\chapter{Introduction}
\label{INTRO}

  {\it What follows is an extract from a thesis latexed with the G.U. thesis.cls
       class file; it is given here as an example for latexing with the phd-report.cls
       class file and is not intended to be an example of a PhD report.}

  High energy collisions of sub-atomic particles can result in events containing
  a high multiplicity of hadronic particles. An example of such an event is the
  production of Z Bosons at LEP, an $e^{+} e^{-}$ collider with a
  center of mass (CM) energy of 91.2 \unt{GeV}, where the subsequent hadronic decay
  of the Z Boson results in two or more jets of particles with a total event
  multiplicity of, on average, about 30 particles mostly charged and
  neutral pions, Figure \ref{FIG_RECON}.

\begin{figure}
%\vspace{-10mm}
\begin{center}
\epsfig{file=figures/dali.ps,height=18cm}
\end{center}
%\vspace{-10mm}
\caption[Reconstruction of a typical hadronic event in the ALEPH detector]
{
\label{FIG_RECON}
         Reconstruction of a typical hadronic event in ALEPH
         detector. The distinguishing feature of hadronic
         events is large multiplicity of visible particles and
         a large fraction of the center of mass energy that
         they carry. The overall reconstructed energy in this
         event is 83 \unt{GeV}, the charged component totals 48 \unt{GeV}
         from 18 tracks.
}
\end{figure}

  Invariant mass spectra, constructed from selected particle tracks,
  reveal the presence of short-lived `mother' particles which present
  themselves as peaks on a combinatorial background.
  One example is the decay of the omega vector meson (mass 782 \unt{MeV/c^2})
  to three pions:
  $\omega \rightarrow \pi^{+} \pi^{-} \pi^{0}$.
  The $\omega$ is reconstructed from measurements of the momentum of
  the daughter particles;
  charged pions are measured in a tracking chamber with good momentum
  resolution while the neutral pion, decaying mainly via the two-photon channel
  $\pi^{0} \rightarrow \gamma \gamma$, is reconstructed from measurements of
  its daughter photons in a calorimeter typically with much poorer momentum
  resolution. Measurements of the $\omega$ and other examples can be found
  in \cite{MEGA_PAPER}.

  The selection of neutral pions
  with a high purity while maintaining also
  a high efficiency can be important in the formation of statistically
  significant mass spectra. Neutral pion reconstruction plays a
  dominant role in the resultant purity and momentum resolution
  of reconstructed mother particles.
  Pion momentum resolution
  tends to be poor due to calorimeter resolution, purity tends to
  be low due to a large combinatorial background from wrong combinations of
  photon pairs, a situation which becomes worse for higher multiplicity events
  and poorer calorimeter resolution. This thesis presents a study of a 
  Ranking method for significantly
  increasing the purity of reconstructed neutral pions.
  This is achieved by identifying and removing background formed
  by incorrect combinations of photon pairs.

  First, an overview of particle physics and particle detectors is given
  in Chapter \ref{DETECTOR}; some emphasis is placed on the ALEPH detector as
  monte-carlo simulations used in this thesis are based on this experiment.
  {\it And so on....}
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