%
\section{An Example of a Complete Detector}
Having discussed in the preceding sections the various 
technologies that are currently in use, I will complete this section 
by illustrating how most of these technologies are combined 
together to make a complete general-purpose detector.  There are 
many detectors in existence that could be considered as examples, 
and I have chosen the Aleph detector at LEP; it is a good example 
to use as it utilizes various relatively well-proven technologies and 
has combined them into a well-functioning detector.

Aleph is a general-purpose detector operating at LEP, designed to 
study the decay modes of the \Zzero\ and to reconstruct $e^+e^-$\ 
collisions up to $\sqrt{s}$\ of 200 GeV (the energies that will be 
accessible by LEP II).  The detector, illustrated schematically in 
Fig. \ref{fig: Aleph schematic}, consists of the following systems:
\begin{itemize}
\item A tracking system comprised of
\begin{itemize}
\item a Si strip vertex detector (2 layers at $R\sim8$\ cm),
\item  an inner tracking chamber comprised of a high precision drift 
chamber with position resolution with $\sigma\sim100\ \mu$,
\item a TPC with outer radius 1.4 m, giving $\sim20$\ space points
along each track  ($\sim60,000$\ channels), and 
\item a solenoid magnet generating a 1.5 T field.
\end{itemize}
Together, these devices give a momentum resolution of:
\begin{eqnarray}
{{\Delta p}\over{p}}\sim 0.001 p \oplus 0.003.
\end{eqnarray}
\item An EM calorimeter consisting of 22 $\Xzero$\ of lead plates 
sandwiched between proportional wire chambers.  The wire 
chamber cathodes are segmented into 3 cm$\times$3 cm pads that 
record the ionization deposited in the calorimeter.  This calorimeter 
is located inside the solenoid magnet in order to avoid the 
degradation in energy resolution that results if $e^\pm$'s and 
$\gamma$'s were to pass through the magnet coil.  The calorimeter 
is organized into $\sim74,000$\ towers, and provides an energy 
resolution of
\begin{eqnarray}
{{\Delta E}\over{E}}\sim {{18\%}\over{\sqrt{E}}}\oplus 1.6\%
\end{eqnarray}
\item A hadron calorimeter that consists of $\sim20$\ layers of Fe 
plates sandwiched between Iarocci tubes--proportional wire 
chambers with cathode readout.  The calorimeter is 7 interaction 
lengths thick and has a resolution of
\begin{eqnarray}
{{\Delta E}\over{E}}\sim {{85\%}\over{\sqrt{E}}}.
\end{eqnarray}
\item A muon identification system located outside the hadron 
calorimeter, consisting of two layers of Iarocci tubes separated 
radially by $\sim0.5$\ m.
\end{itemize}

\begin{figure}
\vspace{3in}
\caption{A  cross section of the Aleph detector. }
\label{fig: Aleph schematic}
\end{figure}

The Aleph design emphasizes excellent charged particle tracking, 
good Em calorimetry with fine segmentation for electron and 
photon identification, and particle identification provided by the 
calorimeter system, the \dEdx measurement from the tracking 
system, and the $\mu$\ system.  The detector has performed well 
during the first full run of the LEP collider, with essentially all of  
the systems being commissioned during that period.  As an 
example of the detailed physics studies that can be performed by 
this detector, I have considered the measurement of the
\Zzero\ branching fraction to \b\bbar, which is a difficult 
measurement to perform as it requires some means of separating 
the \b\ quark jets from the lighter quark jets.  The Aleph 
collaboration chose to do this by tagging \b\ quarks using their 
semi-leptonic decay modes to $e\nu_e X$\ and $\mu\nu_\mu X$.  
This requires the detector to be able to identify electrons and 
muons that are buried in the middle of the lighter quark jet that 
acompanies the lepton daughters.  Fig. \ref{fig: Aleph e ID}\ 
shows the distribution of a variable that characterizes the 
longitudinal profile of a shower observed in the EM calorimeter.  
This variable discriminates between showers that are initiated by 
electrons from those that result from hadrons, and illustrates a clear 
electron signal.  An analysis of the data from the $\mu$\ chambers 
results in a similar rate of such events in the $\mu$\ decay mode.  
The resulting branching fraction is 
\begin{eqnarray} 
BR(Z^\circ \rightarrow b\overline{b}) = ??
\end{eqnarray}
\begin{figure}
\vspace{3in}
\caption{The variable $R=??$\ showing a clear electron signature 
from b quarks in \Zzero\ decays at Aleph. }
\label{fig: Aleph e ID}
\end{figure}


\section{What will the Future Bring?}
In the upcoming years, experimental high energy physics will see 
the commissioning of the HERA $ep$\ collider and along with that 
the ability to accurately measure the structure of the proton using 
both the charged and neutral weak current.  The ongoing program 
at Fermilab's Tevatron collider will see several large collaborations 
continue to study the standard model using large samples of \W\ 
and \Zzero\ bosons, and further the search for the top quark and 
other new phenomena.  The experimental program at LEP will see 
four collaborations studying the detailed properties of the weak 
neutral current, and eventually be able to study $W^+W^-$\ 
production in a very clean environment.  Finally, studies of the 
multi-TeV energy scale will become possible by the end of the 
decade with the completion of the SSC and perhaps the LHC.
In addition to these frontiers, several active fixed-target programs 
will continue to push on specific frontiers that are inaccessible to 
the collider experiments.


