source: trunk/doc/README@ 16

Using ExRootAnalysis package for PGS data analysis
==================================================


Author: Pavel Demin




Introductory remarks
====================


The ExRootAnalysis is a package designed to simplify ROOT tree production and
analysis. The purpose of this tutorial is to present current functionality of
this software.




Quick start with ExRootAnalysis
===============================

Make sure that ROOT is installed and ROOTSYS, PATH, LD_LIBRARY_PATH,
DYLD_LIBRARY_PATH are configured correctly.

Command to unpack the source code:

   tar -zxf ExRootAnalysis.tar.gz

Commands to create shared library for interactive ROOT session:

   cd ExRootAnalysis

   make

Commands to create static library for linking with PGS:

   cd ExRootAnalysis

   make static




Simple analysis using TTree::Draw
=================================


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

Start ROOT and load shared library:

   root
   gSystem->Load("lib/libExRootAnalysis.so");

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

   TFile::Open("pgs_events.root");
   LHCO->Draw("Electron.PT");
   TBrowser browser;

Note 1: LHCO - tree name, it can be learnt e.g. from TBrowser

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

Complete description of all branches can be found in

   ExRootAnalysis/doc/RootTreeDescription.html




Macro-based analysis
====================


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

Basic analysis macro:

{
  // Load shared library
  gSystem->Load("lib/libExRootAnalysis.so");
  gSystem->Load("libPhysics");

  // Create chain of root trees
  TChain chain("LHCO");
  chain.Add("pgs_events.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 *branchJet = treeReader->UseBranch("Jet");
  TClonesArray *branchElectron = treeReader->UseBranch("Electron");
  
  // Book histograms
  TH1 *histJetPT = new TH1F("jet_pt", "jet P_{T}", 100, 0.0, 100.0);
  TH1 *histMass = new TH1F("mass", "M_{inv}(e_{1}, e_{2})", 100, 40.0, 140.0);

  // 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 jet
    if(branchJet->GetEntries() > 0) {

      // Take first jet
      TRootJet *jet = (TRootJet*) branchJet->At(0);
      
      // Plot jet transverse momentum
      histJetPT->Fill(jet->PT);
      
      // Print jet transverse momentum
      cout << jet->PT << endl;
    }

    TRootElectron *elec1, *elec2;
    TLorentzVector vec1, vec2;

    // If event contains at least 2 electrons
    if(branchElectron->GetEntries() > 1) {

      // Take first two electrons
      elec1 = (TRootElectron *) branchElectron->At(0);
      elec2 = (TRootElectron *) branchElectron->At(1);

      // Create two 4-vectors for the electrons
      vec1.SetPtEtaPhiM(elec1->PT, elec1->Eta, elec1->Phi, 0.0);
      vec2.SetPtEtaPhiM(elec2->PT, elec2->Eta, elec2->Phi, 0.0);

      // Plot their invariant mass
      histMass->Fill((vec1 + vec2).M());
    }
  }

  // Show resulting histograms
  histJetPT->Draw();
  histMass->Draw();
}




More advanced macro-based analysis
==================================


ExRootAnalysis/test contains macro Example.C using class ExRootTreeReader to
access data and class ExRootResult to manage histograms booking and output

Here are commands to run this macro:

   cd ExRootAnalysis/test
   $EDITOR test.list
   root
   gSystem->Load("../lib/libExRootAnalysis.so");
   .X Example.C("test.list");

Note: file test.list should contain list of root files that you would like to
analyse (one root file per line)