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.

In this section, we will simulate the 'P + P -> Z+X' process at 7 TeV center-of-mass energy and study the properties of the Z-boson with the help of the [[https://rivet.hepforge.org/analyses/MC_ZINC.html | MC_ZINC ]] Rivet analysis. There are different stages of event generation, each of which plays a crucial role in the correct simulation of Z-bosons at the LHC: * Hard matrix element * Parton Shower * Initial and final state radiation * Multiple parton interactions * Hadronizaion During this exercise, we will see the different contributions to the Z shape observables, like the Z pT spectrum. Download the following input file and move it to your work directory: Input file:{F545117} Alternatively, you can use the following command-line code: ``` wget https://phab-files.hepforge.org/file/download/q5by5q5tocxpquklfi34/PHID-FILE-ekoleb6f3inpmsz6xmbl/LHC-Matchbox.in ``` The input file can be used for Z production and Z+Jets production. For now, all parts except for the hard matrix element are commented out. We will uncomment the necessary parts as we progress with the exercise. Run the input file without any modifications ``` Herwig read LHC-Matchbox.in Herwig run LHC-Matchbox.run -N 10000 ``` It is recommended to change the name of the newly created .yoda file to something meaningful after every new run s.t. you don't have to repeat runs in the end when we compare them with each other. ``` mv LHC-Matchbox.yoda LHC-noShower.yoda ``` Now plot the results with ``` rivet-mkhtml LHC-noShower.yoda ``` Have a look at the produced histograms and try to explain why there is no Z pT and why in the peak region Z pT is just a delta function. You should also have a look at the .log file and understand the first few events. To have a more realistic simulation of the Z-boson, we uncomment some sections in the input file. Include the LO Parton shower BUT without initial and final state radiations. The relevant section in the input file should look like this: ``` ################################################## ## ShowerHandler(s) ################################################## # Initial and final state radiation set /Herwig/Shower/ShowerHandler:DoFSR No set /Herwig/Shower/ShowerHandler:DoISR No cd /Herwig/EventHandlers #set EventHandler:CascadeHandler NULL set EventHandler:CascadeHandler:MPIHandler NULL # switches off multi parton interactions set EventHandler:DecayHandler NULL # switches off particle decays set EventHandler:HadronizationHandler NULL # switches off hadronization ``` Now activate initial and final state radiation and repeat the run. ``` set /Herwig/Shower/ShowerHandler:DoFSR Yes set /Herwig/Shower/ShowerHandler:DoISR Yes ``` Compare your results with the first run. Why do pT spectra change but the reconstructed mass of the Z boson looks roughly the same? For the next task, we use a parton shower matched to NLO. Look at the parton shower selection section from the input file ``` ################################################## ## 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 ``` In the input file, we choose the "read Matchbox/LO-DefaultShower.in" option activating a LO angular ordered parton shower. Now choose the "read Matchbox/MCatNLO-DefaultShower.in" option, to activate the MC@NLO matched shower and run the event again. By now, you should have created several .yoda files. You can compare the corresponding calculations with ``` rivet-mkhtml *.yoda ``` For the final task in this section, activate multiple parton interactions, decays and hadronization options by uncommenting the relevant lines in the input file and compare the LO and the NLO matched shower simulations. You have finished all the main parts of the Herwig tutorial. You can choose to [[https://herwig.hepforge.org/tutorials/gettingstarted/matrixelements.html#some-scale-variations | dive deeper]] into parton shower variations, play around with different cuts and scales or continue with [[https://phab.hepforge.org/w/mcnet2019/thirdrun/ | Third run: minimum bias events and soft physics]]. ==A note on matrix elements== As you can see, the process selection section has been completely deactivated. This is because for a process like `P + P -> Z + X`, Herwig needs to use the external matrix element (ME) providers. However, the use of these tools in the docker/VM would be problematic due to some linking issues. In an installed (and properly linked) version of Herwig, one can easily use these external ME providers by selecting the hard-process in the `Process selection` and choosing the ME providers from the `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 ```

