%
%
%
%  1st lecture of TASI series of Experimental Particle Physics %
\def\mbbar{\overline{b}}
\def\mpbar{\overline{p}}
\def\pbar{$\mpbar$}
\def\mLum{L}
\def\Lum{$\mLum$}
\def\cms{{\rm cm}^{-2}{s}^{-1}}
\def\sqrts{\sqrt{s}}
\def\SppbarS{{\it Sp$\overline{p}$S}}
\def\epsc{\epsilon_c}
\def\Xzero{X_\circ}

\section{Introduction}
These lectures are directed towards an audience
seeking a basic introduction to experimental high energy physics.
The task of covering this field in its entirety is an 
impossible one.  To circumvent this difficulty, I have focused 
on 
an overview of accelerator facilities, and on a discussion of
the principles and techniques of particle detection. 
I will therefore be discussing little ``real'' physics. 

I have tried to be as general as possible, but 
space will limit me to discussing what I view as being the 
most salient issues.
In addition, most of the discussion will be centered on experimental 
techniques at colliding beam facilities, as much of the current experimental 
work is being performed using such machines. 

There is a great deal of literature on the subjects I will be discussing.  In 
particular, ``Experimental Techniques in High Energy Physics'' by T. Ferbel 
\cite{ref: Ferbel}\ is a valuable reference for many of the discussions on 
detector technology.

\section{Accelerator Facilities}
A particle physics experimentalist is 
faced with the task of first bringing the 
necessary initial state particles 
together at the appropriate center of mass energy 
$\sqrt{s}$, and then identifying the specific final state one wishes to study. 
This all has to take place at a rate sufficient to make the desired 
measurement in a reasonable amount of time.  

\subsection{Fixed Target Machines}
Historically, experimental 
particle physics relied heavily on  fixed-target facilities such as
\begin{itemize}
\item the Brookhaven AGS  (30 GeV protons),
\item the Argonne ZGS (10 GeV protons),
\item the CERN PS and SPS (30 and 130 GeV protons),
\item the SLAC linac (30 GeV electrons),
\item the Fermilab synchotron (400 GeV protons), and
\item the Serpukhov proton synchotron.
\end{itemize}
These machines were almost all constructed after the invention of 
``strong-focusing,'' which allows one to 
build rather large accelerators using strings 
of alternating focusing and defocusing magnetic lens.
They have been used 
as sources of intense beams of high-energy protons and electrons, 
which are directed onto a target. 
A detector, located near or around the target measures the properties of
some
or all of the final state constituents, depending on 
the measurement being performed.

These facilities also provide 
``secondary'' beams consisting of more exotic 
particles such as mesons ($\pi$'s and $K$'s), baryons (\pbar 's and 
hyperons like the $\Sigma^\pm$) and leptons ($\mu$'s and neutrinos).  
The primary beam is directed onto a target, 
thereby producing 
a variety of elementary particles.
A particle separator 
placed behind this target selects and collimates 
the desired secondary particles, which are 
then transported to an  experiment and 
directed into a target. 

Such fixed target facilities have several attractions: they can deliver very 
intense primary beams ($10^{12}$\ protons per second
is a typical rate for the 
Fermilab Tevatron) and they provide the only means of making exotic 
particle beams.  The two fundamental figures of merit for such machines 
are their maximum beam energy and the intensity of the beam. 
The rate $R$\ of a process with cross section 
$\sigma$\ in a target with particle density $\rho_t$\ and length $l$\ is 
\begin{eqnarray}
R = \sigma (F A) \rho_t l,
\end{eqnarray}
where $F$\ is the  
flux of beam particles and $A$\ is the cross sectional area of the
beam.
The quantity 
$F A$, the number of beam particles/second, is one of the crucial 
parameters in an experiment.  For example, if we have a Cu target with 
$\rho_t\sim5\times10^{24}$\ protons/cm$^3$\ and length 1 m, and $F_i A 
\sim 10^{10}$\ particles/s, then
\begin{eqnarray*}R & = & 5\times 10^{36} \sigma\quad ({\rm \sigma\ in\ 
cm^2})  \\
  & = & 5000\> \sigma\quad ({\rm \sigma\ in\ nb}).
\end{eqnarray*}

