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\begin{document}

\pagestyle{empty}
\begin{titlepage}

\noindent
\begin{center}
%{\it {\large version of \today}} \\[.3em] 
\begin{small}
\begin{tabular}{llrr}
%Submitted to & \multicolumn{3}{r}{\footnotesize Electronic Access: {\it http://www-h1.desy.de/h1/www/publications/conf/conf\_list.html}} \\[.2em] \hline 
%Submitted to & \multicolumn{3}{r}{\footnotesize {\it www-h1.desy.de/h1/www/publications/conf/conf\_list.html}} \\[.2em] \hline 
Submitted to & & &
\epsfig{file=/h1/www/images/H1logo_bw_small.epsi
,width=2.cm} \\[.2em] \hline
\multicolumn{4}{l}{{\bf
                International Europhysics
                Conference on High Energy Physics},
                July~12,~2001,~Budapest} \\
 {\bf EPS 2001:} 
                 & Abstract:        & {\bf 802}    &\\
                 & Parallel Session & {\bf 5}   &\\
%                 & Plenary Session  & {\bf ??}   &\\[.7em]
\multicolumn{4}{l}{{\bf
               XX International Symposium on Lepton and Photon Interactions}, 
               July~23,~2001,~Rome} \\ 
{\bf LP 2001:}  
                 & Abstract:        & {\bf 495} &\\
                 & Parallel Session & {\bf 11}   &\\
%                 & Plenary Session  & {\bf ??}   &\\ \hline
 & \multicolumn{3}{r}{\footnotesize {\it
    www-h1.desy.de/h1/www/publications/conf/conf\_list.html}} \\[.2em]
\end{tabular}
\end{small}
\end{center}
\vspace*{2cm}

\begin{center}
  \Large {\boldmath \bf Observation of isolated leptons with missing
    $P_T$ and comparison to $W$ production at HERA}

  \vspace*{1cm}
    {\Large H1 Collaboration} 
\end{center}

\begin{abstract}
\noindent
A search for events with a high energy isolated lepton and missing
transverse momentum has been performed at the electron-proton collider
HERA using an integrated luminosity of 13.6 pb$^{-1}$ in $e^-p$
scattering and 101.6 pb$^{-1}$ in $e^+p$ scattering. Within the
Standard Model such events are expected to be due to $W$ boson
production, with subsequent leptonic decay. The analysis has been
tuned to maximise the acceptance of such events, and reject other
Standard Model processes. In $e^-p$ interactions no events are
observed, consistent with the expectation of the Standard Model in
this low luminosity sample. In the $e^+p$ data 18 events are seen in
the electron and muon channel compared to an expectation of 10.5 $\pm$
2.5 dominated by $W$ production (8.2 $\pm$ 2.5). The excess above the
expectation is mainly due to events with transverse momentum of the
hadronic system greater than 25 GeV where 10 events are found compared
to 2.8 $\pm$ 0.7 expected.
\end{abstract}


\end{titlepage}

\pagestyle{plain}

\section{Introduction}
\label{sec:intro}

The HERA collaborations have previously reported
\cite{origisol,isol,wzeus} the observation of events with high $P_T$
isolated leptons and missing transverse momentum in $e^{+}p$
collisions during the period 1994-1997.  The only Standard Model
process which contributes significantly to this event signature is
leptonic $W$ decay. H1 has reported \cite{isol} one $e^-$ event and 5
$\mu^{\pm}$ events compared to expectations from the Standard Model of
2.4$\pm$0.5 and 0.8$\pm$0.2 for the $e^{\pm}$ and $\mu^{\pm}$ channels
respectively, which are dominated by $W$ production. For the same data
taking period ZEUS has reported \cite{wzeus} 3 (0) $e^{+}$
($\mu^{\pm}$) events compared to an expectation of 2.1 (0.8) $W$
events and $1.1\pm0.3$ ($0.7\pm0.2$) background events for the
$e^{\pm}$ ($\mu^{\pm}$) channel.

An extended analysis, developed to preferentially select events
containing a high energy isolated electron or muon and missing
transverse momentum, whilst rejecting other background processes, has
also been presented in \cite{wosaka,nickthesis}. In this paper we
present the results of a similar search using the 36.5~pb$^{-1}$ of
$e^+p$ data taken in 1994-1997 at an electron-proton cms energy of
$\sqrt{s}=300$ GeV , the 13.6~pb$^{-1}$ of $e^-p$ data (1998-1999,
$\sqrt{s}=318$ GeV) and the complete 65.1~pb$^{-1}$ of $e^+p$ data
(1999-2000, $\sqrt{s}=318$ GeV). ZEUS have also updated their analysis
in \cite{zeusosaka}.

