%
%
%
%  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}
The task of discussing the entirety of experimental particle physics is an 
impossible one.  To circumvent this obvious difficulty, I have focused 
these lectures on the following subjects:
\begin{itemize}
\item an overview of accelerator facilities;
\item the ``principles'' of particle detection and recent advancesin 
technology, which I have subdivided into the following topics:
\begin{enumerate}
\item Charged particle detection
\item Calorimetry
\item Particle Identification
\end{enumerate}
\item the outlook for experimental physics in the next decade.
\end{itemize}
In fact, I will be discussing little ``real'' physics in that  most of this 
discussion will focus on the technology we employ to perform particle 
physics experiments today.  I have tried to be as general as possible, but 
space and time limit will limit me to discussing what I view as being the 
most important issues in experimental particle physics.  

There is a great deal of literature on the subjects I will be disussing.  In 
particular, ``Techniques in Experimental Particle Physics'' by Tom Ferbel 
\cite{ref:Ferbel}\ is a valuable reference for many of the discussions on 
detector technology.
\section{The Anatomy of a Particle Physics Experiment}
We can identify several basic ``ingredients'' of a particle physics 
experiment: it should be well-motivated;  it should have the ability to 
produce the desired interaction or final state;
one must have the means to measure the relevant quantities in each 
interaction; and one must have the ability to interpret the 
measurementsquantitatively.  

There are many ways we can motivate an experiment:
\begin{itemize}
\item The first type of motivation is the one that can be  called the ``test of a 
paradigm,'' borrowing the language of Thomas Kuhn \cite{ref:Kuhn}.  In 
this case, one has a specific model that in principle predicts some  number 
of phenomena.  If one can measure these phenomena and confront the 
theoretical prediction, one is then able to either disprove or validate the 
model.  Of course, for this to work, we must have a rasonable model to 
start from.
\item  A second motivation for an experiment is the search for new 
phenomena not necessarily predicted by a model.  Such a search by 
definition must take place on a``frontier;''  in particle physics such frontiers 
are provided bystudying phenomena at the highest possible center of mass, 
by making measurements with much greater sensitvities, or bystudying 
new interactions (\eg\ studying new final states).
\end{itemize}.

Besides these motivations, one also must allow practical considerations to 
enter into the design and motivation of an experiment.  For example, the 
community should try to maximise the use of existing facilities.  Although 
this may compromise the quality of the experiment, it is often the best and 
most immediate way of performing the measurement one is interested in.  
This in fact happens moreoften than we care to admit (\eg\ the upgraded 
Mark II was not theoptimal detector for SLC, but it was available and 
could be upgraded towork on the timescale of SLC's completion).  
However, often one has a specific measurement in mind that constrains the 
design of the experiment to a large degree.  Recent examples of such 
experiments are the apparatus that have been constructed to measure 
$\epsilon^\prime/\epsilon$\ at CERN \cite{ref:NA34}\ and FNAL 
\cite{ref:E731}.

We are often limited by technology and have to perform substantial 
amounts of R\&D to determine whether the experiment can be 
performed.Recent examples of this can be seen in the R\&D effort that has 
been  underway for experiments at SSC and LHC, where the very high 
interaction rate of the collider places severe requirements on all aspectsof 
the experiment.  The design of the experiment has to include a careful 
analysis of how well it will work and how the data will be manipulated to 
achieve  the required precision.This not only means knowing how each 
component of the detector will work, but also how the entire apparatus will 
function as a single system.

\section{Accelerator Facilities}
We are almost always faced with the task of bringing together the 
necessary constituents 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.  Historically, experimental 
particle physics relied heavily on  fixed-target facilities at various 
accelerators:
\begin{itemize}
\item Brookhaven AGS  (30 GeV protons)
\item Argonne ZGS ($\sim 10$\ GeV protons)
\item CERN PS and SPS (30 and 130 GeV protons)
\item SLAC linac ($\sim 30$\ GeV electrons)
\item Fermilab synchotron ($\sim 400$\ GeV protons)
\item Serpukhov proton synchotron
\end{itemize}
These machines were almost all constructed after the invention of``strong-
focusing,'' which allows one to build rather largeaccelerators using strings 
of alternating focusing and defocusing magnetic lens.They were used 
initially as sources of intense beams of high-energy protons and electrons, 
which were then directed onto a target. The experimental apparatus was 
built to detect some or all of the final state constituents, depending on 
exactly the measurement one was trying to make. 