Although the only stable targets are nucleons, one can study more than just 
beam-nucleon interactions in such an environment.  For example, one can 
study $K-\pi$\ elastic scattering through the process   
\begin{eqnarray}
K^- p \rightarrow K^- \pi^+ n.
\end{eqnarray}
This is because the charge-exchange production process is 
empirically known to be 
governed primarily by $\pi$-exchange in the $t$-channel.  One first 
studies the behaviour of the $K^-\pi^+$\ system as a function of $K^-
\pi^+$\ mass and the four-momentum transfer $t$\ between the incoming $K^-
$\ and proton.  This behaviour is then extrapolated back to the unphysical 
region where $t=-m_\pi^2$\  to extract the behaviour of the process $K^-
\pi^+ \rightarrow K^-\pi^+$. A whole generation of spectroscopists 
performed such types of experiments, which  have provided us with some 
of the richest data on light quark spectroscopy \cite{ref: meson}. 

Fixed target physics does have limitations, with the
most severe being that the center of mass energy $\sqrt{s}$\ is 
proportional to $\sqrt{E_{beam}}$, an unfortunately slow scaling 
behaviour. 
In addition, one is always performing an 
experiment involving a specific probe and a nuclear target.  
This implies 
that strong interaction effects  enter into any description of  the 
scattering process, making theoretical comparisons with data more 
problematic.  Many of these problems were overcome with the advent of 
electron storage rings.

\subsection{$e^+e^-$\ Storage Rings}
The development of electron storage rings in the late 1960's and 
early 1970's provided a new way of studying particle interactions. 
For 
the first time, we could study $e^+e^-$\ interactions in as much detail as 
we wished. Many diagrams contribute to $e^+e^-$\ scattering
but the most 
interesting involve the $s$-channel production of a virtual photon ($\gamma$),
 with the 
subsequent coupling of the $\gamma$\ to a fermion-antifermion pair. 
This provides us with a very clean, $J^{PC} = 1^{--}$\ initial state,
avoiding any complications from the strong interaction.
These machines also hold out the hope for even higher \roots\ in these 
collisions, as \roots\ scales linearly with the beam energy.
The beam energy, on the other hand, 
is limited by the RF power required to overcome the 
electrons' energy loss via {\it bremsstrahlung}, which is proportional to 
$\gamma^4$ ($\gamma$\ is the Lorentz factor for the accelerated 
electron). Thus  the size of these machines has to increase dramatically 
as$\sqrt{s}$\ increases.

\subsubsection{The Early Colliders}
The SPEAR ring, built at SLAC and commissioned in 1971, produced the 
first significant results with the discovery of the charmonium resonances 
(which were simultaneously being observed in $\mu\mu$\ final states in a 
fixed-target experiment at Brookhaven) \cite{ref: charm}. 
This ring was filled with one bunch of 
electrons and one bunch of positrons from the SLAC 
linac, providing  two interaction regions in the ring.  The
maximum \roots\ of this collider was 6 GeV but it ran primarily at one of the 
$J/\Psi$\ resonance peaks.  

The rate of events at SPEAR was low enough 
in any one specific channel that a new experimental philosophy and detector 
was required.
It now become more efficient to 
operate a general purpose detector that could simultaneously
study as many final states as 
possible.  Even then, data rates were relatively low and a typical 
detector would take data for one or more years.

The two figures of merit for an $e^+e^-$\ collider are the maximum\roots\ 
(to allow exploration of the energy frontier) and the luminosity of the 
machine, \Lum.  For example, suppose we have two bunches of particles 
and anti-particles with  $N_1$\ and $N_2$\ particles in each bunch 
crossing through each other.  Let $d_x$\ and $d_y$\ be the lateral 
dimensions of the bunch (actually the width of the Gaussian profile) and 
let $\nu$\ be the crossing frequency of the bunches.  Then the rate of a 
process with cross section $\sigma$\ is
\begin{eqnarray*}
R & = & \sigma \left({{\nu N_1 N_2}\over{4\pi d_x d_y}}\right) \\
  & = & \sigma \mLum,
\end{eqnarray*}
where \Lum\ is the luminosity of the machine in units of $\cms$.  
For example, at 
SPEAR where \Lum\ could be  $\sim5\times10^{30}\  \cms$, the 
point cross section ($e^+e^-\rightarrow\mu^+\mu^-$) is $\sim10$\ nb, 
corresponding to a rate of
\begin{eqnarray*}
R & = & 10\ {\rm nb}\  \times\ 5\times10^{30}\  \cms      \\
  & = & 0.05\ {\rm Hz}.
\end{eqnarray*}

SPEAR was initially instrumented with the Mark I detector that 
codiscovered charm.  That detector was followed by the Mark II, 
a general-purpose
detector designed to study charged leptons and photons resulting 
from the decay of charmonium states. A more specialized detector known 
as the Crystal Ball
was subsequently installed in the other interaction region.  It was 
designed to detect the $\gamma$'s resulting from the radiative decay of 
charmonium states. 

