\section{MUON IDENTIFICATION AND TRACKING SYSTEMS}
\label{sec-muons}

  The muon detection strategy at CDF and D\O \ relies on the
penetration power of muons.  Several meters of absorber (including
the calorimeters) absorbs the vast majority of hadrons, and any
charged particle that penetrates this material is inferred to be
a muon.  Because they are at large radius, muon detectors are large,
and to keep costs reasonable, they have very coarse granularity:
typically they are single wire proportional chambers with drift
times in excess of a microsecond.

  In Run~I, the beam crossing time was 3.5~$\mu{\rm s}$, so 
identifying which beam crossing produced the muons could be
done unambiguously.  However, with the 396 and 132~ns crossing
times of Run~II, this will not be true unless changes are made 
to the detector system.  This is particularly important at the
trigger level, since the function of the trigger is to select
events of interest.

   Both detectors have chosen to solve this problem by adding
scintillator counters adjacent to the chambers.  Scintillator
has a response time measured in nanoseconds, so that the 
coincidence between the counter and the chamber unambiguously
determines the bunch crossing.  Because the timing resolution
is substantially better than the minimum required to resolve
bunch crossings, we gain the ability to use timing to reject
certain backgrounds: particularly out of time particles 
produced upstream of the interaction point and cosmic rays.

\subsection{CDF Muon Upgrade}

   The CDF Run~I muon system was composed of several pieces.  The
CMU, or original Central MUon chambers~\cite{CMU}, covered from 
$0 < |\eta| < 0.6$ and were located just behind the central and
hadronic calorimeters, corresponding to approximately 5 hadron
absorbtion lengths.  Behind an additional 2 feet of steel was 
the Central Muon uPgrade, or CMP, another set of drift chambers.
The background from punch-through was an order of magnitude lower
in the CMP than the CMU, at a price of a higher momentum threshold:
just over 2~GeV as compared to 1.4~GeV.  Only 2/3 of $\phi$ of 
CMP was instrumented for Run~I, and the remaining 1/3 of CMU
that did not have CMP coverage dominated the trigger rate.
Forward of CMU and CMP, covering
the region from $0.6 < |\eta| < 1.0$ was the Central Muon 
eXtension, or CMX.  Again, only about 2/3 of $\phi$ for the CMX was
instrumented.  Scintillator counters were installed around the 
CMX and, in the later part of Run~I, behind part of CMP. While 
there are some differences in details of the chamber construction
for the three systems, they are still quite similar: large 
single-wire drift cells.  These detectors served solely to 
identify particles as muons: their momenta was measured in
the central tracking chamber.  

%\begin{figure}[ht]
%\begin{center}
%\epsfxsize=3.5 in
%\epsfbox{cdf-muon-etaphir1.eps}
%\end{center}
%\caption{Run~I CDF Central Muon Coverage.  CMU, CMX and CMP
%detectors are shown.  Light gray hatched areas indicate 
%gaps in the coverage.}
%\label{fig-cdf-muon-run1}
%\end{figure}

   In the forward region ($|\eta| > 1.9$), 
a different technology was chosen, that of electrodeless drift
chambers in between two large steel toroids \cite{FMU}.  For
these muons, momentum was measured by measuring the
muon's deflection in the toroidal fields.

   The Run~I CMU operated in limited streamer mode.  With the
higher event rate (and therefore muon rate) in Run~II, it would
be difficult to maintain efficient streamer formation.  Instead,
it was decided to operate the chambers at lower gain, in the
proportional region.  Doing so requires enhancing the signal 
by installing preamplifiers on the chamber end electronics.  
This was done between Run~I and Run~II.

  Unlike the rest of the CDF muon upgrade, there are no 
scintillators installed near CMU.  There simply isn't any
space to do this.  This is less problematic than it might
appear, even though the CMU drift time is 800~ns, causing
an ambiguity of 3 (7) crossings at 396 (132)~ns.  For a 
high $p_T$ muon, the muon will also be detected by the CMP
and its scintillator, so that scintillator can provide the
bunch identification.  For low $p_T$ muons, such as those
from $J/\psi$ decays, necessary for the $B$ physics programs,
we are interested only in pairs of muons.  The rate of 
a pair of 1.5~GeV tracks in events is sufficiently small
to settle the ambiguity in bunch crossing assignment.

