Part 1: Using MadGraph to generate parton-level events

In the first part of the exercise, we will use the matrix element generator MadGraph5 _amc@NLO, or in short MG5. MG5 can perform automatic matrix element predictions for many processes at leading and next-to-leading order accuracy in QCD. Because of it's ease of use for processes both in and beyond the standard model, it is one of the most widely used software tools to model the hard interaction.

Setting up CMSSW

While MG5 can be used completely standalone, we will still set up a CMSSW area just to make sure that everyone has the same libraries loaded and can profit from pre-installed software such as LHAPDF.

 
source /cvmfs/cms.cern.ch/cmsset_default.sh
scram p CMSSW_7_1_30
cd CMSSW_7_1_30/src
cmsenv
cd -

Task 1: Using standalone MG5

In this part of the exercise, we will learn how to use MG5 to generate W boson events in proton-proton collisions.

Input files

What type of events are produced and what settings are used is defined in the MadGraph steering files, which are called run_card.dat and proc_card.dat.

In CMS, we maintain a large archive of these steering files for all samples that are generated in the official CMS production. The cards are stored in a GitHub repository. Let's check it out:

 
git clone git@github.com:AndreasAlbert/genproductions.git -b CMSDAS2018_26x

The cards we are interested in are located in this subfolder:

 
ls genproductions/bin/MadGraph5_aMCatNLO/cards/examples/wplustest_4f_LO/
wplustest_4f_LO_run_card.dat

wplustest_4f_LO_proc_card.dat

The proc_card file defines process to be generate (hence the name proc). Look at the syntax:

generate p p > w+, w+ > l+ vl

What does this statement mean? Refer to this MG tutorial for documentation on the syntax INSERTLINKHERE. What modifications of this statement can you think of? What could we replace the "w+ > l+ vl" part with?

Setting up Madgraph

We will download MG5 version 2.6.0 from a CMS-hosted mirror of the official release:

wget https://cms-project-generators.web.cern.ch/cms-project-generators/MG5_aMC_v2.6.0.tar.gz
tar xf MG5_aMC_v2.6.0.tar.gz
rm MG5_aMC_v2.6.0.tar.gz
cd MG5_aMC_v2_6_0

While all interaction with MG5 can be entirely scripted (which is useful for larger scale production, as we shall see later), in this exercise it will be instructive to use the interactive command shell provided by MG5.

./bin/mg5

You will be asked whether you want to update to a newer version of MG5. Don't.

To get the generation started, we can now paste the contents of the proc_card line by line. What does each line do? Make sure to pay attention to the console output you get in reply to your input.

Tip: If you want to execute a shell command from inside of the MG5 console, preface it with an exclamation mark, e.g."

!cat  ../genproductions/bin/MadGraph5_aMCatNLO/cards/examples/wplustest_4f_LO/wplustest_4f_LO_proc_card.dat 

The generation can be started by typing launch. MG5 will ask you a few more questions. When asked about the run_card, one can either use a default run card by just pressing ENTER, edit the default run_card by hand or provide a path to run card of one's choice. Please provide the path to the pre-made run_card:

../genproductions/bin/MadGraph5_aMCatNLO/cards/examples/wplustest_4f_LO/wplustest_4f_LO_run_card.dat

-- AndreasAlbert - 2018-08-02

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