Several other storage machines came online during the 1970's.  
SPEAR was followed by DORIS, a similar 
storage ring located at the DESY facility in Hamburg, Germany. 
The PETRA 
storage ring at DESY began operation in 1978, providing collisions with 
$\sqrts$\ as large as 46 GeV at luminosities of $\sim10\times10^{30}\ 
\cms$.  The ring accelerated a pair of electron and a pair of positron 
bunches, providing four interaction regions 
that were instrumented with
(i) the PLUTO detector, which was moved from DORIS, and which 
was later replaced by CELLO, (ii) the JADE detector,
(iii) the MARK J detector, and (iv) the TASSO detector.
Several of these were specialized to investigate particular physics topics 
(\eg\ two photon final states). 
The physics program at PETRA continued till 1986, and it has been reviewed 
by Marshall \cite{ref: Marshall}.

The PEP storage ring at SLAC was commissioned in 1979 as a 
high-luminosity $e^+e^-$\ collider with $\sqrt{s}\sim30$\ GeV and 
\Lum$\sim50\times10^{30}\ \cms$.  The ring collided three bunches of 
electrons on three bunches of positrons, which provided for six interaction 
regions.  These regions were instrumented with
(i) the Mark II detector from SPEAR,
(ii) the DELCO detector,
(iii) the High Resolution Spectrometer (HRS),
(iv) the MAC (Magnetic Calorimeter) detector,
(v) the TPC detector (Time Projection Chamber), and
(vi) the ASP experiment (ASP is an acronym for Anomolous Single 
Photon).
Most of these experiments used general-purpose 
detectors capable of studying a broad range of physics processes.  These 
were all prolific experiments, with several of them are still analysing data.

The most recent general-purpose $e^+e^-$\ colliders have been 
TRISTAN, which  produces $e^+e^-$\ collisions with a maximum 
$\sqrt{s}$\ of $\sim60$\ GeV and a maximum luminosity of 
$\sim10\times10^{30}\ \cms$, and BEPC, a high-luminosity 
collider optimized to be a charmonium factory. 
The TRISTAN facility has four interaction regions that are instrumented with
(i) the AMY detector,
(ii) the VENUS detector,
(iii) the  TOPAZ detector, and
(iv) the SHIP detector.
The first three detectors have been extending the studies performed at PEP 
and PETRA, while SHIP is a specialized device designed to search for 
heavily ionizing particles (such as monopoles).

BEPC, located in Beijing, 
is an $e^+e^-$\ storage facility designed for charmonium studies, and is the 
first such accelerator project in China.  It provides collisions 
up to $\sqrt{s}\sim6$\ GeV at a design luminosity of 
$\sim10\times10^{30}\ \cms$, well in excess of the maximum luminosity 
of the SPEAR ring \cite{ref: BEPC}.  

\subsubsection{e$^+$e$^-$\ Colliders for $B$\ Physics}
Motivated by  the discovery of the $\Upsilon$\ in 1977
\cite{ref: upsilon}, 
the CESR storage ring at Cornell and the DORIS storage ring at DESY 
were upgraded to produce $e^+e^-$\ collisions at $\sqrt{s}\sim10$\ GeV
and have since been dedicated to the study of bottom hadrons. 

The CESR ring can operate at a $\sqrt{s}$\ of $\sim12$\ GeV 
with a maximum luminosity of $\sim90\times10^{30}\ \cms$.  The facility 
has two interaction regions, one being instrumented by CLEO, a 
general-purpose magnetic spectrometer, and the other being 
instrumented with CUSB, a $\gamma$\ detector designed to study 
radiative $\Upsilon$\ decays.  This ring is still operational with an upgraded 
version of the CLEO detector.