For Run II, a significant planned changed to the CMP and CMX is
to improve the coverage.  CMP will be complete in $\phi$,
and CMX will be free of large gaps except for a $30^o$
section on the east side to accomodate the solenoid's
cryogenics.  Additionally, scintillator will be added
to the CMP to select the correct bunch crossing, as 
mentioned above.  Each counter is twice as wide as a 
chamber and half as long.  
The $\eta - \phi$ coverage of the various muon detectors is shown in 
Fig.~\ref{fig-cdf-muon-run2}. 

%The arrangement of the scintillators
%and chambers is shown in Fig.~\ref{fig-cdf-muon-cmpcsp}.

%\begin{figure}[ht]
%\begin{center}
%\epsfxsize=3.5 in
%\epsfbox{cdf-muon-cmpcsp.eps}
%\end{center}
%\caption{A muon detected by the CMP and CSP.  (IMU and CMX
%are similar)  As it travels from left to right, it penetrates
%several feet of steel, then traverses a set of chambers, and
%finally a counter.  While the counters do not overlap over
%the length of the chamber, the two layers are slighly offset
%to prevent mechanical interferences.}
%\label{fig-cdf-muon-cmpcsp}
%\end{figure}

\begin{figure}[h]
\begin{center}
\epsfxsize=3.5 in
\epsfbox{cdf-muon-etaphir2.eps}
\end{center}
\caption{Run~II CDF Central Muon Coverage.  CMU, CMX, CMP
and IMU detectors are shown.  The remaining gaps in the
coverage are the bottom $90^\circ$ of the IMU, because of
the shielding motion system and the east $30^\circ$ of the
CMX, because of the cryogenic plant for the solenoid.}
\label{fig-cdf-muon-run2}
\end{figure}

The Run~I forward muon system was determined to be unlikely
to operate at Run~II rates.  Rather than replace it with
a new system that covered the same rapidity, CDF instead 
elected to replace it with a system that expanded the
muon coverage in the central-intermediate region 
(The IMU, or Intermediate Muon system).  A
set of drift chambers similar to those for the CMP and
CMX was constructed and installed in a barrel around the
toroids, which have had their coils removed, additional
shielding added (see below) and been moved 5~m closer
to the interaction point.  A layer of counters (again,
twice as wide and half as long as a chamber) surrounds
this barrel to identify crossings.  Also, a layer of
counters is mounted between the toroids from $\eta=1.2$
to $\eta=1.5$.  This requires that any muon candidate
identified in this region to form a coincidence with
another counter behind a substantial amount of steel:
it is highly unlikely that a coincidence could be 
``faked" by a single particle on any trajectory.  Momentum
of the muons in this region is measured in the central
tracker (similar to more central muons) rather than using
independent toroidal magnets.  This improves the momentum
resolution, as well as providing a higher degree of
uniformity of all CDF muons. The locations of these 
new detectors in the $\eta-\phi$ plane for Run~II 
is shown in  Fig.~\ref{fig-cdf-muon-run2}.


%In comparison, the CDF shielding is 
%somewhat simpler, largely because the 
%muon coverage stops at $\eta=1.5$, and the backgrounds are smaller in
%the central region than the forward region.  The steel from the
%toroids provides substantial shielding to begin with, and 
%rings of additional steel were welded in positions designed such
%that neither a primary particle nor a secondary produced in 
%interactions with the beampipe has a path to an active detector
%unobstructed by steel.


\subsection{D\O's Central and Forward Muon Detectors and Forward Shielding}

The D\O \ muon detector received a substantial upgrade due to the realization
that the performance of the Run~I proportional drift chambers, particularly
in the forward region, would deteriorate unacceptably with Run~II 
luminosities. 

The muon system has three large toroid magnets, one central and 
two forward, which act as absorber in addition to the calorimeter, 
provide a structure on which to mount the muon detectors, and provide 
a bend to the muons so the detectors can measure the momentum. 
The calorimeters contain between 7 and 10 interaction lengths of material, 
depending on the pseudorapidity. The thickness of the calorimeters 
plus the toroid magnets varies between 13 to 14 interaction lengths for 
$|\eta|\le 0.9$, 10 to 15 interaction lengths for $0.9 \le |\eta| \le 1.2$,
and 18 to 20 interaction lengths for $1.2 \le |\eta| \le 2.0$.  Muons 
with momentum greater than $\sim 1.4$ GeV/c ($\sim 3.5$ GeV/c) penetrate
the calorimeter (toroid magnet) at $\eta=0$. 

