Matched Cross-Section

Hi,

Your process has some QCD tradition.
you have some event with 0j in the final state and some with 1j in the final state
the parton shower (pythia) also adds jets in the final state.
Therefore you have some double counting between the 0j and the 1j sample.

Removing such double counting is not easy and you need to cross-check that this is done correctly.
This is why after the pardon-shower we have to reject events and the cross-section is therefore corrected from this double counting.
This procedure is called matching and merging.
This is why you have a second cross-section (actually that one is correct if the matching/merging works which is not guarantee)

More details on how to use matching and merging and the physical meaning:
https://cp3.irmp.ucl.ac.be/projects/madgraph/wiki/IntroMatching

I insist that the check of consistency of the matching procedure (i.e. the fact that you have put the correct parameter in the run_card) is not done automatically.
This is your responsibility to look at DJR plot and to assess if the transition between the two regime is correct or not.
Only after that step, you can claim that the second cross-section is correct.

Cheers,

Olivier

> On Feb 6, 2016, at 00:32, sgurruti <email address hidden> wrote:
>
> New question #284688 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/284688
>
> Hello,
> I am using the following code to generate some processes:
>
> set group_subprocesses Auto
> set ignore_six_quark_processes False
> set gauge unitary
> set complex_mass_scheme False
> import model sm
> define p = g u c d s b u~ c~ d~ s~ b~
> define j = g u c d s b u~ c~ d~ s~ b~
> define all = g u c d s u~ c~ d~ s~ a ve vm vt e- mu- ve~ vm~ vt~ e+ mu+ t b t~ b~ z w+ h w- ta- ta+
> define l+ = e+ mu+
> define l- = e- mu-
> define vl = ve vm vt
> define vl~ = ve~ vm~ vt~
> generate p p > all vl vl~
> add process p p > all all vl vl~
> output CrossSection_SM_2nu
> launch
> pythia=ON
> pgs=ON
> delphes=ON
> set WH 0.0041
> set ebeam1 7000
> set ebeam2 7000
> set nevents 200000
> launch CrossSection_SM_2nu -i
> print_results --path=./CrossSection_SM_2nu/cross_section_sm_2nu.txt --format=short
>
> As a result, Madgraph calculate the cross section, but it tells me the following message:
>
> << === Results Summary for run: run_01 tag: tag_1 ===
>
> Cross-section : 2707 +- 2.133 pb
> Nb of events : 200000
> Matched Cross-section : 1248 +- 3.174 pb
> Nb of events after Matching : 92180 >>
>
> What does it mean "Matched Cross-section"? What is the difference between both cross sections and the number of events?
>
> Thank you!
> Sebastian
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

I see, thank you!

Best regards,
Sebastian