The DORIS storage ring at DESY began life as a low-energy $e^+e^-$\ 
storage ring in the early 1970's. It was upgraded a decade later
to produce collisions at 
a $\sqrt{s}$ of $\sim11$\ GeV with a maximum luminosity of 
$\sim30\times10^{30}\ \cms$. 
The ring has two interaction regions, one 
instrumented by ARGUS, a general-purpose detector, and 
the other by the Crystal Ball, the $\gamma$\ detector initially installed
at SPEAR.  The ring is still 
operational, although it is overshadowed by
recent increases in the CESR luminosity and the commissioning of HERA.

There are now several groups studying the possibility of building a very
high-luminosity asymmetric $e^+e^-$\ collider that would be devoted to
high-rate $B$ physics.  Such a $B$\ factory could be built as an enhancement 
to the CESR or PEP facilities in the next decade \cite{ref: B Factories}.

\subsubsection{$Z^\circ$\ Factories}
The latest generation of electron colliders have been built to produce
$e^+e^-$\ collisions with $\sqrt{s}$\ equal to the mass of the 
\Zzero\ boson.  These are
\begin{itemize}
\item the SLC, a linear collider that produces $e^+e^-$\ collisions up to 
$\sqrt{s}\sim100$\ GeV  by colliding very intense bunches of electrons and 
positrons that have been accelerated in the SLAC linac, and
\item LEP (Large Electron Project), which in its first phase is producing 
collisions at $\sqrt{s}$\ up to 100 GeV and a luminosity of 
$\sim3\times10^{30}\ \cms$.
\end{itemize}

SLC is the first generation of machines using the linear 
collider concept (see Fig. \ref{fig: SLC}).
Large bunches of electrons and positrons are accelerated in  the SLAC 
linear accelerator. 
The $e^+$\ and $e^-$\ bunches are transported through separate ``arcs'' that 
come together at the final focus region.  After the collision, the bunches 
are extracted and directed into a beam dump.
This scheme avoids the cost of accelerating the 
electrons and positrons in a synchotron, where the power radiated by 
{\it bremsstrahlung}\ radiaton is much higher.  However, one must now 
accelerate large bunches of electrons and positrons and focus them into a 
small beam spot in order to obtain a reasonable luminosity.  
At the SLC, the transverse size of the bunches are only a 
few microns, which is achieved by paying careful attention to the beam 
optics in the linac and collider.  
\begin{figure}[t]
\vspace{4in}
\caption{A schematic of the SLAC linear collider and the 
Large Electron Project at CERN.  The two figures differ in scale
by a factor of 5. }
\label{fig: SLC}
\end{figure}

SLC had a
design luminosity of $\sim 1\times10^{30}\ \cms$.  Unfortunately, it has 
experienced various technical problems and has not been able to attain 
this luminosity, due in part to persistent alignment problems, 
but it has the  unique feature of being able to 
produce polarized beams.  The machine has one interaction region, 
which until recently has accomodated the Mark II 
detector from PEP.
An entirely new detector called SLD has been 
constructed and will 
continue the physics program at the SLC. 

LEP is a conventional $e^+e^-$\ storage ring with a circumference of 28 
km.  It can store beams with energies in excess 
of 50 GeV in order to produce \Zzero's, and  a subsequent 
phase (LEP II) of the project will see the beam energy doubled in order 
to produce
$W^+W^-$\ events. Phase I of the LEP collider was completed in 
fall of 1989, along with four general-purpose detectors, known as 
ALEPH, DELPHI, L3 and OPAL.
Each detector has been optimized to study \Zzero\ decays, but they 
employ different philosophies and technology.  
For example, the OPAL (Omni-Purpose 
Apparatus for LEP) detector is perhaps most conventional in design, and 
has been viewed as the ``conservative'' detector, while the
DELPHI  detector incorporates several new technologies
that have the potential for providing 
better performance, but are also unproven and pose certain risks. Together, 
these collaborations have collectively recorded $10^6$\ \Zzero\ decays by 
fall 1990, and are expected to continue their studies over the next several 
years.

LEP II, which will have stored beams with twice the energy of LEP I, 
is scheduled to be completed by 1994, at which time the collaborations 
will begin a detailed study of $W^+W^-$\ production.