It is also possible to fashion``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 was directed onto a target, thereby producing 
essentially every possible species of elementary particle.A device 
following the target selected and collimated the desired secondary particles 
and they were then transported to the experiment where they were 
directed into a target. 

Such fixed target facilities have several attractions: they can deliver very 
intense primary beams($10^{10}$\ 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 energies and the intensity of the beam. Suppose 
we want to produce a rate $R$\ of an interaction with cross section 
$\sigma$\ in a target with particle density $\rho_t$\ and length $l$. If the 
flux of beam particles is $F_i$\ with cross section $A$, then
\begin{eqnarray}
R = \sigma N_i F_i = \sigma (F_i A) \rho_t l
\end{eqnarray}
so $F_i A$, the number of beam particles/second, is one of the crucial 
parameters in the 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 by studying the process   
\begin{eqnarray}
K^- p \rightarrow K^- \pi^+ n.
\end{eqnarray}
This is because the 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 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 ofexperiments, which  have provided us with some 
of the richest data on light quark spectroscopy. 

Fixed target physics does have many limitations, however. One of the 
most severe is the fact that the center of mass energy $\sqrt{s}$\ is 
proportional to $\sqrt{E_{beam}}$, an unfortunately slow scaling 
behaviour. Another problem in fixed-target physics is the fact that as one 
increases the beam energy, the final state constituents are Lorentz-boosted 
into an increasingly smaller forward cone. This leads to diffculty in 
recognizing the final state and accurately reconstructing the four-
momentum of the final state particles. Finally, one always is performing an 
experiment involving a specific probe and a nuclear target.  This implies 
that one always has to deal with the strong interaction effects  in  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 
early1970's provided an entirely new way of studying particle physics. For 
the first time, we could study $e^+e^-$\ interactions in as much detail as 
we wished. Many diagrams contribute to $e^+e^-$\ scattering, with the 
dominant ones listed in Fig.~\ref{fig:e+e- diagrams}, but the most 
interesting involve the $s$-channel production of a virtual photon, with the 
subsequent coupling of the $\gamma$\ to a fermion-antifermion pair. 
These machines also held out the hope for even higher \roots\ in these 
collisions, as \roots\ scales linearly with the beam energy.
\begin{figure}
\vspace{4in}
\caption{The diagrams contributing to $e^+e^-$\ interactions. }
\label{fig:e+e- diagrams}
\end{figure}

The SPEAR ring, built at SLAC and commissioned in 1971, produced the 
first signifcant results with the discovery of the charmonium resonances 
(which were simultaneously being observed in$\mu\mu$\ final states in an 
experiment at Brookhaven). This ring was filled with one bunch of 
electrons and 1 bunch of electrons injected into the ring from the SLAC 
linac, providing  two interaction regions that could be instrumented.  The 
maximum \roots\ of this ring 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 rather new philosophy of detector 
design and experiment was required;  it had now become more efficient to 
have a general purposedetector that could study as many final states as 
possible simultaneously.  Even then, data rates were relatively low and so 
typically detectors would take data for several years collecting as large a 
sample as possible.  

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\quad \bigl({{\nu N_1 N_2}\over{4\pi d_x d_y}}\bigr) \\
  & = & \sigma \mLum,
\end{eqnarray*}
where \Lum\ is the luminosity of the machine ($\cms$).  For example, at 
SPEAR where the maximum  \Lum\ is $\sim5\times10^{30}\  \cms$, 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*}
On the other hand, at the Tevatron collider, where \Lum\ is 
$\sim1\times10^{30}\ \cms$\ and the total cross section is 50 mb, the 
``physics'' rate is
\begin{eqnarray*}
R = 50~000\ {\rm Hz}.
\end{eqnarray*}
In this latter case, a very large fraction of this cross sectionis arises from 
``soft'' \pbarp\ interactions and most of it is usually ignored.  The 
interesting physics cross section at the Tevatron is of order 50 nb 
(depending on what you define as interesting), and this rate corresponds to 
0.05 Hz.  

