double mtm (double pt, double m) {
  return sqrt (pt*pt + m*m) - m;
}


int ttree_project_fun () {
  TFile* fin = new TFile ("auau_1.23AGeV_8dsts.root");
  TTree* tin = (TTree*) fin->Get ("wars_tree");
  TCut cSelectProtons ("mass>650 && mass<1200");
  tin->Draw ( "mtm (pt,mass)" , cSelectProtons ); 
  return 0;
}