README.session2 for GlueX Tutorial Jan. 7, 2011

12/8/2010 DL


The goals of this session are:

A. Run a DANA(JANA) program to reconstruct events
B. Inspect root file and generate TSelector for tree
C. Analyze the reconstructed events using customized ROOT TSelector

===========================================================


-------------------------------------------------
A. Run a DANA(JANA) program to reconstruct events
-------------------------------------------------

1. Run hd_root with the phys_tree plugin to reconstruct rho
   events and write results into ROOT tree.

   > ln -s ../HandsOnSession1/sim_rho_smeared.hddm
   > hd_root -PPLUGINS=phys_tree sim_rho_smeared.hddm -o sim_rho_smeared.root


-------------------------------------------------
B. Inspect root file and generate TSelector for tree
-------------------------------------------------

1. Inspect contents of root file.

   > root sim_rho_smeared.root
   ...
   root [1] PHYSICS->cd()
	root [2] recon->Draw("Npip")
	root [3] recon->Draw("pip.p.Theta()")
	

2. Use root to generate a TSelector skeleton class from the phys_tree
   object.

   > root sim_rho_smeared.root
   ...
   root [1] PHYSICS->cd()
	root [2] recon->MakeSelector("mk_hists")
	root [3] .q


-------------------------------------------------
C. Analyze the reconstructed events using a ROOT TSelector
-------------------------------------------------

1. Edit the mk_hists.C and mk_hists.h files to produce histograms.

   > nedit mk_hists.h
	   {Add declaration of TH1D *mass_rho to class declaration}

   > nedit mk_hists.C
      {Add the following line to the mk_hists::Begin method}

	      mass_rho = new TH1D("mass_rho", "Mass of #pi^{+} + #pi^{-} for all combos", 200, 0.0, 1.4);


	   {Add the following lines to the mk_hists::Process method}
		   
			GetEntry(entry);
			if(Npip>kMaxpip)Npip = kMaxpip;
			if(Npim>kMaxpim)Npim = kMaxpim;
			
			// Copy local elements into TLorentzVector objects
			TLorentzVector *pip = new TLorentzVector[Npip];
			for(Int_t i=0; i<Npip; i++){
				pip[i] = pip[i].SetPxPyPzE(pip_p_fP_fX[i], pip_p_fP_fY[i], pip_p_fP_fZ[i], pip_p_fE[i]);
			}
			TLorentzVector *pim = new TLorentzVector[Npim];
			for(Int_t i=0; i<Npim; i++){
				pim[i] = pim[i].SetPxPyPzE(pim_p_fP_fX[i], pim_p_fP_fY[i], pim_p_fP_fZ[i], pim_p_fE[i]);
			}
			
			for(Int_t i=0; i<Npip; i++){
				for(Int_t j=0; j<Npim; j++){
					TLorentzVector rho = pip[i] + pim[j];
					mass_rho->Fill(rho.M());
				}
			}


3. Run the selector and plot the pi+ pi- mass
	
   > root sim_rho_smeared.root
   ...
   root [1] PHYSICS->cd()
	root [2] recon->Process("mk_hists.C")
	root [3] mass_rho->Draw();