This is the simulation of a `P + P -> Z + jet` production event at the LHC for 7 TeV center-of-mass energy, using the [[ https://rivet.hepforge.org/analyses/ATLAS_2011_I945498.html | ATLAS_2011_I945498 ]] and [[ https://rivet.hepforge.org/analyses/MC_ZINC.html | MC_ZINC ]] Rivet plugins for data analysis.
Please download the following input file and move it to your work directory:
Input file:
{F543471}
Alternatively, you can use the following command-line codes:
```
mkdir LHC-Z+jet
cd LHC-Z+jet
wget https://phab-files.hepforge.org/file/data/n36vm5xhxosaddw7xe4x/PHID-FILE-etk5vfa3ii64fngxon2w/LHC-Z_jet.in
```
Before running this event, let us take a look at the input file
```
vi LHC-Z+jet.in
```
As you can see, the process selection section has been completely deactivated. This is because for a process like `P + P -> Z + jet`, Herwig needs to use the external matrix element (ME) providers. However, the use of these tools in the docker/VM would be problematic. In an installed version of the Herwig, one can easily use these external ME providers by selecting the hard-process in the `Process selection` and choosing the ME providers from `Matrix element library selection`.
```
##################################################
## Process selection
##################################################
## Model assumptions
# read Matchbox/StandardModelLike.in
# read Matchbox/DiagonalCKM.in
## Set the order of the couplings
# cd /Herwig/MatrixElements/Matchbox
# set Factory:OrderInAlphaS 1
# set Factory:OrderInAlphaEW 1
## Select the process
# do Factory:Process p p -> Z0 j
##################################################
## Matrix element library selection
##################################################
## Select a generic tree/loop combination or a
## specialized NLO package
# read Matchbox/MadGraph-GoSam.in
# read Matchbox/MadGraph-MadGraph.in
# read Matchbox/MadGraph-NJet.in
# read Matchbox/MadGraph-OpenLoops.in
```
Fortunately, Herwig comes equipped with a set of Pre-calculated ME (you can see the complete list of these at Herwig_directory/share/Herwig/LHC.in):
```
##################################################
# Pre-calculated ME for hadron-hadron collisions
##################################################
cd /Herwig/MatrixElements/
# Z+jet
insert SubProcess:MatrixElements[0] MEZJet
```
Now, we can run the Z+jet event using
```
Herwig read LHC-Z+jet.in
Herwig run LHC-Z+jet.run -N 10000
```
and plot the results by
```
rivet-mkhtml LHC-Z+jet.yoda
```
Please change the name of the produced yoda files
```
mv LHC-Z+jet.yoda Z+jet_MCNLO.yoda
```
and take a look at the parton shower selection section
```
##################################################
## Matching and shower selection
##################################################
read Matchbox/MCatNLO-DefaultShower.in
# read Matchbox/Powheg-DefaultShower.in
## use for strict LO/NLO comparisons
# read Matchbox/MCatLO-DefaultShower.in
## use for improved LO showering
# read Matchbox/LO-DefaultShower.in
# read Matchbox/MCatNLO-DipoleShower.in
# read Matchbox/Powheg-DipoleShower.in
## use for strict LO/NLO comparisons
# read Matchbox/MCatLO-DipoleShower.in
## use for improved LO showering
# read Matchbox/LO-DipoleShower.in
# read Matchbox/NLO-NoShower.in
# read Matchbox/LO-NoShower.in
```
The "read Matchbox/MCatNLO-DefaultShower.in" option is chosen by default, activating an MC@NLO matched angular ordered parton shower. Please choose the "read Matchbox/MCatLO-DefaultShower.in" option, to choose the MC@LO and try running the event again:
```
Herwig read LHC-Z+jet.in
Herwig run LHC-Z+jet.run -N 10000
mv LHC-Z+jet.yoda Z+jet_MCLO.yoda
```
Repeat this process again, this time with "read Matchbox/LO-NoShower.in" option and rename the yoda file as
```
mv LHC-Z+jet.yoda Z+jet_NOPS.yoda
```
At this point, you are in the position of 3 separate Z+jet production events, each with different parton shower settings. You can compare your findings by plotting these yoda files altogether:
```
rivet-mkhtml Z+jet_MCNLO.yoda:MC_NLO Z+jet_MCLO.yoda:MC_LO Z+jet_NOPS.yoda:NO_PS
```
and check out your result by
```
firefox rivet-plots/index.html
```
Switching things on and off
-------------------------------
At this point, we are going to modify our copy of the LHC-Z+jet.in input file and make a few more runs.
