void addBranch() {
TFile f("kapu.root","update");
Float_t  pt[16];
TTree *t3 = (TTree*)f.Get("Tchep");
TBranch *newBranch = t3-> Branch("pt",&pt,"pt");
//read the number of entries in the t3
Int_t nentries = (Int_t)t3->GetEntries();
/*for (Int_t i = 0; i < nentries; i++){
new_v= gRandom->Gaus(0,1);
newBranch->Fill();
}
*/
t3->Write("",TObject::kOverwrite);     // save only the new version of the tree
}