#include<stdio.h>
#include<math.h>
#include "TH1.h"
#include "TCanvas.h"

using namespace std;


void test(){

// Book histogram for distribution
TH1 *hist_costh = new TH1F("costh", "cosTheta", 20, -1.0, 1.0);


hist_costh->GetXaxis()->SetLimits(-1.0, 1.0);
//hist_costh->GetXaxis()->SetRangeUser(-1.0, 1.0);

  // Load shared library  
 gSystem->Load("/home/pramod/Packages/MG5_aMC_v2_7_3/ExRootAnalysis/libExRootAnalysis.so"); // Use your ExRootAnalysis path here
 gSystem->Load("libPhysics");
 gSystem->Load("libMathCore");
// Create chain of root trees
 TChain chain("LHEF");
 chain.Add("go-1.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 *branchParticle = treeReader->UseBranch("Particle");

//print the number of Events in the file
cout << "Number of evetns = " << numberOfEntries << endl;


// Loop over all event
  for(Int_t entry = 0; entry <numberOfEntries; ++entry) {
// Load selected branches with data from specified event
treeReader->ReadEntry(entry);


// Create pointer to store the location of l- and l+
int e_pos;

// Initialize the pointers
e_pos=0;



// Loop over all particle in a given event
// Skip 1st two particles as they are incoming particles
for(Int_t i = 2; i < branchParticle->GetEntries(); ++i) {
	// enlisting the index of generated particles
// i'th particle of one event
	      	TRootLHEFParticle *p = (TRootLHEFParticle*) branchParticle->At(i);

//electron
if (p->PID == 11){ 
	e_pos=i;
		}
		            
                                                        } 
// Acess final state particles with found positon   							
TRootLHEFParticle *e1 = (TRootLHEFParticle*) branchParticle->At(e_pos);


TLorentzVector vece;
vece.SetPtEtaPhiM(e1->PT, e1->Eta, e1->Phi, e1->M);
hist_costh->Fill(vece.CosTheta());
                                 
}
        hist_costh -> Draw();

} //main func ends here