This paper is organised as follows. Section \ref{sec:monte} describes
the Standard Model processes that contribute to the signal and to the
background.  Section \ref{sec:exper} describes the H1 detector and
experimental conditions. Section \ref{sec:lephad} outlines the lepton
identification criteria and the hadronic reconstruction methods. The
selection requirements and the results for the electron\footnote{In
  this paper ``electron'' refers generically to both electrons and
  positrons.  Where distinction is required the terms $e^-$ and $e^+$
  are used.}  and muon channels are described in sections
\ref{sec:lepsel} and \ref{sec:results}. Section \ref{sec:kinem}
discusses the kinematics of the selected events. The paper is
summarised in section \ref{sec:summary}.

\section{Standard Model Processes} 
\label{sec:monte}


In this section the processes within the Standard Model that are
expected to contribute to signal and background are outlined.  These
processes are described in detail in \cite{isol}.

\begin{itemize}
\item {$W$ production}
  
  The generator EPVEC \cite{epvec}, which uses leading order QCD
  calculations, was used to estimate the Standard Model expectation
  for $W$ production. The total W production cross section amounts to
  1.1 pb for an electron-proton cms energy $\sqrt{s}=300$ GeV and 1.3
  pb for $\sqrt{s}=318$ GeV. Final state parton showers are simulated
  using the PYTHIA framework \cite{epvecmcw}. An error of 30\% is
  quoted for this cross section calculation.  This is due mainly to
  uncertainties in the photon parton density functions and the scale
  at which the calculation is performed.

\item {Charged Current (CC) processes : $ep\to\nu X$}
  
  A CC DIS event can mimic the selected topology if a particle in the
  hadronic final state is misinterpreted as a candidate isolated
  lepton.  The generator DJANGO \cite{django} was used to calculate
  this contribution. Studies of CC events where a candidate electron,
  passing loose selection requirements, is found show that events of
  this kind are described by the expectation within a systematic error
  of 30\% (see section~\ref{sec:lephad}).

\item {Neutral Current (NC) processes : $ep\to eX$} 
  
  The scattered electron in a NC DIS event can be identified as an
  isolated high lepton, but missing transverse momentum can only be
  produced by fluctuations in the shower or detector response to the
  final state particles, or by limited geometrical acceptance.  The
  generator DJANGO \cite{django} was used to calculate this
  contribution. The generator PYTHIA \cite{pythia} was used to
  calculate the QCD photoproduction process $\gamma p\to X$.
  Production of heavy flavours is included in DJANGO and PYTHIA
  \cite{aroma}. Studies of NC events with reconstructed missing
  transverse momentum show that events of this kind are described by
  the expectation within a systematic error of 30\% (see
  section~\ref{sec:lephad}).

  
\item {Lepton pair production in two photon ($\gamma\gamma$)
    interactions}
 
  Inelastic lepton pair production can mimic the selected topology if
  one lepton escapes detection, and measurement error causes apparent
  missing momentum. The generator LPAIR \cite{lpair} was used to
  calculate this contribution. Studies of events with two identified
  leptons show that events of this kind are described by the
  expectation within a systematic error of 75\%.


\end{itemize}

In order to determine acceptances and background contributions for the
selected events, the detector response to events produced by the above
programs is simulated in detail using a program based on
GEANT~\cite{GEANT}. The simulated events are then subjected to the
same reconstruction and analysis chain as the real data.

\section{Experimental Conditions} 
\label{sec:exper}

A detailed description of the H1 detector can be found in
\cite{h1det}. Only those components of particular importance to this
analysis are described here.


The inner tracking system consisting of central and
forward\footnote{The forward direction and the positive $z$-axis are
  taken to be that of the proton beam direction.} tracking detectors
(drift chambers) are used to measure the charged particle
trajectories and to determine the interaction vertex.  A magnetic
field of 1.15 Tesla allows the measurement of the particle transverse
momenta.

Electromagnetic and hadronic final state particles are absorbed in a
highly segmented LAr calorimeter \cite{calo}, which is 5 to 8 interaction
lengths deep depending on the polar angle of the particle.

The calorimeter is surrounded by a superconducting coil and an iron
yoke instrumented with streamer tubes. Tracks of penetrating charged
particles, such as muons, which escape the calorimeter are
reconstructed from their hit pattern in the streamer tubes with an
efficiency of greater than $90\%$. The instrumented iron is also used
as a backing calorimeter to measure the energy of hadrons that are not
stopped in the LAr calorimeter.

In the forward region of the detector two sets of three drift chamber
layers (the forward muon system) detect muons and measure their
momenta in a toroidal magnetic field. Also in the forward direction,
around the beam-pipe, is the plug calorimeter which measures hadronic
showers.