\subsection{Hadron Colliders}

Although $e^+e^-$\ colliders produce very clean initial states, they 
cannot compete on the energy frontier with hadron colliders.  
Whereas the $\sqrts$\ of an $e^+e^-$\ machine is 
severely limited by the rapid rise 
in beam energy loss due to synchotron radiation, this effect is negligible at 
a hadron collider (because of the difference in mass of the accelerated 
particle).  The $\sqrts$\ at a hadron collider is 
limited by the strengths of the 
magnetic fields required to contain the hadron beams (alternatively, they 
are limited by the radius of the machine).  The price paid for this 
increase in available $\sqrt{s}$\ is that one no longer is colliding 
elementary fermions, but two systems that consist of several 
strongly-interacting partons.  

A consequence of this is that a high 
energy collision between two of the partons is accompanied by the 
``underlying event'' that results from the disassociation of the hadrons 
during the collision process.  The experience at hadron colliders in the past 
decade have shown that one can take the
effects of the underlying event into account, 
but the resulting physics analyses are
nevertheless complicated by its presence. 

Another consequence is
that the interaction 
cross section at a hadon collider is dominated by ``soft'' collisions, so that 
the more interesting high-\Pt\ events occur at rates that are many orders of 
magnitude smaller.  For example, the total cross section at a hadron 
collider is of order 50 mb, while a typical electroweak production cross 
section (such as \W\ or \Zzero\ production) is 
on the order of 10 nb, constituting only 
$10^{-6}$\ of all events.  
To study these relatively rare interactions, an 
experimentalist must actively reject these backgrounds by using a
sophisticated detector trigger.  

\subsubsection{The CERN ISR and \SppbarS}
 The idea of a hadron collider was first pursued with the construction of the 
CERN ISR (Intersecting Storage Ring), a proton-proton collider with  
$\sqrt{s}\sim50$\ GeV.    In retrospect, most of the experiments at this 
machine ended up studying the relatively murky physics processes 
underlying the ``softer'' hadron-hadron collisions.  
The relatively low $\sqrt{s}$\ available for high-\Pt\ collisions 
made them quite rare compared with the rate of ``minimum-bias'' 
interactions resulting from the disassociation of the hadrons. 
A major step in overcoming these problems was taken 
with the development of the \SppbarS, a proton-antiproton collider 
constructed in the tunnel originally used to house the SPS  proton 
synchotron.  

The primary obstacle to the development of a \pbarp\ collider was
the difficulty of 
generating and accelerating a large bunch of anti-protons. 
This was solved by the technique of ''stochastic cooling.''  
Small bunches of \pbar's are placed in  a storage ring (the ``accumulator'') 
and collected for several hours.  
The average position 
of the \pbar\ bunch at one point in the ring
is used to apply a small correction to the bunch orbit on 
the other side of the ring when the \pbar\ bunch passes.  
This correction reduces the phase 
space of the \pbar\ bunch.
When enough \pbar's have been accumulated and 
their bunch reduced to the appropriate size, 
they are extracted from the accumulator and injected into the proton 
storage ring and accelerated to collision energy.

The CERN \SppbarS\ collider was the first machine to employ this 
technique.  The collider was able to produce 
collisions with a $\sqrt{s}$\ of 630 GeV at an eventual luminosity of 
$\sim3\times10^{30}\ \cms$\ in two 
interaction regions.  One region was instrumented with the UA1  
detector, a general purpose device 
consisting of a large charged particle tracking 
volume immersed in a dipole field and almost completely surrounded by a 
calorimeter \cite{ref: UA1}.  
The other region was instrumented with the UA2 detector, 
which consisted of a non-magnetic tracking system partially contained 
within a calorimeter \cite{ref: UA2}.  
Perhaps the most revolutionary aspect of the UA1 
experiment was the focus on having almost complete (in the jargon, 
``hermetic'') calorimetry coverage in order to be able to detect neutrinos 
by the momentum imbalance observed in the calorimeter.  This aspect 
gave the experiment the 
necessary tools to disover the \W\ boson, and subsequently its neutral 
current partner the \Zzero\ \cite{ref: WZ}.  

The experimental program at the \SppbarS\ is now almost complete.
The UA2 detector will be completing a final data run during winter, 1991,
and no future runs are currently planned. 

\subsubsection{The Second Generation of Hadron Colliders}
By the time the \SppbarS\ collider was nearing completion, the Isabelle
proton collider at Brookhaven National Laboratory 
was under construction in the US.  
It was designed to collide protons 
together with $\sqrts=300$\ GeV,  but the development of the 
superconducting dipole magnets fell so far behind schedule that the project 
was cancelled in the mid-1980's.  