The D\O \ detector relies on layers of scintillation counters and drift 
chambers to identify muons and measure their position and momentum. The 
detector layers inside the magnets are referred to as the ``A-Layer", while 
the layers outside the magnets are referred to as the ``B" and ``C-Layers". 
The upgrade of the central region ($|\eta| \le 1$) consists of the addition of
scintillation counter coverage to the A-Layer and C-Layer. The upgrade of the 
forward region ($1 \le |\eta| \le 2$) consists of three layers of entirely new
scintillation counters and mini-drift tubes. In addition, new massive 
shielding structures isolate the muon detectors from backgrounds generated near
the beampipe and accelerator elements.

\subsubsection{CENTRAL MUON SYSTEM DRIFT CHAMBERS}
The central muon drift chambers were retained Run~I but their electronics 
have been replaced. The drift chambers are made from extruded aluminum cells
of 4-inch width and lengths up to 228 inches. The wires in the cells 
are parallel to the field in the toroid magnets so that the bending of the 
track in the toroids takes place in the drift ordinate. 

Individual drift chambers (PDT's) in the C- and B-Layers consist of three 
staggered decks of up to 24 cells each.  Drift chambers in the A-Layer consist
of four staggered decks of 24 cells each, except for the ones on the bottom, 
which are three deck PDT's. The top and bottom of each cell has a copper-clad 
cathode pad. The copper has a milled cut separating it into an inner and 
outer pad such that the width of the inner pad alternately increases and 
decreases along its length. The wavelength of the vernier is 24 inches. 
Pairs of wires are connected through a delay chip at the end away from the 
front-end electronics.  On passing through a cell, a muon will cause a 
hit in the cell and a hit in the neighbor cell which is some time later 
depending on the muon's proximity to the far end.  Charge is accumulated on 
the inner and outer pads of the cell through which the muon passed. The drift
time is derived from the sum of the two cells times.  The distance along 
the wire is derived from the difference. The normalized difference of the 
integrated pad charge provides the distance along a pad wavelength. 

The Run~I A-Layer PDT's were rebuilt so as to increase their effective 
lifetimes.  The ends of the PDT's were removed.  The cathode pads, which 
outgassed a dielectric that coated the anode wires in Run~I, were replaced 
with new G-10 pads which don't.  The lifetime of the B- and C-Layer PDT's 
is long enough so that aging won't pose a problem for those detectors.

The PDT's use a drift gas composed of 80\% argon, 10\% CF$_4$, and 
10\% CH$_4$. The maximum drift time is approximately 500 ns, longer 
than the bunch spacing. This poses no problem for the electronics, which 
records each hit in as many crossings in which it could have occurred. 
The time measurements are made in 1.8 ns bins.  The drift ordinate resolution
is $\sim 500 \mu {\rm m}$ per hit, limited by the fluctuations in the drift
time due to the gas. 

The muon momentum is calculated from the bend in the toriod magnet as 
determined from the difference in slopes between the line formed from the 
interaction point and the A-Layer hits and the line through the B- and 
C-Layer hits. The momentum resolution is expected to be 
$\sigma{1/p}=0.18/p \otimes 0.005$ with p in GeV/c. This momentum resolution
is worse than that expected from the fiber tracker measurement, especially 
at low $p_T$.  

\subsubsection{CENTRAL MUON SYSTEM SCINTILLATION COUNTERS}
An important part of the upgrade is two new layers of scintillation counters.  
These detectors not only tag the bunch crossing from which the muons originate
for the slow drift chambers, but also reject background particles which leave
hits at times other than expected from a muon originating at the interaction 
point. 