SPEAR was initially instrumented with the Mark I detector that co-
discovered charm.  That detector was followed by theMark 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,operated in the other interaction region.  It was specially 
designed to detect the $\gamma$'s resulting from the radiative decay of 
charmonium states. SPEAR was soon followed by DORIS, a similar 
storage ring located at the DESY facility in Hamburg, Germany. Electron 
storage rings are limited by the RF power required to overcome the 
electrons' energy loss via Bremsstrahlung, which is proportional to 
$\gamma^4$ ($\gamma$\ is the Lorentz factor for the accelerated 
electron). Thus  the size of these machines have to increase dramatically 
as$\sqrt{s}$\ increases.

Several storage machines came online during the 1970's.  The PETRA 
storage ring at DESY began operation in ??, 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, which allowed four interaction regions to operate simultaneously.  
These four regions were instrumented with:
\begin{itemize}
\item the PLUTO detector, which was moved from DORIS, and which 
was later replaced by CELLO
\item the JADE detector
\item  the MARK J detector
\item the TASSO detector
\end{itemize}
Several of these were specialized to investigate particular physics topics 
(\eg\ two photon final states. 

The physics program at PETRA continued till 1986, it has been reviewed 
by R.~Marshall, Rep. Prog. Phys.{\bf 52} (1989) 1329.

The PEP storage ring at SLAC was commissioned in 1979 as a high-
luminosity $e^+e^-$\ collider with $\sqrt{s}\sim30$\ GeV and a luminosity 
of \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:
\begin{itemize}
\item the Mark II detector from SPEAR,
\item the DELCO detector,
\item the High Resolution Spectrometer (HRS),
\item the MAC (Magnetic Calorimeter) detector ,
\item the TPC detector (Time Projection Chamber), and
\item the ASP experiment (ASP is an acronym for Anomolous Single 
Photon).
\end{itemize}
All of these detectors, except for ASP, were/are ``general-purpose'' 
detectors designed to study a broad range of physics processes.  These 
were all prolific experiments, with several of them are still analysing data.

\section{e$^+$e$^-$\ Colliders for $B$ Physics}
Motivated by  the discovery of the $\Upsilon$\ in 1977, two $e^+e^-
$\ storage rings have been devoted to the study of bottom hadrons: the 
CESR storage ring at Cornell, and the DORIS storage ring at DESY.  Both 
rings started life as lower energy colliders but increased the energy of their 
beams in order to be able to study the $\Upsilon$\ resonances.

The CESR ring can operate at a maximum $\sqrt{s}$\ of $\sim12$\ GeV 
with a maximum luminosity of $\sim90\times10^{30}\ \cms$.  The facility 
has two interaction regions with one being instrumented by the CLEO, a 
general-purpose magnetic spectrometer, and the other being initially 
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 ?? GeV $e^+e^-$\ 
storage ring in ??, but was renovated in the early 1980's in order to achieve 
a $\sqrt{s}$ of $\sim11.2$\ GeV with a maximum luminosity of 
$\sim30\times10^{30}\ \cms$.  The ring has two interaction regions, one of 
which has been instrumented by ARGUS, a general-purpose detector, and 
the other by the Crystal Ball, the $\gamma$\ detector moved from SPEAR 
in order to study the radiative $\Upsilon$\ decays.  This ring is still 
operational, although it is now having difficulty competing with the CESR 
ring, given the recent increases in the CESR luminosity.

\subsection{The Latest Generation of e$^+$e$^-$\ Colliders}
The latest generation of electron colliders have been built to provide the 
very highest energy $e^+e^-$\ collisions.  These facilities include:
\begin{itemize}
\item Tristan, which  is a conventional $e^+e^-$\ collider with a maximum 
$\sqrt{s}$\ of $\sim60$\ GeV and a maximum luminosity of 
$\sim10\times10^{30}\ \cms$.
\item BEPC, a high-luminosity collider designed as a charmonium factory 
that was recently commissioned in Beijing, China.
\item SLC, a linear collider that produces $e^+e^-$\ collisions up to 
$\sqrt{s}\sim95$\ 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 a $sqrt{s}$\ of $\sim92$\ GeV at a luminosity of $\sim??$.
\end{itemize}

The Tristan facility has four interaction regions that are instrumented with 
the following detectoris:
\begin{itemize}
\item AMY
\item VENUS
\item SHIP (Search for Highly Ionizing Particles), and 
\item TOPAZ
\end{itemize}
These four detectors have been extending the studies performed at PEP 
and PETRA.

BEPC is $e^+e^-$\ storage facility designed for charmonium studies, is the 
first such accelerator project in China.  It is designed to provide 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.  It has two interaction regions, instrumented with....

