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\begin{titlepage}

\noindent
%Date:     \today  , Version:  1.06        \\
% Editors:  Gerhard Brandt, Christian Veelken, Stefania Xella, CD\\
% Referees: Hinrich Meyer, David South \\
%Comments by
\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 
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\epsfig{file=/h1/www/images/H1logo_bw_small.epsi,width=2.cm} \\[.2em] \hline
\multicolumn{4}{l}{{\bf
                32nd International Conference 
                on High Energy Physics, ICHEP04},
                August~16,~2004,~Beijing} \\
                 & Abstract:        & {\bf  12-0188 }    &\\
                 & Parallel Session & {\bf 4,12}   &\\ \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}
\begin{Large}

{\bf Search for Events with Tau Leptons in $ep$ Collisions at HERA}

\vspace{2cm}

H1 Collaboration

\end{Large}
\end{center}

\vspace{2cm}


\begin{abstract}
  The production of tau leptons in $ep$ collisions is studied with the
  H1 detector at HERA. The identification algorithm is based on the
  search for isolated charged tracks associated to narrow hadronic
  jets detected in the calorimeters, a typical signature of the
  one-prong hadronic tau decay. Using this identification procedure, a
  search for events with high $P_{T}$ isolated tau leptons and missing
  transverse momentum is performed using a data sample collected with the H1
  detector at HERA, corresponding to an
  integrated luminosity of 108 pb$^{-1}$. This search complements the
  previously published observation of isolated electrons and muons in
  events with missing transverse momentum. In addition, a 
  search for events with tau lepton pairs produced in elastic
  photon-photon collisions is performed.
\end{abstract}

\vspace{1.5cm}

\end{titlepage}

%\newpage

%======================================
\section{Introduction}
%======================================

The HERA collaborations H1 and ZEUS have previously reported the
observation of events with isolated high energy electrons and muons in
events with significant missing transverse momentum, $P_T^{miss}$.
\cite{Ahmed:1994aw,Adloff:1998aw,Breitweg:1998aw,Andreev:2003pm} .
The dominant Standard Model (SM) contribution to this topology is the
production of real $W$ bosons with subsequent leptonic decay. Such
events can also be a signature of new phenomena beyond the Standard
Model, for example the production of single top quarks via Flavour
Changing Neutral Currents (FCNC)~\cite{Aktas:2003yd,zeus_singletop}.

\par

In this paper we present a search for events with tau leptons using
the H1~detector. The analysis uses the data collected between 1996 and
2000, corresponding to an integrated luminosity of $108$ pb$^{-1}$. The
hadronic decays of tau leptons are searched for in events with
significant missing transverse momentum. A search for events
containing tau--pairs produced in photon--photon collisions is also
presented here.

\par
 
Another search for events with isolated tau leptons and missing
transverse momentum at HERA was recently published by the ZEUS
Collaboration~\cite{tauzeus}.

\par

This paper is organised as follows. Section 2 describes the physics
processes contributing to signal and background in the search of
events with high $P_T$ tau leptons. Section 3 describes the H1
detector and experimental conditions relevant to the analysis.
Section 4 describes the search for events with tau leptons and missing
transverse momentum. Section 5 presents a search for events with tau
lepton pairs, followed by conclusions.

%======================================
\section{Processes and their Monte Carlo Simulation}
%======================================

% The processes within the Standard Model that are expected to lead to a
% final state containing a tau lepton and missing transverse momentum,
% due to penetrating particles escaping detection in the H1 detector,
% are briefly outlined in this section.

The processes within the Standard Model that contribute to the searched topology are briefly outlined in this section.
\par

The following SM processes produce events containing genuine isolated
tau leptons and genuine missing transverse momentum in the final
state:

% 
%
% DS : not needed..
%The Standard Model processes that produce events containing an high
%$P_T$ isolated tau and total missing transverse momentum, are
%outlined below.

%\par 
%\noindent

\begin{itemize}
\item {\it Production of W bosons: }\\
  In the Standard Model, the main source of isolated tau leptons with
  high transverse momenta is the production of $W$ bosons with
  subsequent decay $W\rightarrow \tau \nu_\tau$. The production of the
  electroweak vector bosons $W^{\pm}$ as well as $Z^0$ is modelled
  using the EPVEC~\cite{EPVEC} generator. The NLO QCD corrections to
  $W$ production~\cite{SPIRA} are taken into account by weighting the
  events as a function of the rapidity and transverse momentum of the
  $W$ boson~\cite{PRIVSPI}.

