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Reference Guide
pythiaExample.C File Reference

Detailed Description

Using Pythia6 with ROOT.

To make an event sample (of size 100) do

shell> root
root [0] .L pythiaExample.C
root [1] makeEventSample(1000)

To start the tree view on the generated tree, do

shell> root
root [0] .L pythiaExample.C
root [1] showEventSample()

The following session:

shell> root
root [0] .x pythiaExample.C(500)

will execute makeEventSample(500) and showEventSample()

Alternatively, you can compile this to a program and then generate 1000 events with

./pythiaExample 1000

To use the program to start the viewer, do

./pythiaExample -1

NOTE 1: To run this example, you must have a version of ROOT compiled with the Pythia6 version enabled and have Pythia6 installed. The statement gSystem->Load("$HOME/pythia6/libPythia6"); (see below) assumes that the directory containing the Pythia6 library is in the pythia6 subdirectory of your $HOME. Locations that can specify this, are:

Root.DynamicPath resource in your ROOT configuration file
(/etc/root/system.rootrc or ~/.rootrc).
Runtime load paths set on the executable (Using GNU ld,
specified with flag `-rpath').
Dynamic loader search path as specified in the loaders
configuration file (On GNU/Linux this file is
For Un*x: Any directory mentioned in LD_LIBRARY_PATH
For Windows: Any directory mentioned in PATH

NOTE 2: The example can also be run with ACLIC:

root > gSystem->Load("libEG");
root > gSystem->Load("$ROOTSYS/../pythia6/libPythia6"); //change to your setup
root > gSystem->Load("libEGPythia6");
root > .x pythiaExample.C+
#include "TApplication.h"
#include "TPythia6.h"
#include "TFile.h"
#include "TError.h"
#include "TTree.h"
#include "TClonesArray.h"
#include "TH1.h"
#include "TF1.h"
#include "TStyle.h"
#include "TLatex.h"
#include "TCanvas.h"
#include "Riostream.h"
#include <cstdlib>
using namespace std;
#define FILENAME "pythia.root"
#define TREENAME "tree"
#define BRANCHNAME "particles"
#define HISTNAME "ptSpectra"
#define PDGNUMBER 211
// This function just load the needed libraries if we're executing from
// an interactive session.
void loadLibraries()
#ifdef __CINT__
// Load the Event Generator abstraction library, Pythia 6
// library, and the Pythia 6 interface library.
gSystem->Load("$ROOTSYS/../pythia6/libPythia6"); //change to your setup
// nEvents is how many events we want.
int makeEventSample(Int_t nEvents)
// Load needed libraries
// Create an instance of the Pythia event generator ...
TPythia6* pythia = new TPythia6;
// ... and initialise it to run p+p at sqrt(200) GeV in CMS
pythia->Initialize("cms", "p", "p", 200);
// Open an output file
TFile* file = TFile::Open(FILENAME, "RECREATE");
if (!file || !file->IsOpen()) {
Error("makeEventSample", "Couldn;t open file %s", FILENAME);
return 1;
// Make a tree in that file ...
TTree* tree = new TTree(TREENAME, "Pythia 6 tree");
// ... and register a the cache of pythia on a branch (It's a
// TClonesArray of TMCParticle objects. )
TClonesArray* particles = (TClonesArray*)pythia->GetListOfParticles();
tree->Branch(BRANCHNAME, &particles);
// Now we make some events
for (Int_t i = 0; i < nEvents; i++) {
// Show how far we got every 100'th event.
if (i % 100 == 0)
cout << "Event # " << i << endl;
// Make one event.
// Maybe you want to have another branch with global event
// information. In that case, you should process that here.
// You can also filter out particles here if you want.
// Now we're ready to fill the tree, and the event is over.
// Show tree structure
// After the run is over, we may want to do some summary plots:
TH1D* hist = new TH1D(HISTNAME, "p_{#perp} spectrum for #pi^{+}",
100, 0, 3);
char expression[64];
char selection[64];
sprintf(selection,"%s.fKF==%d", BRANCHNAME, PDGNUMBER);
// Normalise to the number of events, and the bin sizes.
hist->Scale(3 / 100. / hist->Integral());
hist->Fit("expo", "QO+", "", .25, 1.75);
TF1* func = hist->GetFunction("expo");
func->SetParNames("A", "- 1 / T");
// and now we flush and close the file
return 0;
// Show the Pt spectra, and start the tree viewer.
int showEventSample()
// Load needed libraries
// Open the file
TFile* file = TFile::Open(FILENAME, "READ");
if (!file || !file->IsOpen()) {
Error("showEventSample", "Couldn;t open file %s", FILENAME);
return 1;
// Get the tree
TTree* tree = (TTree*)file->Get(TREENAME);
if (!tree) {
Error("showEventSample", "couldn't get TTree %s", TREENAME);
return 2;
// Start the viewer.
// Get the histogram
TH1D* hist = (TH1D*)file->Get(HISTNAME);
if (!hist) {
Error("showEventSample", "couldn't get TH1D %s", HISTNAME);
return 4;
// Draw the histogram in a canvas
TCanvas* canvas = new TCanvas("canvas", "canvas");
TF1* func = hist->GetFunction("expo");
char expression[64];
sprintf(expression,"T #approx %5.1f", -1000 / func->GetParameter(1));
TLatex* latex = new TLatex(1.5, 1e-4, expression);
return 0;
void pythiaExample(Int_t n=1000) {
#ifndef __CINT__
int main(int argc, char** argv)
TApplication app("app", &argc, argv);
Int_t n = 100;
if (argc > 1)
n = strtol(argv[1], NULL, 0);
int retVal = 0;
if (n > 0)
retVal = makeEventSample(n);
else {
retVal = showEventSample();
return retVal;
Christian Holm Christensen

Definition in file pythiaExample.C.