1. [[ https://herwig.hepforge.org/tutorials/gettingstarted/onoffthings.html | Shower handlers ]]
These commands control the major modules of the event generator. You can switch them on and off, rerun the event and observe the produced effects of the results.
```
##################################################
## ShowerHandler(s)
##################################################
cd /Herwig/EventHandlers
set EventHandler:CascadeHandler:MPIHandler NULL
# set EventHandler:DecayHandler NULL
# set EventHandler:HadronizationHandler NULL
set /Herwig/Shower/ShowerHandler:Interactions QCDandQED
```
- The Multy Particle Interaction (MPIHandler) is switched off by default. Turn it on and off, and run the event in each state and see what changes in the results.
- You can change the type of parton shoer by changing the value in
```
set /Herwig/Shower/ShowerHandler:Interactions QCDandQED
```
The options are `QCDandQED`, `QCD` and `QED`. Try to check out the difference they make in the event.
2. [[ https://herwig.hepforge.org/tutorials/hardprocess/matchbox.html#scale-choices | Scale choices ]]
Your choice for the scale of the hard-process is made in this section. The primary option for a Z+jet production event, "the fixed scale option", is pre-selected here. You can choose other options and/or change the value of the fixed scale and see the effect.
```
##################################################
## Scale choice
##################################################
cd /Herwig/MatrixElements/Matchbox/
## Fixed scale
set Factory:ScaleChoice Scales/FixedScale
set Scales/FixedScale:FixedScale 91.1876*GeV
## The transverse momenta of coloured particles
# set Factory:ScaleChoice Scales/HTScale
## The transverse momenta of all outgoing non-jet particles
# set Factory:ScaleChoice Scales/HTPrimeScale
## For processes involving a pair of final-state leptons
# set Factory:ScaleChoice Scales/LeptonPairMassScale
## The transverse component of the momentum of the lepton pair
# set Factory:ScaleChoice Scales/LeptonPairPtScale
## The transverse momentum of the hardest jet
# set Factory:ScaleChoice Scales/MaxJetPtScale
## The partonic centre-of-mass energy
# set Factory:ScaleChoice Scales/SHatScale
## For processes involving the production of a t-tbar pair
# set Factory:ScaleChoice Scales/TopPairMassScale
## The transverse mass of the t-tbar pair
# set Factory:ScaleChoice Scales/TopMTScale
```
3. [[ https://herwig.hepforge.org/tutorials/hardprocess/matchbox.html#cuts | Cuts ]]
You can impose any constraint that you want on your event. To find out the compete list of cuts and how to use them, see Herwig documentaion on [[ https://herwig.hepforge.org/tutorials/hardprocess/matchbox.html#cuts | cuts ]].
```
##################################################
## Cut selection
## See the documentation for more options
##################################################
cd /Herwig/Cuts/
## cuts on jets
read Matchbox/DefaultPPJets.in
insert JetCuts:JetRegions 0 FirstJet
insert JetCuts:JetRegions 1 SecondJet
# set /Herwig/Cuts/JetFinder:Variant Kt
# set /Herwig/Cuts/JetFinder:ConeRadius 0.4
# set /Herwig/Cuts/FirstJet:PtMin 20.*GeV
# do /Herwig/Cuts/FirstJet:YRange -5.0 5.0
```