#include <TFile.h>
#include <TH1.h> 
#include <TTreeReader.h>
#include <TTreeReaderValue.h>
#include <TTreeReaderArray.h>
TH1F *PTrans=new TH1F("Pt","Transverse Momentum",10000,0,100);
TFile*MyFile=TFile::Open("dropbox/AMPT_DATA/Au_ampt0000001/ampt_afterART.root");
TTreeReader myReader("AMPT",MyFile);
TTreeReaderArray<Float_t> myPx(myReader,"Px");
TTreeReaderArray<Float_t> myPy(myReader,"Py");
while(myReader.Next()){
  for( Float_t​ ​px:Px){
    for( Float_t​ ​py:Py){
      PTrans->Fill(px+py);
}}}