The LAr calorimeter is the main trigger for high transverse momentum
events. The trigger efficiency is $\approx 98\%$ for events with an
electron which has transverse momentum above 10 GeV. For events with
high missing transverse momentum, determined from an imbalance in
energy measured in the calorimeter, $P^{\rm calo}_{T}$, the efficiency
is $98\%$ when $P^{\rm calo}_{T}>25$~GeV and is $50\%$ when $P^{\rm
  calo}_{T} = 12 $~GeV \cite{eplus}.

\section{Lepton identification and Hadronic Reconstruction}
\label{sec:lephad}

An electron candidate is defined by the presence of a compact and
isolated electromagnetic cluster of energy in the LAr calorimeter,
with the loose requirement of an associated track, having a distance
of closest approach to the cluster of less than 12 cm.  Electrons
found in regions between calorimeter modules containing large amounts
of inactive material are excluded \cite{eplus}. The energy of the
electron candidate is measured from the calorimeter cluster.

Muons are identified by requiring tracks in the instrumented iron, the
forward muon system, the inner tracking system or any combination.
Muon candidates formed from inner tracks alone must be associated with
a pattern typical of a minimum ionising particle in the LAr
calorimeter. All candidates with an inner track link are required to
have transverse momentum above 1 GeV and candidates formed solely by a
track in the instrumented iron are rejected for $\theta<25^\circ$.

A stronger muon identification is required to select the final sample
of muon candidates. If the muon candidate is not associated with a
forward muon track it must have an inner track link and either a track
segment or an energy deposit in the instrumented iron. The momentum of
the muon candidate is measured from the curvature of the track in the
inner tracking system.

The hadronic final state is measured by combining calorimeter energy
deposits with low momentum tracks as described in \cite{eplus}. The
calibration of the final hadronic energy scale is made by comparing
the transverse momentum of the precisely measured positron to that of
the hadronic final state in a large NC event sample. The ratio of
these two measurements is used to correct the hadronic energy
measurement as a function of the hadronic polar angle and transverse
momentum, typically by less than 10\%.

\section{Selection of Isolated Electron and Muon Events}
\label{sec:lepsel}

Multiple cut requirements must be imposed to remove the background
arising from NC, CC, $\gamma p$ and $\gamma\gamma$ interactions,
whilst maintaining a good acceptance of Standard Model $W$ decay
events.  These are listed in tables \ref{cutsa} and \ref{cutsb} and
are defined in terms of the quantities described below.

\begin{itemize}
\item $P^{\rm calo}_{T}$, the missing transverse momentum measured in the
  calorimeter.
\item $P^{miss}_{T}$, the total missing transverse momentum 
  reconstructed from calorimeters and track detectors.
\item $P^l_T$, the transverse momentum of an identified muon or electron.
\item $P^X_T$, the transverse momentum of the hadronic system, defined
as all reconstructed particles apart from identified isolated leptons.
\item $\theta_l$, the polar angle of the muon or electron, where
  $\theta_l=0^o$ is defined as the direction of the incoming proton
  beam.
\item $\delta_{miss}=2E_{e}-\sum E_i(1-\cos \theta_i)$, where $E_i$
  and $\theta_i$ denote the energy and polar angle of each detected
  particle in the event and $E_e$ is the electron beam energy. For an
  event where only longitudinal momentum in the proton direction is
  undetected $\delta_{miss}=0$.
\item $\Delta \phi_{l-X}$, the difference in azimuthal angle between
  the lepton and the vector that balances the vector of the hadronic
  final state $X$. NC events typically have low values of $\Delta
  \phi_{l-X}$.
\item $V_{ap}/V_{p}$, a measure of the azimuthal balance of the event.
  It is defined as the ratio of the anti-parallel to parallel
  components of the measured calorimetric transverse momentum, with
  respect to the direction of the missing calorimetric transverse
  momentum~\cite{eplus}.  Events with one or more particles, that do
  not deposit much energy in the calorimeter ($\mu$, $\nu$), generally
  have low values of $V_{ap}/V_{p}$.
\item ${\zeta}^{2}_{l}=4 E^{l}E_e \cos^2 \theta_l/2$, where $E^l$ is
  the energy of the final state lepton. For NC events, where the
  scattered electron is misinterpreted as a decay lepton from the $W$,
  ${\zeta}^{2}_{l}$ is equal to the four momentum transfer squared
  $Q^2$. Since the NC cross section falls steeply with $Q^2$, these
  events generally have small values of ${\zeta}^{2}_{l}$. Conversely,
  leptons from $W$ decay generally have high values of ${\zeta}^{2}_{l}$.
\item $E_{cone}$, the energy contained within a cone of radius 1 in
  $\eta$-$\phi$ space around the electron, that is not associated to
  the electron.
\end{itemize}