The A-Layer receives 630 ``A$\phi$" counters, each approximately 32" long 
and spanning ~4.5 degree in azimuthal angle. There are 9 counters in a row 
spanning $-1.0 \le \eta \le 1.0$.  Each row has a slight overlap with a 
neighboring tow so as to minimize the cracks between counters. The counters 
are made from 1/2 inch thick Bicron scintillator plates with many Bicron 
BCF'92 wavelength shifting fibers 
embedded in deep grooves. The fibers collect and transmit the light to a 
single photomultiplier tube (PMT).  
%The counters are encased in light-tight 
%aluminum boxes. The PMT's are encased in light-tight magnetic shields. 
The counters have a time resolution of $\sigma = 4$ ns and are expected to 
discriminate between muons produced in the collisions and the background, 
which is particles backscattered from the calorimeter exit, and which 
arrives ~14 ns later than would a muon. 
The ``A$\phi$" counters span 93\% of the azimuthal angle. There is a gap 
in the A-Layer coverage where the calorimeter is supported by the detector 
platform.

A layer of scintillation counters has been added on the outside of the 
muon toroid magnet. The 240 ``Cosmic Cap" C-Layer scintillation counters 
were deployed late in the second half of 
Run~I and previously have been described in 
detail~\cite{d0scintnim}.  These counters are between 81.5 and 113 inches 
long and 25 inches wide. Eight of them are mounted on the outside of each 
C-Layer PDT on the top and sides of the central muon detector. 
Underneath the toroid magnet, the three layer coverage is broken up because 
of the support structure for the central platform and toroid magnets. 
120 New ``Cosmic Bottom" counters are arrayed on bottom C-Layer and 
B-layer PDT's. 
The Cosmic Cap and Cosmic Bottom counters are made from 1/2 inch thick 
Bicron scintillator with Bicron BCF'91A wavelength shifter fibers embedded
in grooves. Each counter is readout with two photomultiplier tubes. 
One advantage of using two PMT's is that coincidental tube noise is 
improbable. Another is the immediate redundancy available in case one
of the PMT's fails. 

The scintillation counters have an LED pulser calibration system capable
of providing a clocked, timed, amplitude-controlled photon pulse. Each PMT 
is connected by a light-shielded optical fiber to a light-tight box which 
houses bundles of LED's glued into a clear acrylic block. A single box 
may provide the photon pulse for up to 100 PMT's. The stability of the 
photon pulse is monitored with a light-sensitive diode housed in the 
clear block. This system allows the timing and amplitude to be 
monitored and controlled. 

The muon system has a wide range of options available for triggering. 
Triggers may be composed of coincidences of in-time hits in scintillation 
counters, hits in layers of the PDT's, and tracks found in the central
tracker.  Low-$p_T$ muons would rely on hits only in the A-layer detectors
and fiber tracker. High-$p_T$ muons would also use scintillation counters
and PDT's in the B- and C-Layers.

\subsubsection{FORWARD MUON SYSTEM DRIFT TUBES}

\begin{figure}[ht]
\begin{center}
\epsfxsize=4.5 in
\epsfbox{d0-mdt.eps}
\end{center}
\caption{Dzero forward muon mini-drift tube (MDT) plane. The octant
boundaries are shown.}
\label{fig-d0-mdt}
\end{figure}


The Mini-Drift Tube (MDT) system~\cite{abramov}
is comprised of three planes of drift-tubes, with one plane in front of, and 
two planes behind the forward toroid magnet.
 The layers are divided into octants with 
tubes of different length depending on position in the octant. 
As in the central region, the MDT A-Layer has four decks of drift tubes and 
the B and C-Layers have three decks each. The drift tubes, made from long 
aluminum extrustions of eight 1 cm. square cells, are contained in  plastic 
sleeves.  Wires in the cells are oriented parallel to the magnetic field 
of the forward toroid magnet.  The sleeves of tubes are mounted on an 
aluminum support structure which also provides mechanical support for the 
infrastructure. A plane of MDT counters is shown in Fig.~\ref{fig-d0-mdt}. 

\begin{figure}[ht]
\begin{center}
\epsfxsize=4.5 in
\epsfbox{d0-mupixels.eps}
\end{center}
\caption{Two D\O \ forward muon pixel octants.}
\label{fig-d0-mupixels}
\end{figure}


The MDT's use a non-flammable gas mixture composed of 90\% CF$_4$ and 
10\% CH$_4$, with a maximum drift time of about 60 ns.  The momentum
resolution is limited by multiple Coulomb scattering in the iron toroid
and the hit resolution of the detector. The MDT electronics uses a coarse 
digitization of the drift time (18.8 ns time bin).  The momentum resolution
is roughly $\sigma(p_T)/p_T = 0.2 $.  Importantly, it is on par with the 
resolution of D\O 's central  tracker in the forward region where the full 
coverage of the fiber tracker's layers has ended. 
    
