void skim() {

   //Get old file, old tree and set top branch address
   auto oldfile = TFile::Open("output_Coulomb_LER_study_100.root");
   TChain* chain=new TChain("tree");
   chain->Add("output_Coulomb_LER_study_100.root");
   // Deactivate all branches
   chain->SetBranchStatus("*",0);
   // Activate 4 branches only: our skim
   chain->SetBranchStatus("StrSimHits",1);

   //Create a new file + a clone of old tree header. Do not copy events
   TFile * newfile = TFile::Open("small.root","recreate");
   TTree * newtree = chain->CloneTree(0);
 
  
   // Here we copy the branches
   newtree->CopyEntries(chain);

  
   //newtree->CopyEntries(oldtree);

   // Flush to disk
   newfile->Write();

   // Clean
   // delete oldfile;
   // delete newfile;
}