The isolation of identified leptons with respect to jets or other
tracks in the event, is quantified using:
\begin{itemize}
\item their distance $D_{jet}$ to the closest hadronic jet in the
  pseudorapidity-azimuth plane ($\eta$ - $\phi$), defined by
  $D_{jet}=\sqrt{(\Delta\eta_{track-jet})^{2}
    +(\Delta\phi_{track-jet})^{2}}$ (for this purpose jets are
  reconstructed using an inclusive $k_T$ algorithm \cite{kt1,kt2} with
  $R<1$ and $E^{min}_{T}=5$~GeV). If there is no jet in the event,
  $D_{jet}$ is defined with respect to the polar and azimuthal angle
  of the hadronic final state $X$.
\item their distance $D_{track}$ to the closest track in $\eta$ -
  $\phi$, defined in an analogous way to $D_{jet}$, where all tracks
  with a polar angle greater than $5^{o}$ are considered.
\end{itemize}

The dominant background source in the electron channel arises from NC
events which have significant $P_{T}^{\rm calo}$ due to fluctuations
in the measurement of the final state particles. To reduce this
background, events with NC topology (azimuthally balanced, with the
lepton and hadronic system diametrically opposite in the transverse
plane) are rejected. For low values of $P_{T}^{\rm calo}$, where the
NC background is largest, the requirement on $\zeta^{2}_l$ is
tightened. A further requirement that the lepton candidate be isolated
from the hadronic final state is imposed to reject CC events.  Events
which have in addition to the isolated electron one or more isolated
muons are not considered in the electron channel, but may contribute
in the muon channel.

The dominant backgrounds in the muon channel are due to the inelastic
two photon process $ep\to e\mu^+\mu^-X$ and photoproduction events
which contain a muon. The final sample is selected by rejecting
azimuthally balanced events and events where more than one muon is observed.
Elastic lepton pair production is suppressed by rejecting events which
have low track multiplicities and where the muon is found to be
opposite in azimuth to the hadron system.

To verify that the backgrounds that contribute to the two channels are
well understood, event samples are defined according to less stringent
criteria. For the electron channel these criteria are those shown in
table \ref{cutsa}, but requiring $\zeta^2>500$ GeV$^2$ and the
$\frac{V_{ap}}{V_p}$ cut from table \ref{cutsb}. For the muon channel
these criteria are also those shown in table \ref{cutsa}, lowering the
$P_T^{\mu}$ threshold to 5 GeV, dropping the $P_T^{miss}$ requirement,
but also requiring the $\frac{V_{ap}}{V_p}<$ cut from table
\ref{cutsb}. Only events with at least one isolated muon satisfying
the stronger selection requirements (see section \ref{sec:lepsel}) are
allowed in the muon sample. The events thus selected in the $e^+p$
data sample are shown in figure \ref{fig:econ} for the electron
channel and figure \ref{fig:mcon} for the muon channel.  Also included
in the figures are the Standard Model expectation and the expectation
from $W$ production alone. The electron channel is dominated by NC
events and the muon channel is dominated by the two photon process
$\gamma\gamma\to\mu^+\mu^-$. All quantities that are used for the
final selection cuts are found to be well described in both shape and
normalisation by the Standard Model expectation.  Similar agreement
between the data and simulation is obtained for the $e^-p$ data
sample.

Further studies of individual background processes have also been
performed. One example is a subset of the muon sample described above,
defined by requiring at least two isolated muons, at least one of
which satisfies the stronger selection requirements. This sample is
shown in figure \ref{fig:studies}(a,b). The expectation is dominated
by the two photon process $\gamma\gamma\to\mu^+\mu^-$. The data sample
is shown to be well described in both shape and normalisation by the
Standard Model expectation.


A second sample was selected from the inclusive sample of CC
events\cite{eplus}, with the additional requirement that the electron
finder (see section \ref{sec:lephad}) found a candidate electron. This
sample is shown in figure \ref{fig:studies}(c,d). The expectation is
dominated by radiative CC events or CC events where a final state
particle fakes an electron cluster. The data sample is shown to be
well described in both shape and normalisation by the Standard Model
expectation.