\subsubsection{FORWARD MUON SYSTEM SCINTILLATION COUNTERS}
Three layers of ``Pixel" scintillation counters~\cite{abramov}
are added to the forward region (approximately $1.0 \le |\eta| \le 2.0$). 
Their primary role is muon triggering.  The $\sim 4800$ Pixel counters 
have segmentation $0.1 \times 4.5^\circ$ in $\Delta \eta \times \Delta \phi$.  
Most of the trapezoidal shaped pixel counters are made from 0.5" thick Bicron 
scintillator with wavelength shifting bars. They are each read out by a 
single PMT. The counters are held in protective aluminum containers with a 
steel fastener, on one corner, which mates to the magnetic shield of the PMT. 
A few of the counters have special space constraints and are made with 
wavelength shifting fiber so as to allow more flexibility in the orientation 
of the PMT and magnetic shield.  The same kind of LED pulser calibration 
system used in the central muon scintillators is used for the Pixel counters. 

The counters are assembled into octants of $\sim 100$ counters each.
The octants provide mechanical support for the counters and their 
infrastructure.  These are, in turn, mounted directly onto the A- and 
B-Layer sides of the forward toroid magnets and onto the inside face of the 
C-Layer support frame.  Two octants are shown in Fig.~\ref{fig-d0-mupixels}. 

Forward muon triggers are formed from coincidences in the three layers of
scintillation counters consistent with a muon of a given momentum. The 
MDT's provide a $p_T$ measurement at trigger level, especially important
in the fiducial region where the fiber tracker has reduced coverage.

\subsubsection{SHIELDING}
D\O 's experience in Run~I was that hits in the muon detectors were, in 
general, due to backgrounds generated at high $\eta$ from the interaction
of the beam jets with forward elements of the detector and accelerator 
hardware, such as the beampipe and low-beta quadrupole magnet. One way to
reduce those backgrounds is to shield the muon detectors from the sources.
The shielding is built in several large, movable sections extending from the 
endcap calorimeters, through the forward toroid magnets, to the Tevatron
tunnels. The shields themselves totally contain the accelerator elements
within the collision hall, including the low-beta quadrupole magnet, inside
a case of 20 inches of iron, six inches of polyethylene, and two inches of
lead.  The shielding is shown in Fig.~\ref{fig-d0det}. 
CDF uses the steel from the former toroid magnets as shielding
in addition to shielding installed in the tunnel for both experiments.  
Note that the D\O \  muon system covers a larger pseudorapidity than
CDF's, and is perhaps more sensitive to these sorts of backgrounds.

%
%Figure~\ref{fig-d0-shield} shows an elevation view of one half of the D\O \ 
%upgrade detector with emphasis on the muon shielding.
%
%The shielding is 
%built in several large, movable mechanical sections. The first section, 
%extending from the calorimeter to the B-Layer side of the end toroid magnet, 
%is comprised of a square 20" hole for the beampipe and detector elements, 
%followed by 16" of iron, 6" of polyethylene, and 2" of lead. The second 
%section, spanning the region between forward the B- and C-Layers, is comprised
%of a square 25" hole for the low-beta quadrupole magnet, followed by 20" of 
%iron, 6" of polyethylene, and 2" of lead. The third and fourth sections 
%occupy space in the forward C-Layer. They are C-shaped, keyed in the  
%middle to form a rectangle with the same cross section as the B-layer 
%shielding.  The two halves of the third section can be moved apart to
%allow the 2nd section to be inserted in the gap as the forward muon toroid
%magnet is advanced towards the C-layer, allowing access to the A-Layer, 
%calorimeter and tracking. The total weight of the shielding on each 
%side of the detector is 141 tons.
% 35t, 47.3t, 16.9t ,21t, 20t
%
%
%\begin{figure}
%\begin{center}
%\epsfxsize=5.5 in
%\epsfbox{d0-shield.eps}
%\end{center}
%\caption{Dzero shield.}
%\label{fig-d0-shield}
%\end{figure}

