void AddBranch() {
  // TFILE
  TFile f("EXISITING_FILE","update");
  // Variable for filling
  int isMuonStream;
  // Exisiting ttree
  TTree *t = (TTree*)f.Get("mini");
  // New branches to add
  TBranch *newBranch = t->Branch("isMuonStream",&isMuonStream,"isMuonStream/i");
  //read the number of entries in the tree
  int nentries = (int)t->GetEntries();
  // Loop
  for (int i = 0; i < nentries; i++){
    // Calc value
    isMuonStream = 1;
    // Fill new branch
    newBranch->Fill();
  }
  // Save the updated version of the ttree
  t->Write("",TObject::kOverwrite);
  // Close file
  f.Close();

}