Following the selection criteria described above, the efficiencies as
a function of $P_T^X$ are shown in table \ref{acc}. These efficiencies
were calculated using the EPVEC generator for $W$ production.  The
overall efficiency to select $W \to e \nu$ events is $44\%$ and to
select $W \to \mu \nu$ events is $15\%$. The difference in efficiency
between the two channels is due to the cut on $P_{T}^{\rm calo}$ which
acts as a cut on $P_{T}^{X}$ because the muon deposits little energy
in the calorimeter. There is thus little efficiency in the muon
channel for $P_{T}^{X}<12$~GeV. For values of $P_{T}^{X}>12$~GeV the
efficiencies of the two channels are comparable at around $44\%$.

\begin{table}[t]
  \begin{center}
    \begin{tabular}{|c||c|} \hline
      Variable  & Requirement\\ \hline
      \hline                                                   
      $P_T^l$ & $ >$ 10 GeV\\
      \hline
      $\theta_l$ & $5^o<\theta_l<145^o$\\
      \hline
      $P_T^{\rm calo}$ & $>$ 12 GeV\\
      \hline
      $D_{jet}$ & $>$ 1.0\\
      \hline
      $P_T^{miss}$ & $>$ 12 GeV\\
      \hline
    \end{tabular}
  \end{center}
  \caption{Selection requirements common to both channels.}
  \label{cutsa}
\end{table}

\begin{table}[h]
\begin{center}
\begin{tabular}{|c||c|c|} \hline
 Variable  & Electron  & Muon \\ \hline
\hline
$\zeta^2$ & $>500$ GeV$^2$ & - \\
          & $>5000$ GeV$^2$ for $P_T^{\rm calo} <$ 25 GeV & -\\
\hline
$\frac{V_{ap}}{V_p}$ & $<$ 0.5 ($<$ 0.15 for $P_T^e <$ 25 GeV) & $<$ 0.4 ($<$ 0.15 for $P_T^{\rm calo} <$ 25 GeV)\\
\hline
$D_{track}$ & $>$ 0.5 for $\theta_e \ge 45^o$ &  $>$ 0.5 \\
            & $>$ 0.5 for $\frac{E_{cone}}{E^l}>$ 0.05 &  \\
\hline                                                   
$P_T^X$ &  -  &$>$ 12 GeV\\ 
\hline
$\Delta\phi_{e-X}$ & $>$ 20$^o$ & $>$ 10$^o$ for $\le$ 2 central tracks\\
\hline
\# isolated $\mu$ & 0 & 1\\
\hline
$\delta_{miss}$ & $ >$ 5 GeV $^\dagger$ & - \\
\hline
\end{tabular}
\begin{flushright}
  $^\dagger$ {\it if only one $e$ candidate is detected, which has the same charge as the beam  lepton.}
\end{flushright}
\end{center}
\caption{Selection requirements specific to each channel.}
\label{cutsb}
\end{table}



\section{Results}
\label{sec:results}

For the $e^-p$ data sample no events are observed in either the
electron or muon channels. This compares to the Standard Model
expectations of 1.46 $\pm$ 0.30 events in the electron channel and 0.32
$\pm$ 0.09 in the muon channel.

In the $e^+p$ data sample after all selection requirements 10
candidate events are observed in the electron channel compared to 6.08
$\pm$ 1.83 expected from $W$ production and 1.83 $\pm$ 0.45 from
background sources. One of the candidate events in the electron
channel is observed to contain an $e^-$. This event was first reported
and discussed in \cite{isol}.  Five of the other candidate events
contain an $e^+$.  The charges of the electrons in the remaining
events are unmeasured since the electrons are produced at low polar
angles and they shower in the material of the tracking detectors. Four
of the events in the electron channel have been observed since
\cite{wosaka}.

For the $e^+p$ data sample in the muon channel 8 candidate events are
observed compared to 2.11 $\pm$ 0.63 expected from $W$ production and
0.46 $\pm$ 0.27 from other Standard Model sources.  Five of the muon
events observed in the $e^+p$ data sample are those first reported and
discussed in \cite{isol}. Five of the events have positively charged
muon, 2 have negative muons and one is not determined.



\section{Event kinematics}
\label{sec:kinem}

Distributions of the selected events in lepton polar angle,
acoplanarity, transverse mass and $P_T^X$ are shown in figure
\ref{fig:emfinal}. The figure shows the electron and muon channels
combined. Also included is the Standard Model expectation for $W$
production. The events are generally found at low values of lepton
polar angle and evenly distributed in acoplanarity in agreement with
the expectation.  The reduction in the expectation at low and high
acoplanarity is due to the selection cuts.  The events are distributed
in a Jacobian peak consistent with the nominal W mass, as expected
from Standard W production. At $P_T^X <$ 25 GeV there is good
agreement between the expectation and the observed events. At higher
values of $P_T^X$ the data lie above the expectation.