By this time, construction of the Fermilab Tevatron Collider was well
underway.  This device was designed around the Fermilab
synchotron and had the ability  to produce collisions with 
$\sqrts=1800$\ GeV.  The 
design luminosity of $\sim 1\times10^{30}\ \cms$ was somewhat lower 
than that achieved at the \SppbarS, but the increased production cross sections 
for high-\Pt\ processes more than compensated for the luminosity 
differences.  

The collider has six interactions regions, with two being 
instrumented with large, general-purpose detectors.  One of them is CDF, 
which had its first major physics run in 1988 \cite{ref: CDF}.  
It  consists of a cylindrical 
tracking volume immersed in a solenoidal magnetic field and almost 
completely surrounded by calorimetry, except for $3^\circ$\ 
holes around the beamline.  The other detector, D0, takes its name from 
the location of its interaction region \cite{ref: D0}.  
It is also a general purpose detector, 
with the emphasis being placed on good calorimetry.  In order to avoid 
compromising the hermeticity of the calorimeter, D0 does not 
contain a magnet for momentum analysis.  The \Pt\ of 
muons are measured using a set of toroidal magnets located outside the 
calorimeter.  The CDF detector is currently completing the analysis of a  
successful data run performed in 1988-89, and both detectors and the 
Tevatron collider are being prepared for the next run that is currently 
scheduled to start in fall 1991.

The rate of interactions involving electroweak processes at a hadron 
machine are higher than at a comparable $e^+e^-$\ collider.  However, 
they are mixed together with a very large interaction rate from ``minimum 
bias'' processes and from hard QCD interactions. For example, I have 
listed in Tab. \ref{table: rates}\ the approximate Tevatron cross 
sections for a variety of physics processes.
\begin{table}
\vskip .2 in
\tabskip=1em plus2em minus.5em
\def\thickfill{\leaders\hrule height1.5pt\hfill}
\vbox{\offinterlineskip
\halign to \hsize{
\vrule width1.5pt#&\strut\quad#\hfil&\vrule#&\strut\hfil#\quad&\vrule 
width1.5pt#\cr
\omit\span\omit\span\omit\span\omit\span\omit \thickfill\cr
height 4 pt &\omit&&\omit& \cr
&Process      &&   Rate (nb)                         &\cr
height 4 pt &\omit&&\omit& \cr
 \omit\span\omit\span\omit\span\omit\span\omit \hrulefill\cr
height 4 pt &\omit&&\omit& \cr
&Minimum Bias                         &&  50,000,000 &\cr
height 4 pt &\omit&&\omit& \cr
&Inclusive \b\ quark production       && $\sim30,000$&\cr
height 4 pt &\omit&&\omit& \cr
&Inclusive $e^\pm$\ (\Pt$>12$\ \GeVc) &&      10     &\cr
height 4 pt &\omit&&\omit& \cr
&$W^\pm \rightarrow e^\pm \nu_e$      &&       2     &\cr
height 4 pt &\omit&&\omit& \cr
&\Zzero$\rightarrow e^+e^-$           &&      0.2    &\cr
height 4 pt &\omit&&\omit& \cr
 \omit\span\omit\span\omit\span\omit\span\omit \thickfill\cr
}}
\caption{Cross sections for several processes at the Fermilab
Tevatron collider.}
\label{table: rates}
\end{table}\
The challenge to the experimentalists is to separate what one is 
calling interesting physics from the uninteresting rates. Both UA1, UA2 
and CDF have shown that this is possible. As the energy of the 
collider increases, the problems in some sense become easier as the 
relative rate of ``interesting'' physics grows with $\sqrt{s}$. 

\subsection{ep Colliders}
High-energy $ep$\ collisions are a unique way of studying 
phenomena such as 
proton structure, the 
electroweak interaction, and the possible production of new 
particles (leptoquarks, supersymmetry, etc.).
The earliest studies were done with high energy electrons interacting 
on a fixed nuclear target at SLAC.
The HERA 
facility at DESY will be the first true electron-proton
colliding beam machine.  
It is designed to collide 
30 GeV $e^-$'s with 820 GeV protons with a design luminosity of
$15\times 10^{30} \cms$.
HERA has 
four interaction regions, with two being instrumented 
with the ZEUS and H1 detectors \cite{ref: HERA}.
Both of these are state-of-the-art devices with particular emphasis 
being placed on precision calorimetry.
This focus comes from the desire to make precision measurements of the 
charge current interaction that produces final states consisting of
high energy jets and neutrinos.

