.! $ROOTSYS/test/eventexe 1000 1 1 1 

TString dir = "$ROOTSYS/test/Event.root";
gSystem->ExpandPathName(dir);
const auto filename = gSystem->AccessPathName(dir) ? "./Event.root" : "$ROOTSYS/test/Event.root";

TFile oldfile(filename);
TTree *oldtree;
oldfile.GetObject("T", oldtree);

for (auto activeBranchName : {"event", "fNtrack", "fNseg", "fH"}) {
   oldtree->SetBranchStatus(activeBranchName, 1);
}

TFile newfile("small.root", "recreate");
auto newtree = oldtree->CloneTree(0);

newtree->GetBranch("fH")->SetFile("small_fH.root");
newtree->CopyEntries(oldtree);

newtree->Print();
newfile.Write();