The scattered positron is tagged in three of the eighteen events,
allowing the lepton-neutrino mass to be reconstructed, under the
assumption that there is only one neutrino in the final state and
there is no initial state QED radiation. All three events yield masses
that are consistent with the $W$ mass, having values of
$82^{+19}_{-12}$, $71^{+10}_{-11}$ and $77^{+22}_{-15}$ GeV.  From
Standard Model $W$ production it is expected that approximately 25\%
of events have a scattered positron in the acceptance range of the
detector.

The transverse mass and the transverse momentum of the hadronic system
of the selected events are compared to W production in
figure~\ref{fig:ptxmlnu}. Events are seen with high $P_T^X$, atypical
of Standard Model W production. 

The significance of missing transverse momentum and acoplanarity has
been studied with data using a sample of NC events having similar
transverse momentum and lepton polar angle to the events selected. The
transverse momentum is reconstructed using the calorimetric deposit
for comparison with the electron events and using the electron track
momentum for comparison with the muon candidates.  The deviations from
zero in the $P_T^{miss}$ and acoplanarity plane for NC events quantify
the experimental smearing. The distribution of this sample is
presented in figure~\ref{fig:dphi_ptmis}. This study clearly shows
that the observed acoplanarities and missing momenta in the selected
events are inconsistent with a measurement error and confirms the
existence of a non-detected particle in those events.


Details of the dependence of the event rate for the $e^+p$ data sample
as a function of the transverse momentum of the hadronic final state,
$P_{T}^{X}$, are given in table \ref{etab1} and \ref{mtab1} for the
electron and muon channels. The combined results for the electron and
muon channels are given in table \ref{emtab}.  At $P_{T}^{X}<25$~GeV
eight events are seen in agreement with the expectation for $W$
production from the Standard Model.  At $P_{T}^{X}>25$ GeV ten events
are seen six of which are seen at $P_{T}^{X}>40$ GeV where the
expectation for Standard Model $W$ production is very low.



\section{Summary}
\label{sec:summary}


A search for events containing an isolated high energy electron or
muon, and missing transverse momentum has been performed on an $e^+p$
data sample corresponding to an integrated luminosity of 101.6
pb$^{-1}$ and on an $e^-p$ data sample corresponding to an integrated
luminosity of 13.6 pb$^{-1}$.  The selection has been optimised to
increase the acceptance of events with a high energy isolated electron
or muon and missing transverse momentum. The analysis also extends to
lower values of hadronic transverse momentum $P_{T}^{X}$ than have
been previously published.

No events are observed in the $e^-p$ data, consistent with the
expectations of 1.46 $\pm$ 0.30 and 0.32 $\pm$ 0.09 for the electron
and muon channels respectively in this low luminosity data sample.
Eighteen events are found in the $e^+p$ data sample which are
kinematically consistent with $W$ production, 10 in the electron
channel and 8 in the muon channel.  One electron event and 5 of the
muon events were first reported and discussed in \cite{isol}.  The
expected rates from the Standard Model are 7.92 $\pm$ 1.88 and 2.56
$\pm$ 0.69 for the electron and muon channels respectively in the
$e^+p$ data sample.

The event rate in both channels is consistent with the expected rate
at low values of $P_T^X$. At $P_T^X>$ 25 GeV, however, the 10 observed
events exceed the Standard Model prediction of 2.82 $\pm$ 0.73.

\section*{Acknowledgements}

We are grateful to the HERA machine group whose outstanding efforts
have made and continue to make this experiment possible.  We thank the
engineers and technicians for their work in constructing and now
maintaining the H1 detector, our funding agencies for financial
support, the DESY technical staff for continual assistance, and the
DESY directorate for the hospitality which they extend to the non DESY
members of the collaboration.

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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\newpage
\begin{table}[ht]
\begin{center}
\begin{tabular}{|c||c|c|} \hline
 & \multicolumn{2}{c|}{Efficiency}\\ \hline
  & $e$ & $\mu$\\ \hline
\hline
$P_T^X>0$~GeV  & 0.44 & 0.15 \\ \hline
$P_T^X>12$~GeV & 0.40 & 0.39 \\ \hline
$P_T^X>25$~GeV & 0.39 & 0.49 \\ \hline
$P_T^X>40$~GeV & 0.39 & 0.53 \\ \hline
\end{tabular}
\end{center}
\caption{Efficiencies of the final selection for $W$ decay in the electron and 
muon channels as a function of the $P_T^X$ requirement. These efficiencies were calculated 
with the EPVEC generator.}
\label{acc}
\end{table}