\subsection{Summary of Existing Colliders}
I summarize in Tab. \ref{table: coll}\ the parameters of
existing colliders.  
One can see reflected in this table the quest for colliders with both
higher energies and higher luminosities.

Looking somewhat into the future, the next generation of machines will be
the new hadron colliders that will take a giant step forward in both
center of mass energy and luminosity.
The SSC, being constructed in Texas, will collide protons
together at $\sqrt{s}$\ of 40 TeV with a luminosity of $10^{33}$\ $\cms$.
The LHC, a proton collider that is under consideration at CERN, will
yield collisions with $\sqrt{s}$\ of 17 TeV at a luminosity of 
$10^{34}$\ $\cms$.
Both of these machines will stretch the capabilities of the
physicists responsible for designing the accelerator and the detectors. 

\begin{table}
%
\tabskip=1em plus2em minus.5em\def\thickfill{\leaders\hrule 
height1.5pt\hfill}\vbox{\offinterlineskip\halign to \hsize{\vrule 
width1.5pt#&\strut\quad#\hfil&\vrule#&\strut\hfil#\hfil&\vrule#&\strut\hfil
#\quad&\vrule#&\strut\hfil#\hfil&\vrule width1.5pt#\cr 
\omit\span\omit\span\omit\span\omit\span\omit% 
\span\omit\span\omit\span\omit\span\omit \thickfill\cr
height 4 pt &\omit&&\omit&            &\omit&&\omit& \cr
&Facility    &&    Beams 
&& \quad$\sqrt{s}$ (GeV)\hfil && \Lum ($10^{30}\ \cms$)&\cr
height 4 
pt &\omit&&\omit&            &\omit&&\omit& \cr 
\omit\span\omit\span\omit\span\omit\span\omit% 
\span\omit\span\omit\span\omit\span\omit \thickfill\cr
height 4 pt 
&\omit&&\omit&            &\omit&&\omit& \cr
&SPEAR       &&   $e^+e^-
$   &&  6      &&    5                        &\cr
height 4 pt &\omit&&\omit&            
&\omit&&\omit& \cr&BEPC        &&   $e^+e^-$   &&  6      &&   10                        
&\cr
height 4 pt &\omit&&\omit&            &\omit&&\omit& \cr&CESR        
&&   $e^+e^-$   &&  12     &&   90                        &\cr
height 4 pt 
&\omit&&\omit&            &\omit&&\omit& \cr&DORIS       &&   $e^+e^-
$   &&  11     &&   30                        &\cr
height 4 pt &\omit&&\omit&            
&\omit&&\omit& \cr&PEP         &&   $e^+e^-$   &&  30     &&   50                        
&\cr
height 4 pt &\omit&&\omit&            &\omit&&\omit& \cr&TRISTAN     
&&   $e^+e^-$   &&  60     &&   10                        &\cr
height 4 pt 
&\omit&&\omit&            &\omit&&\omit& \cr&SLC         &&   $e^+e^-$   
&& 100     &&    1                        &\cr
height 4 pt &\omit&&\omit&            
&\omit&&\omit& \cr&LEP         &&   $e^+e^-$   && 120     &&   10                        
&\cr
height 4 pt &\omit&&\omit&            &\omit&&\omit& \cr 
\omit\span\omit\span\omit\span\omit\span\omit% 
\span\omit\span\omit\span\omit\span\omit \hrulefill\cr
height 4 pt 
&\omit&&\omit&            &\omit&&\omit& \cr&S\pbarp S   &&   \pbarp     
&&  630    &&    3                        &\cr
height 4 pt &\omit&&\omit&            
&\omit&&\omit& \cr&Tevatron    &&   \pbarp     &&  1800   &&    2                        
&\cr
height 4 pt &\omit&&\omit&            &\omit&&\omit& \cr&HERA        
&&   $ep$       &&  220    &&   15                        &\cr
height 4 pt 
&\omit&&\omit&            &\omit&&\omit& \cr 
\omit\span\omit\span\omit\span\omit\span\omit% 
\span\omit\span\omit\span\omit\span\omit \thickfill\cr}}
\caption{A summary of the existing particle accelerators and
colliders. }
\label{table: coll}
\end{table}