Simulation of a dijet production event at the LHC for 7 TeV center-of-mass energyIn this section, using the [[ we will simulate the 'P + P -> Z+X' process at 7 TeV center-of-mass energy and study the properties of the Z-boson with the help of 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 directoryMC_ZINC.html | MC_ZINC ]] Rivet analysis. There are different stages of event generation, each of which plays a crucial role in the correct simulation of Z-bosons at the LHC: Input file: * Hard matrix element * Parton Shower * Initial and final state radiation * Multiple parton interactions * Hadronizaion {F542868}During this exercise, we will see the different contributions to the Z shape observables, like the Z pT spectrum. Download the following input file and move it to your work directory: Input file:{F545117} Alternatively, you can use the following command-line codes: ``` mkdir wget https://phab-files.hepforge.org/file/download/q5by5q5tocxpquklfi34/PHID-FILE-ekoleb6f3inpmsz6xmbl/LHC-dijetMatchbox.in ``` cd LHC-dijetThe input file can be used for Z production and Z+Jets production. For now, all parts except for the hard matrix element are commented out. We will uncomment the necessary parts as we progress with the exercise. wget https://phab-files.hepforge.org/Run the input file/download/eh5glykvcuwpul6yvwzp/PHID-FILE-g4u3mgzeahyqyc7mcgvj/LHC-dijet.in without any modifications ``` Herwig read LHC-Matchbox.in You can run the dijey event,Herwig run LHC-Matchbox.run -N 10000 ``` It is recommended to change the name of the newly created .yoda file to something meaningful after every new run s.t. usingyou don't have to repeat runs in the end when we compare them with each other. ``` mv LHC-Matchbox.yoda LHC-noShower.yoda ``` Now plot the results with ``` Herwig readrivet-mkhtml LHC-dijet.innoShower.yoda ``` Have a look at the produced histograms and try to explain why there is no Z pT and why in the peak region Z pT is just a delta function. You should also have a look at the .log file and understand the first few events. You should see an error message appearing on your consoleTo have a more realistic simulation of the Z-boson, we uncomment some sections in the input file. Include the LO Parton shower BUT without initial and final state radiations. The relevant section in the input file should look like this: ``` Error: MatchboxFactory: No matrix elements have been found.################################################## ## ShowerHandler(s) ################################################## # Initial and final state radiation set /Herwig/Shower/ShowerHandler:DoFSR No Please check if your order of Alpha_s and Alpha_ew have the right value.set /Herwig/Shower/ShowerHandler:DoISR No cd /Herwig/EventHandlers #set EventHandler:CascadeHandler NULL set EventHandler:CascadeHandler:MPIHandler NULL # switches off multi parton interactions set EventHandler:DecayHandler NULL # switches off particle decays set EventHandler:HadronizationHandler NULL # switches off hadronization ``` 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 oNow activate initial and final state radiation and repeat the run. There is a section in the input files for choosing the possible external ME providers:``` set /Herwig/Shower/ShowerHandler:DoFSR Yes set /Herwig/Shower/ShowerHandler:DoISR Yes ``` Compare your results with the first run. Why do pT spectra change but the reconstructed mass of the Z boson looks roughly the same? For the next task, we use a parton shower matched to NLO. Look at the parton shower selection section from the input file ``` ################################################## ## 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 ``` In the input file, we choose the "read Matchbox/LO-DefaultShower.in" option activating a LO angular ordered parton shower. Now choose the "read Matchbox/MCatNLO-DefaultShower.in" option, to activate the MC@NLO matched shower and run the event again. By now, you should have created several .yoda files. You can compare the corresponding calculations with ``` rivet-mkhtml *.yoda ``` For the final task in this section, activate multiple parton interactions, decays and hadronization options by uncommenting the relevant lines in the input file and compare the LO and the NLO matched shower simulations. You have finished all the main parts of the Herwig tutorial. You can choose to [[https://herwig.hepforge.org/tutorials/gettingstarted/matrixelements.html#some-scale-variations | dive deeper]] into parton shower variations, play around with different cuts and scales or continue with [[https://phab.hepforge.org/w/mcnet2019/thirdrun/ | Third run: minimum bias events and soft physics]]. ==A note on matrix elements== As you can see, the process selection section has been completely deactivated. This is because for a process like `P + P -> Z + X`, Herwig needs to use the external matrix element (ME) providers. However, the use of these tools in the docker/VM would be problematic due to some linking issues. In an installed (and properly linked) version of Herwig, one can easily use these external ME providers by selecting the hard-process in the `Process selection` and choosing the ME providers from the `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 ``` 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.