source: git/README_4LHCb@ 9e991f8

Delphes 4 LHCb
==============

The card "delphes_card_prelLHCb.tcl" contains a preliminary parametrization of the LHCb detector. 

- ParticlePropagator

particles are propagated in a constant B field.

- ChargedHadronMomentumSmearing/ElectronEnergySmearing/MuonMomentumSmearing

charged particles momenta are smeared according to detector resolution

- TrackMerger

charged particles are merged into single collection for simpler future processing

- ImpactParameterSmearing

charged particles transverse IP are smeared according to known LHCb tracking
performance.


- IdentificationMap

This module is a recent addition in order to map particle misindentification rates
and reconstruction efficiencies. 

An example is given in the card but can be expanded if needed. 

- ECAL/HCAL

Calorimeter modules are used to parametrize the energy response and angular
resolution of neutral objects such as photons/neutral hadrons. 

- TreeWriter 

user specifies here which collections are stored in the output. 
By default tracks, neutral hadrons and photons are stored in this card.
Tracks contain muons, electrons, and charged hadrons. 


Quick start with Delphes
========================

1) Compile:

   make

2) Simulate p p -> b b~ events 

  tar -xzvf pp2bb.hep.tgz
  ./DelphesSTDHEP examples/delphes_card_prelLHCb.tcl delphes_output.root pp2bb.hep


For more details, please visit:

https://cp3.irmp.ucl.ac.be/projects/delphes



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::Open("delphes_output.root");
   Delphes->Draw("Track.PT");
   TBrowser browser;

Note 1: Delphes - tree name, it can be learned e.g. from TBrowser

Note 2: Track - branch name; PT - variable (leaf) of this branch

Complete description of all branches can be found in

   doc/RootTreeDescription.html

This information is also available at

   https://cp3.irmp.ucl.ac.be/projects/delphes/wiki/WorkBook/RootTreeDescription


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 *branchTrack = treeReader->UseBranch("Track");

  // Book histograms
  TH1 *histTrackPT = new TH1F("track pt", "track P_{T}", 50, 0.0, 20.0);

  // 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 track
    if(branchTrack->GetEntries() > 0)
    {
      // Take first track
      Track *track = (Track*) branchTrack->At(0);
      
      // Plot track transverse momentum
      histTrackPT->Fill(track->PT);
      
      // Print electron transverse momentum and Particle Data Group ID
      cout << track->PID<< "   " << track->PT << endl;
    }

  }

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