The First run

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For the first run, we are going to simulate a LEP event with the default setup and look at the output files.

First, create a folder

mkdir herwig-tutorial
cd herwig-tutorial

and copy the input file LEP.in that you will find under {Herwigpath}/share/Herwig/ or here into that folder.

You can download the input file via

wget  https://phab-files.hepforge.org/file/download/d3y555lituggiomcwatj/PHID-FILE-hirg3s3rz7xph4yuaij6/LEP.in

Have a look at the input file with the tool of your choice and try to understand what process is being simulated.

gedit LEP.in
vi LEP.in
less LEP.in
cat LEP.in
emacs LEP.in

Then execute the following commands in order to simulate 10000 LEP events:

Herwig read LEP.in
Herwig run LEP.run -N 10000

In the next step, have a look at the "LEP.log" file that displays the first 10 events of that run. Each event is made up of individual steps that reflect the treatment of the event as it passes through the various stages of the generator (hard subprocess, parton shower, hadronization and decays). Try to make sense of the numbers displayed and try to find the individual steps in the event record (the LEP.log file).

Looking at numbers is tedious. In order to make our life easier we now try to compare the simulation with data from the ALEPH experiment.

For that purpose, we need to modify the LEP.in input file and include the following part

cd /Herwig/Generators
read snippets/Rivet.in
insert /Herwig/Analysis/Rivet:Analyses 0 ALEPH_1996_S3486095

right above the line "saverun LEP EventGenerator".

Now we need to run Herwig again. The steps are the same as above but now we should have created a "LEP.yoda" file. A yoda file is a file created by YODA which is a lightweight common system for MC event generator validation analyses, particularly as the core histogramming system in Rivet. In order to plot this yoda file, we use the following command

rivet-mkhtml LEP.yoda

This creates a folder named "rivet-plots" that collects all the created histograms of the ALEPH analysis. You can display the analysis in your browser by opening the file.

rivet-plots/index.html

Have a look at the created histograms and try to roughly understand what is being measured.
Also, change the name of the .yoda file to LEP-default.yoda with

mv LEP.yoda LEP-default.yoda

Switching off the Parton Shower

In the next step, we switch off the Parton Shower and compare the simulation to the one that includes the Parton Shower.

Add the following lines to the input file

cd /Herwig/EventHandlers
set EventHandler:CascadeHandler NULL

right below

set EventGenerator:EventHandler:LuminosityFunction:Energy 91.2

Then, run Herwig again with 10000 Events

Herwig read LEP.in
Herwig run LEP.run -N 10000

and plot the two yoda files together with

rivet-mkhtml LEP-default.yoda LEP.yoda

Now, have a look at the histograms. Why is the mean charged multiplicity larger without a Parton Shower?

Optional: You can also include more Rivet analyses.