Delphes

Delphes is a C++ framework, performing a fast multipurpose detector response simulation.

More details can be found on the Delphes website https://delphes.github.io

Quick start with Delphes

Commands to get the code:

wget http://cp3.irmp.ucl.ac.be/downloads/Delphes-3.5.1.tar.gz

tar -zxf Delphes-3.5.1.tar.gz

Commands to compile the code:

cd Delphes-3.5.1

make

Finally, we can run Delphes:

./DelphesHepMC3

Command line parameters:

./DelphesHepMC3 config_file output_file [input_file(s)]
  config_file - configuration file in Tcl format
  output_file - output file in ROOT format,
  input_file(s) - input file(s) in HepMC format,
  with no input_file, or when input_file is -, read standard input.

Let's simulate some Z->ee events:

wget http://cp3.irmp.ucl.ac.be/downloads/z_ee.hep.gz
gunzip z_ee.hep.gz
./DelphesSTDHEP cards/delphes_card_CMS.tcl delphes_output.root z_ee.hep

or

curl -s http://cp3.irmp.ucl.ac.be/downloads/z_ee.hep.gz | gunzip | ./DelphesSTDHEP cards/delphes_card_CMS.tcl delphes_output.root

For more detailed documentation, please visit https://delphes.github.io/workbook

Configure Delphes on lxplus.cern.ch

git clone https://github.com/delphes/delphes Delphes

cd Delphes

source /cvmfs/sft.cern.ch/lcg/views/LCG_105/x86_64-el9-gcc12-opt/setup.sh

make

Install Delphes from conda-forge

Delphes is also available as a pre-built binary package from conda-forge.

The scripts used to build this package are available at

https://github.com/conda-forge/delphes-feedstock

Command to install the package in a conda environment:

conda install --channel conda-forge delphes

Commands to run the Z->ee example from the 'Quick start' section above in a conda environment:

wget http://cp3.irmp.ucl.ac.be/downloads/z_ee.hep.gz
gunzip z_ee.hep.gz
DelphesSTDHEP $CONDA_PREFIX/cards/delphes_card_CMS.tcl delphes_output.root z_ee.hep

where the CONDA_PREFIX environment variable is automatically set when the conda environment is activated.

The cards and examples directories from this repository are also installed under $CONDA_PREFIX.

Simple analysis using TTree::Draw

Now we can start ROOT and look at the data stored in the output ROOT file.

Start ROOT and load Delphes shared library:

root -l
gSystem->Load("libDelphes");

Open ROOT file and do some basic analysis using Draw or TBrowser:

TFile *f = TFile::Open("delphes_output.root");
f->Get("Delphes")->Draw("Electron.PT");
TBrowser browser;

Notes:

• Delphes is the tree name. It can be learned e.g. from TBrowser.
• Electron is the branch name.
• PT is a variable (leaf) of this branch.

Complete description of all branches can be found in doc/RootTreeDescription.html.

This information is also available at this link.

Macro-based analysis

Analysis macro consists of histogram booking, event loop (histogram filling), histogram display.

Start ROOT and load Delphes shared library:

root -l
gSystem->Load("libDelphes");

Basic analysis macro:

{
  // Create chain of root trees
  TChain chain("Delphes");
  chain.Add("delphes_output.root");

  // Create object of class ExRootTreeReader
  ExRootTreeReader *treeReader = new ExRootTreeReader(&chain);
  Long64_t numberOfEntries = treeReader->GetEntries();

  // Get pointers to branches used in this analysis
  TClonesArray *branchElectron = treeReader->UseBranch("Electron");

  // Book histograms
  TH1 *histElectronPT = new TH1F("electron pt", "electron P_{T}", 50, 0.0, 100.0);

  // Loop over all events
  for(Int_t entry = 0; entry < numberOfEntries; ++entry)
  {

    // Load selected branches with data from specified event
    treeReader->ReadEntry(entry);

    // If event contains at least 1 electron
    if(branchElectron->GetEntries() > 0)
    {
      // Take first electron
      Electron *electron = (Electron*) branchElectron->At(0);

      // Plot electron transverse momentum
      histElectronPT->Fill(electron->PT);

      // Print electron transverse momentum
      cout << electron->PT << endl;
    }

  }

  // Show resulting histograms
  histElectronPT->Draw();
}

More advanced macro-based analysis

The examples directory contains ROOT macros Example1.C, Example2.C and Example3.C.

Here are the commands to run these ROOT macros:

root -l
.X examples/Example1.C("delphes_output.root");

or

root -l examples/Example1.C'("delphes_output.root")'