Particle Physics Data Analysis

(Hewlett Packard Laboratory, 3rd Floor)



Given some data from high energy e+e- interactions, and some skeleton computer programs:

1. Introduction

This HTML file is located at

This experiment involves writing a computer program on a Hewlett Packard (HP) computer to read and analyse some data generated by high energy electron-positron interactions at CERN detected by the OPAL Detector at LEP (Large Electron-Positron Collider). A reasonable competency in programming in Fortran, C or C++ is essential. The HPs are Unix workstations, and therefore some experience of the Unix operating system would be desirable, but not essential. It will help to be taking the 3rd Year courses in nuclear and particle physics.

Typically, the electrons and positrons have 45 GeV energy each. They collide head on, making 90 GeV available in the laboratory. Various interactions take place. Sometimes the electron and positrons simply scatter elastically. At other times they annihilate and another lepton-antilepton pair is created. Often a quark-antiquark pair is created; the quark and anti-quark then combine with many more quarks and antiquarks to produce "ordinary" particles, such as pions and protons (collectively known as hadrons). The data you will analyse have been selected to be rich in hadrons - a so-called multi-hadron sample - and can be assumed to be predominantly caused by the production of quark-antiquark pairs.

It will be your job to analyse these data by writing a computer program to read the computer files and classify the interactions. You will be able to infer some properties of the original particles (electron and positron) and the produced quarks. You will also be able to infer characteristics of the so-called fragmentation process whereby quarks finally produce hadrons. You may be able to identify particular quark types. You may see the effects of gluon radiation.

2. The Data

The data are in file ~mbcheja/3rd_year_lab/pp_analysis/3rd_Year_Lab_Data. This is a large file. Don't try to copy it. You can read it. The best way is to make a link from your working directory with the command:

ln -s ~mbcheja/3rd_year_lab/pp_analysis/3rd_Year_Lab_Data .

(The final dot is important; it means ``this directory'', i.e., make a link of the same name in this directory.)

The data are a highly selected summary of part of the data held on the Particle Physics Group's research computer cluster. Moreover the format has been converted to a ``human readable'' form. The first few lines of the data file look like:

Run       4035, Event        202
        19 charged tracks (px, py, pz)
        18 electromagnetic clusters (E, theta, phi)
        12 hadron clusters (E, theta, phi)

Then follow:

Then the pattern is repeated.

The electromagnetic clusters can be caused either by electrons or photons. Electrons will also show up as charged tracks.

The hadron clusters are caused by hadrons --- pions, protons, neutrons and the like. Charged hadrons will also show up as charged tracks.

3. The Method

Your job is to write a computer program in Fortran, C or C++ to read the data. There are sample programs in ~mbcheja/3rd_year_lab/pp_analysis/skeleton.c (C language) or .cc (C++).

While developing your program, restrict the processing to the first few events to save computing time. When you have completed the development, you can read the full file.

4. The Analysis

Begin by calculating a few simple quantities, e.g.:

While doing this, think what physical explanations there might be, and what properties of the particles you might be able to infer.

Do the clusters have similar properties?

The particles are produced in jets.

There is a prediction that if quarks have a spin of one half, the angular distribution of jets (the probability of a jet going in a particular direction per unit solid angle) is proportional to 1 + cos^2 theta, where theta is the angle to the intial electron direction.

The jets are produced from the original quarks by a process of further quark-antiquark production, leading to formation of "ordinary" particles, which acquire some trasverse momentum on account of the internal motion of quarks in the particles. What can you say about the kinetic energy of quarks in particles?

4. References

John Allison, 19th September 1997