\begin{table}[htbp]
\begin{center}
\begin{tabular}{|c||c|c||c|c|} \hline
          Electron &  Data & SM expectation & $W$ & Other SM processes\\ \hline
\hline  
 $P_{T}^{X}>0$~GeV  & 10 & 7.92 $\pm$ 1.88  & 6.08 $\pm$ 1.83  & 1.83 $\pm$ 0.45 \\
\hline  
 $P_{T}^{X}>12$~GeV & 5 & 2.57 $\pm$ 0.65  & 2.11 $\pm$ 0.63  & 0.46 $\pm$ 0.16  \\
\hline
 $P_{T}^{X}>25$~GeV & 4 & 1.29 $\pm$ 0.33  & 1.05 $\pm$ 0.32  & 0.24 $\pm$ 0.11  \\
\hline
 $P_{T}^{X}>40$~GeV & 2 & 0.41 $\pm$ 0.12  & 0.40 $\pm$ 0.12  & 0.01 $\pm$ 0.01  \\
\hline
\end{tabular}
\end{center}
\caption{Observed and predicted event rates in the electron channel for all $e^+p$ data.}
\label{etab1}        
\begin{center}
\begin{tabular}{|c||c|c||c|c|} \hline
              Muon &  Data & SM expectation & $W$ & Other SM processes\\ \hline
\hline  
 $P_{T}^{X}>12$~GeV & 8 & 2.56 $\pm$ 0.69  & 2.11 $\pm$ 0.63  & 0.46 $\pm$ 0.27  \\
\hline
 $P_{T}^{X}>25$~GeV & 6 & 1.54 $\pm$ 0.41  & 1.29 $\pm$ 0.39  & 0.25 $\pm$ 0.13  \\
\hline
 $P_{T}^{X}>40$~GeV & 4 & 0.58 $\pm$ 0.16  & 0.53 $\pm$ 0.16  & 0.05 $\pm$ 0.03  \\
\hline
\end{tabular}
\end{center}
\caption{Observed and predicted event rates in the muon channel for all $e^+p$ data.}
\label{mtab1}        
\end{table}

\begin{table}[ht]
\begin{center}
\begin{tabular}{|c||c|c||c|c|} \hline

       Electron and Muon & Data & SM expectation & $W$ & Other SM processes\\ \hline
\hline  
 $P_{T}^{X}>0$~GeV  & 18 & 10.48 $\pm$ 2.53  & 8.19 $\pm$ 2.46 & 2.29 $\pm$ 0.59 \\
\hline  
 $P_{T}^{X}>12$~GeV & 13 & 5.14 $\pm$ 1.31  & 4.22 $\pm$ 1.27  & 0.92 $\pm$ 0.33  \\
\hline
 $P_{T}^{X}>25$~GeV & 10  & 2.82 $\pm$ 0.73  & 2.34 $\pm$ 0.70  & 0.48 $\pm$ 0.18  \\
\hline
 $P_{T}^{X}>40$~GeV & 6  & 0.99 $\pm$ 0.28  & 0.93 $\pm$ 0.28  & 0.06 $\pm$ 0.04  \\
\hline
\end{tabular}
\end{center}
\caption{Observed and predicted event rates in the electron and muon channels combined for all $e^+p$ data. Only the electron channel contributes for
$P_{T}^{X}<12$ GeV.}
\label{emtab}        
\end{table}


\begin{figure}[ht]
\setlength{\unitlength}{1cm}
\begin{picture}(12.0,7.0)
\put(0.,0.0)
{\epsfig{file=/x04/usr/malden/osaka/econ_9400_ptcal.eps,width=7.0cm}}
\put(8.0,0.0)
{\epsfig{file=/x04/usr/malden/osaka/econ_9400_ptx.eps,width=7.0cm}}
\end{picture}
\begin{picture}(12.0,7.0)
\put(0.,0.0)
{\epsfig{file=/x04/usr/malden/osaka/econ_9400_acop.eps,width=7.0cm}}
\put(8.0,0.0)
{\epsfig{file=/x04/usr/malden/osaka/econ_9400_dtrack.eps,width=7.0cm}}
\end{picture}
\begin{picture}(12.0,7.0)
\put(0.,0.0)
{\epsfig{file=/x04/usr/malden/osaka/econ_9400_q2e.eps,width=7.0cm}}
\put(8.0,0.0)
{\epsfig{file=/x04/usr/malden/osaka/econ_9400_dmiss.eps,width=7.0cm}}
\end{picture}
\caption{A comparison of the $e^+p$ data selected with the loose requirements 
  in the electron channel compared to the combined Standard
  Model expectation (open histogram). The total error on the Standard
  Model expectation is given by the shaded band. The $W$ production
  component of the Standard Model expectation is given by the shaded
  histogram.}
\label{fig:econ} 
\end{figure}

