text: #
  # Plot dimuon mass distribution for SWC HEP 2015
  #
  from ROOT import TFile, TBrowser, TH1D
  
  
  file_events = TFile("test_data/events.root")
  tree = file_events.Get("events")
  nEvents = tree.GetEntries()
  print 'Number of events = '+ str(nEvents)
  
  # xrange(n) = 0,1,2,...,n-1
  #for iEv in xrange(nEvents):
  #    tree.GetEntry(iEv)
  #    nParticles = tree.nPart
  #    print 'Number of particle = ' + str(nParticles)
  
  hist_dimuon_mass_nBins = 100
  hist_dimuon_mass_massMin = 0
  hist_dimuon_mass_massMax = 500
  
  h = TH1D("histo_dimuon_mass","di-muon mass spectrum;m [GeV^{2}/c];Entries", hist_dimuon_mass_nBins, hist_dimuon_mass_massMin, hist_dimuon_mass_massMax )
  h.Draw()
  #raw_input("Press return to exit.")
  