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:
{F542868}
Alternatively, you can use the following command-line codes:
```
mkdir LHC-dijet
cd LHC-dijet
wget https://phab-files.hepforge.org/file/download/eh5glykvcuwpul6yvwzp/PHID-FILE-g4u3mgzeahyqyc7mcgvj/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.