means calculate cross section with considering the parton shower). I get the cross section , meanwhile i get the lhe file for this calculation .

It should be noted that I put
PYTHIA6Q=shower
p Tj > 25 GeV,
|η|j < 2.5
p Tl > 20 GeV,
|η | l< 2.5,
p ν T > 25 GeV,
m WT > 40 GeV
the last two parameter , i cant put the run_card.dat file ,so i put those parameter on cuts.f fortran file. in this calculation i get the correct total coress section .but this differential cross section is not correct , compare to the result of paper ,difference is more large? how can i get the correct differential cross section value same like paper result ? extra question about the using parton shower, for using parton shower , it is eneough modify the run_card.dat file or is there some parameter we should modify? in the madgraph , when install pythia8 , why there appear so many error massage on pythia8_install.log? ?(HEPTools/pythia/pythia8_install.log)
thanks
Best.

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2017-10-24
2017-10-24
 Olivier Mattelaer (olivier-mattelaer) said on 2017-10-24: #1

Hi,

1) the syntax: "generate p p > e ve~ c [NLO]" does not exists.
I have try to run such syntax in the lastest version of the code and it indeed crashes.

2) The "pythia=ON" is actually a LO syntax, so is this something that you type in the interface? or just a way to express yourself?

3) how can i get the correct differential cross section value same like paper result ? extra question about the using parton shower, for using parton shower , it is eneough modify the run_card.dat file or is there some parameter we should modify? in the madgraph , when install pythia8 , why there appear so many error massage on pythia8_install.log? ?(HEPTools/pythia/pythia8_install.log)

You should first be aware that you have different distribution for NLO curve (without Parton-shower) and NLO+PS (with parton-shower) in this second stage, the plot of the distribution can only be done AFTER parton-shower since the files generated before parton-shower are NOT physical.

4) extra question about the using parton shower, for using parton shower , it is eneough modify the run_card.dat file or is there some parameter we should modify? in the madgraph , when install pythia8 , why there appear so many error massage on pythia8_install.log? ?(HEPTools/pythia/pythia8_install.log)

The handling of the various shower are done in the shower_card, you can see that all options do not apply on all the parton-shower.
So I would suggest to check carefully the content of that file.

Cheers,

Olivier