\chapter{Introduction - How the TRD works}

The main purpose of the ALICE \ac{TRD} is to provide electron identification in the central barrel for momenta above 1~\GeVc.
Below this momentum electrons can be identified via specific energy loss measurement in the TPC.
Above 1~\GeVc transition radiation (TR) from electrons passing a radiator can be exploited in concert with the specific energy loss in
a suitable gas mixture to obtain the necessary pion rejection capability.
In conjunction with data from the ITS and the TPC it is possible to study the production of light and heavy vector-meson
resonances and the dilepton continuum both in pp as well as in Pb-Pb collisions.
Exploiting the excellent impact parameter resolution of the ITS it is furthermore possible to reconstruct open charm
and open beauty in semi-leptonic decays.

The TRD was designed to derive a fast trigger for charged particles with high momentum.
It is part of the Level 1 trigger and can significantly enhance the recorded $\Upsilon$-yields, high-$p_T$ J/$\Psi$, the
high-mass part of the dilepton continuum as well as jets.
The design parameters of the TRD are the following:

\begin{description}

\item[Pion rejection capability]

This is governed by the signal-to-background ratio in the measurement of J/$\Psi$ production and its $p_T$ dependence.
This led to the design goal for the pion rejection capability of a factor 100 for momenta above 1~\GeVc in central Pb-Pb collisions.
The measurement of the lighter vector-mesons and the determination of the continuum between the J/$\Psi$ and the $\Upsilon$
will only be feasible when this level of rejection can be reached.

\item[Position and momentum resolution]

It is crucial to be able to match to the TPC in order to exploit the combined momentum resolution
leading to an overall mass resolution of about 100~\MeVcc at the $\Upsilon$-mass (for B=0.4~T).
The required pointing accuracy for electrons needs to be on the level of a fraction of a TPC pad.
The anticipated momentum resolution of the TRD of 3.5 (4.7)\% at 5~\GeVc (depending on multiplicity)
will crucially determine the sharpness of the trigger threshold in $p_T$ as well as the capability
to reject fake tracks.

\item[Radiation length]

It has to be minimized in order to reduce Bremsstrahlung leading to incorrect momentum determination
or loss of electrons and to reduce photon conversions resulting in increased occupancy as well as incorrect matching.

\item[Detector granularity]

In the bending direction this is governed by the desired momentum resolution and in longitudinal direction
by the need to correctly identify and track electrons through all layers of the detector even at the largest
anticipated multiplicities.
For the desired quality of the reconstructed dilepton pair signal (mostly dictated by the tracking efficiency) this
leads to pads with an average area of about 6~cm$^2$, a tracking efficiency of 90\% can thus be achieved for single tracks.
Including secondary particles this leads to a maximum occupancy of 34\% at the highest simulated multiplicity density of
$dN_{ch}/d\eta$=8000.

\end{description}

\section{Principles}