\begin{figure}[ht]
\setlength{\unitlength}{1cm}
\begin{picture}(12.0,7.0)
\put(0.,0.0)
{\epsfig{file=/x04/usr/malden/osaka/mcon_9400_thmu.eps,width=7.0cm}}
\put(8.0,0.0)
{\epsfig{file=/x04/usr/malden/osaka/mcon_9400_ptmu.eps,width=7.0cm}}
\end{picture}
\begin{picture}(12.0,7.0)
\put(0.,0.0)
{\epsfig{file=/x04/usr/malden/osaka/mcon_9400_acop.eps,width=7.0cm}}
\put(8.0,0.0)
{\epsfig{file=/x04/usr/malden/osaka/mcon_9400_ptmiss.eps,width=7.0cm}}
\end{picture}
\begin{picture}(12.0,7.0)
\put(0.,0.0)
{\epsfig{file=/x04/usr/malden/osaka/mcon_9400_ptcal.eps,width=7.0cm}}
\put(8.0,0.0)
{\epsfig{file=/x04/usr/malden/osaka/mcon_9400_ptx.eps,width=7.0cm}}
\end{picture}
\caption{A comparison of the $e^+p$ data selected with the loose requirements in the muon channel compared to the combined Standard Model expectation (open histogram). The total
  error on the Standard Model expectation is given by the shaded band.
  The $W$ production component of the Standard Model expectation is
  given by the shaded histogram.}
\label{fig:mcon} 
\end{figure}

\begin{figure}[ht]
\setlength{\unitlength}{1cm}
\begin{picture}(12.0,7.0)
\put(0.,0.0)
{\epsfig{file=/x04/usr/malden/osaka/mcon_lpair_9400_thmu.eps,width=7.0cm}}
\put(8.0,0.0)
{\epsfig{file=/x04/usr/malden/osaka/mcon_lpair_9400_ptx.eps,width=7.0cm}}
\large
\put(5.5,5.0){(a)}
\put(13.0,5.0){(b)}
\end{picture}
\begin{picture}(12.0,7.0)
\put(0.,0.0)
{\epsfig{file=/x04/usr/malden/osaka/cc_study_pte.eps,width=7.0cm}}
\put(8.0,0.0)
{\epsfig{file=/x04/usr/malden/osaka/cc_study_theta.eps,width=7.0cm}}
\large
\put(5.5,5.0){(c)}
\put(13.0,5.0){(d)}
\end{picture}
\caption{Comparison of a $\gamma\gamma\to\mu^+\mu^-$ sample ((a) and (b)) and a charged current sample including a candidate electron ((c) and (d)) with the Standard Model expectation. The error on the Standard Model expectation is given by the shaded band.} 
\label{fig:studies} 
\end{figure}

\begin{figure}[ht]
  \setlength{\unitlength}{1cm}
\begin{picture}(12.0,7.0)
\put(0.,0.0)
{\epsfig{file=/x04/usr/malden/budapest/emharsh_buda_theta.eps,width=7.0cm}}
\put(8.0,0.0)
{\epsfig{file=/x04/usr/malden/budapest/emharsh_buda_acop.eps,width=7.0cm}}
\end{picture}
\begin{picture}(12.0,7.0)
\put(0.,0.0)
{\epsfig{file=/x04/usr/malden/budapest/emharsh_buda_mt.eps,width=7.0cm}}
\put(8.0,0.0)
{\epsfig{file=/x04/usr/malden/budapest/emharsh_buda_ptx.eps,width=7.0cm}}
\end{picture}
\caption{A comparison of the final $e^+p$ data selection in the electron and muon channels combined compared to the combined Standard Model expectation (open histogram).  The total error on the Standard Model expectation is given by the shaded band. The $W$
  production component of the Standard Model expectation is given by
  the dashed histogram.}
\label{fig:emfinal} 
\end{figure}

\begin{figure}[ht]
\setlength{\unitlength}{1cm}
  {\epsfig{file=/x04/usr/malden/budapest/mtptx_budapest.eps,width=18.0cm}}    
  \caption{A comparison of the final data sample in each channel, showing the distribution in $P_T^X$ and $M_T^{l\nu}$. The smaller dots represent the distribution of Standard Model W Monte Carlo events with a luminosity 500 times that of the data sample.}
\label{fig:ptxmlnu} 
\end{figure}

\begin{figure}[ht]
\setlength{\unitlength}{1cm}
{\epsfig{file=/x04/usr/malden/budapest/dphi_ptmis_budapest.eps,width=18.0cm}}    
  \caption{A comparison of the final data sample in each channel, showing the distribution in $\Delta\phi_{l-X}$ and $P_T^{miss}$. The dots represent the distribution of  NC data events.}
\label{fig:dphi_ptmis} 
\end{figure}

\end{document}
