Simulation of a dijet production event at the LHC for 7 TeV center-of-mass energy, using the [[ https://rivet.hepforge.org/analyses/ATLAS_2012_I1082936.html | ATLAS_2012_I1082936 ]] Rivet plugin for data analysis.
Please download the following input file and move it to your work directory:
Input file:
{F542878}
Alternatively, you can use the following command-line codes:
```
mkdir LHC-dijet
cd LHC-dijet
wget https://phab-files.hepforge.org/file/data/2gbec5zz2h73aj2cvlmf/PHID-FILE-4mojzqvmtcggvujgnqmp/LHC-dijet.in
```
You can run the dijey event, using
```
Herwig read LHC-dijet.in
```
You should see an error message appearing on your console
```
Error: MatchboxFactory: No matrix elements have been found.
Please check if your order of Alpha_s and Alpha_ew have the right value.
```
This indicated that the Matchbox does not contain this specific process and you must use either an external matrix element (ME) provider of a [[ https://phab.hepforge.org/w/mcnet2019/leshouches/ | Les Houches input file ]]. Let us consider the former option at the moment. We will return to the Les Houches case later on.
There is a section in the input files for choosing the possible external ME providers:
```
##################################################
## 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
```
Please uncomment the "read Matchbox/MadGraph-OpenLoops.in" option. This would tell the event handler to use MadGraph and OpenLoops for the tree-level and one-loop MEs, respectively. You can see that there are a number of different options to choose from, based on the event that you want to simulate.
Now, you can try running the event again:
```
Herwig read LHC-dijet.in
```
You will see a message, informing you that the external ME provider is engaged and will create a parton level amplitude for you:
```
Initializing external amplitudes.
>>> Compiling MadGraph amplitudes. This may take some time -- please be patient.
>>> In case of problems see ./Herwig-scratch/Build/MadGraphAmplitudes/MG.log for details.
```
This process may take several minutes to complete, based on the number of channels involved in the event. You can reduce the time by changing the shower selection options:
```
##################################################
## 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. By choosing the "read Matchbox/MCatLO-DefaultShower.in" option, you can speed up the process with the price of lowering the accuracy.
After the "read" command is completed, you can run the event by:
```
Herwig run LHC-dijet.run -N 10000
```
and plot the results by
```
rivet-mkhtml LHC-dijet.yoda
```