The most recent electron colliders to come on line have been SLC and 
LEP, both of which have been designed to produce $e^+e^-$\ collisions at 
the \Zzero\ pole.  SLC is the first generation of machines using the linear 
collider concept;  the electron and positron bunches are accelerated in a 
linear accelerator, where it is possible to accelerate very large bunches of 
particles to high energies.  These bunches are then collided and discarded 
(which is why this device is also referred to as a single-pass collider.  InTo 
avoid having to construct two separate linacs to accelerate the particles, 
SLC employs the SLAC linac to accelerate both bunchs of particles and 
then transports the $e^+$\ and $e-$\ bunch through separate ``arcs'' that 
come together at the final focus region.  After the collision, the bunches 
are extracted by a kicker magnet and directed into a beam dump. The 
advantage of this scheme is that it avoids the cost of accelerating the 
electrons and positrons in a synchotron, where the power radiated by 
Bremsstrahlung radiaton is much higher.  However, one must now 
accelerate large bunches of electrons and positrons and focus them into a 
small beam spot.  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.  

SLC can operate at a maximum $\sqrt{s}$\ of $\sim100$\ GeV with a 
design luminosity of $\sim 1\times10^{30}\ \cms$.  Unfortunately, it has 
had several problems starting up and has not been able to attain this
design luminosity due to various persistent problems in aligning the 
machine and finding the best operating conditions for the machine.
It faces severe competition from LEP, but is unique it that it can 
produce polarized beams.  The interaction region can only accomodate a 
single detector, which until recently has been the venerable Mark II 
detector, upgraded in order to deal with the class of events that are 
produced by \Zzero\ decays.  An entirely new detector called SLD will 
roll in during fall 1990 and continue the physics program.  

LEP is a conventional $e^+e^-$\ storage ring with a circumference of 28 
km.  It was designed to be able to store electrons with energies in excess 
of 100 GeV, in order to be able to produce both \Zzero's and in a later 
phase $W^+W^-$\ events. Phase I of the LEP collider was completed in 
fall of 1989, which allowed collision energies of between 90 and 100 
GeV. Four detectors were built to be ready at LEP turn-on:
\begin{itemize}
\item OPAL
\item ALEPH
\item L3
\item DELPHI
\end{itemize}
All of these detectors are generable purpose devices,although they 
emphasize different philosophies.  For example, the OPAL (Omni-Purpose 
Apparatus for LEP) detector is perhaps most conventional in design, and 
has been viewed as the ``conservative'' detector.  On the other hand, the 
DELPHI  detector design incorporates several new technologies (such as 
Ring Imaging Cerenkov Counters), which have the potential for providing 
better performance, but are also unproven and pose certain risks. Together, 
these collaborations have already recorded$>200,000$\ \Zzero\ decays by 
fall 1990, and are expected to continue their studies in the next several 
years.

\section{Hadron Colliders}

Although the $e^+e^-$\ colliders produce very clean initial states,they 
cannot compete on the energy frontier with hadron-hadron colliders.  
Whereas the $\sqrts$\ of an $e^+e^-$\ collider 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 one pays for this 
increase in available $\sqrt{s}$\ is that one no longer is colliding 
elementary fermions, but are colliding two systems that consist of several 
strongly-interacting partons.  The practical effect of this is that a high 
energy collision between two of the partons is accompanied by the 
``underlying event'' that results from the diassociation of the hadrons 
during the collision process.  The experience at hadron colliders in the past 
decade have shown that one can experimentally take into account the 
effects of the underlying event, but the resulting physics analysis is 
nevertheless complicated by its presence. 

Another effect that is not present at an $e^+e^-$\ 
collider is that the interaction 
cross section at a hadon collider is dominated by ``soft'' interactions, so that 
the more interesting high-\pt\ events occur at rates that are many orders of 
magnitude smaller.  For example, a typical total cross section at a hadron 
collider is of order 10 mb, while a typical Electroweak production cross 
section (such as \W\ or \Zzero\ production) is 
on the order of 10 nb, a factor of 
$10^{-6}$.  To study these relatively rare high-\pt\ interactions, an 
experimentalist must actively reject these backgrounds, partly by designing 
a sophisticated detector trigger.  

 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 processes.  The inherent problem 
was that the relatively low $\sqrt{s}$\ available for high-\pt\ collisions 
made them quite rare compared with the rate of the ``minimum-bias'' 
interactions resulting from the disassociation of the hadrons as they collided 
with each other.  A major step in overcoming these problems was taken 
with the development of the S\ppbar S, a proton-antiproton collider 
constructed in the tunnel originally used to house the SPS  proton 
synchotron.  

The most significant problem faced by this project was the the difficulty of 
generating enough anti-protons that could be accelerated and injected into 
the storage ring. 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.  In order to coalesce these \pbar's into a 
small bunch (\ie\ reducing the emittance of the beam) the average position 
of the \pbar\ bunch is used to apply a small correction to the bunch orbit in 
the other side of the ring.  This correction, on average, reduces the phase 
space of the \pbar\ bunch (which is why this technique is known as 
``stochastic cooling'').  When enough \pbar's have been accumulated and 
when their bunch has been cooled to the appropriate size and emmittance, 
they are then extracted from the accumulator and injected into the proton 
storage ring and accelerated to collision energy.

The CERN S\pbarp S\ collider was the first machine to employ this 
technique of stochastic coolling.  The collider was able to produce 
collisions with a $\sqrt{s}$\ of 630 GeV at a luminosity of 
$\sim3\times10^{30}\ \cms$.  The collider had two instrumented 
interactions regions.  One of the regions was instrumented with the UA1  
detector, a general purpose detector consisting of a large charged tracking 
volume immersed in a dipole field and almost completely surrounded by a 
calorimeter.  The other region was instrumented with the UA2 detector, 
which consisted of a non-magnetic tracking system partially contained 
within a calorimeter.  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, 
which was not shared by the UA2 detector, gave the experiment the 
necessary tools to disover the \W\ boson, and subsequently its neutral 
current partner, the\Zzero.  Although UA2 almost immediately confirmed 
these discoveries, the inability to confidently detect the neutrino resulting 
from the leptonic decay of the \W\ made it difficult for that experiment to 
compete with UA1.

By the time the \SppbarS\ collider was under construction, a similar hadron 
collider was under construction in the US.  Known as Isabelle and located 
at Brookhaven Natonal Laboratory, it was designed to collide protons 
together with $\sqrts=300$\ GeV.  Unfortunately, the development of the 
superconducting dipole magnets fell so far behind schedule that the project 
was effectively cancelled in the mid-1980's.  However, by this time, 
construction of a \pbarp\ collider at Fermilab was well underway.  This 
device, known as the Tevatron collider, was designed around the Fermilab 
synchotron and had the ability  to achieve $\sqrts=1800$\ 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 currently 
instrumented with large, general purpose detectors.  One of them is CDF, 
which had its first major physics run in 1988.  It  consists of a cylindrical 
tracking volume immersed in a solenoidal magnetic field and almost 
completely surrounded by calorimetry, except for a $3^\circ$\ degree 
holes around the beamline.  The other detector, D0, takes its name from 
the interaction region it occupies.  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, the detector does not 
employ a magnetic field to perform 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, which is currently 
scheduled to start in fall 1990.

The rate of interactions involving electro-weak 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:Tevatron rates}\ the approximate Tevatron cross 
sections for a variety of physics processes
\begin{figure}
\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
}}
\label{table:Tavatron rates}
\end{figure}
Thus, 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 and that as the energy of the 
collider increases, the problems in some sense become easier as the 
relative rate of ``interesting'' physics is inherently larger. 

\subsection{ep Colliders}
High-energy $ep$\ collisions are a unique way of studying several physics 
phenomena
\begin{itemize}\item proton structure
\item electroweak interaction
\item search for new particles (leptoquarks, supersymmetry, etc.)
\end{itemize}
There have been several proposals to build an$ep$\ collider, but the 
HERA facility at DESY will be the first one.  The collider is designed 
\begin{itemize}\item will collide 30 GeV $e^-$'s with 820 GeV 
$p$'s\item design \Lum\ is $15\times 10^{30} \cms$\item 210 bunches in 
the proton ring\item four interaction regions, with two instrumented 
initially\begin{itemize}\item ZEUS\item H1Both of these are state-of-the-
art detectors, with particularemphasis on calorimetry.Detectors reflect the 
asymmetry in the beam energies.\end{itemize}\end{itemize}\vfil%%

\section{SUMMARY of Existing Colliders}
\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{tab: collider summary}
\end{table}