%\noindent

\item {\it Lepton pair production: }\\
  The contribution from tau pair production is dominated by
  two--photon processes where at least one of the produced taus decays
  hadronically.  For this process, the genuine missing transverse
  momentum is low and should be aligned with the tau pair direction in
  the transverse plane for high $P_T$ tau pair production. 
  The production of muons can also contribute as
  a background to the searched topology if one of the muons fakes a
  tau signature. The contribution from lepton pair production is calculated        using the
  GRAPE~\cite{GRAPE} generator.
\end{itemize}

Other processes contribute to the selected sample through
misidentification or mismeasurement.  A fake tau lepton, fake missing
transverse momentum or both can be reconstructed and may lead to the
topology of interest. The following processes are considered:

% Other processes can contribute to the investigated final state through
% misidentification of leptons as hadrons or through fake missing
% transverse momentum due to measurement fluctuations. The following
% processes do not yield a true $\tau$ lepton, and therefore  contribute to the background:
%\noindent

\begin{itemize}
\item {\it Charged current deep inelastic scattering: } \\
  The main background to the tau and $P_T^{miss}$ topology is the
  charged current (CC) deep inelastic scattering process, which
  contains genuine large missing transverse momentum due to the
  scattered neutrino. In these events, a hadronic jet can have low
  multiplicity and mimick the tau signature. This background
  contribution is calculated using the DJANGO~\cite{DJANGO} program.

%\noindent
  

\item {\it Neutral current processes:} \\
  Events containing no genuine missing transverse momentum may
  contribute to the searched topology via an energy measurement
  fluctuation which may induce significant $P_T^{miss}$. This is the case for
  the neutral current (NC) and multi--jet photoproduction ($\gamma p$)
  processes. In NC events the scattered electron can also mimick a
  narrow hadronic jet, while for the $\gamma p$ events one of the jets
  in the final state may have low multiplicity and fake the tau
  candidate. The background contribution from NC deep inelasic
  scattering is modelled using the RAPGAP~\cite{RAPGAP} generator.
  Multi--jet events with photon virtualities $Q^2<4$~GeV$^2$ are
  modelled with the PYTHIA Monte Carlo generator~\cite{PYTHIA}. Both
  generators rely on first order QCD matrix elements and use
  leading--log parton showers and string
  fragmentation~\cite{JETSET74}. Both light and heavy quark flavours
  are generated. The GRV LO (GRV--G LO) parton densities in the proton
  (photon) are used~\cite{SFGRVGLO}.


% \item {\it Neutral current deep inelastic scattering: } \\
% The background contribution from NC deep inelastic scattering is
% estimated using the RAPGAP~\cite{RAPGAP} generator. These events
% can pass the signal selection because the scattering lepton 
%  could be mismeasured, failing to be identified as an electromagnetic particle and/or
% creating tails in the missing transverse momentum which, due to the
% high cross section for this process, can bring this process up to
% the level of a significant background.
% %\noindent
% \item {\it Photoproduction of jets: }\\
% Multi--jet events with photon virtualities $Q^2<4$~GeV$^2$ are
% generated with the PYTHIA Monte Carlo program~\cite{PYTHIA}. The
% generator relies on first order QCD matrix elements and use
% leading--log parton showers and string fragmentation~\cite{JETSET74}.
% Both light and heavy quark flavours are generated.

\end{itemize}

Finally, as a prototype for physics beyond the Standard Model,
the anomalous production of single top quarks at HERA is
considered:

%\par
%\noindent

\begin{itemize}
\item {\it Anomalous single top production: } \\
  The single top production via flavour changing netral currents at HERA  fulfills the signal selection, if the decay of the top quark in a bottom quark and a $W$ boson is followed by the decay of the $W$ into $\tau ~\nu_{\tau}$.  Genuine missing transverse
  momentum is generated by the presence of neutrinos in the event.
  This process is expected to have a high $P_T^X$ due to the presence
  of the b-quark jet from top decay.  It is simulated using the generator
  ANOTOP~\cite{Aktas:2003yd} and is used for qualitative comparisons with
  the results of the present analysis.
\end{itemize}

All generated events are passed through the full
GEANT~\cite{Brun:1987ma} based simulation of the H1 apparatus and are
reconstructed using the same program chain as for the data.

% A summary
%of the samples used is given in table~\ref{MC}.


% 
% In the presenta analysis,  the $\tau$ lepton is identified in the hadronic channel. Due to the low multiplicity of its hadronic decays, the $\tau$ signature is a narrow hadronic jet. Due to escaping neutrinos, the "visible" momentum of the tau leptons, measured form the hadrons, is reduced.
% At the same time an imbalance in the transverse momentum should be observed, with extra missing transverse momentum along the $\tau$ direction, if no other neutrinos are produced in the event.
% \par
% Events that contain $\tau$ leptons and significant missing transverse momentum, in addition to the genuine neutrinos from tau decays, are produced in the Standard Model (SM) by the direct $W$ production, followed by the decay $W\rightarrow \tau \bar{\nu_\tau} $.  In those events one narrow hadronic jet is accompanied by large missing transverse momentum. The presence of other hadrons in the events is possible, but large acoplanarities are usually expected between the tau candidate and the hadronic syste
%  vector bosons $W^{\pm}$ at HERA is modelled using the EPVEC~\cite{EPVEC} generator.
% The NLO  QCD corrections to $W$ production~\cite{SPIRA} are taken into account
%  by weighting the events as a function of the rapidity and transverse momentum of the $W$ boson~\cite{PRIVSPI}.
% \par
% The main background to this topology $\tau+P_T^{miss}$ is the charged current (CC) deep inelastic scattering process, which genuinely present large missing transverse momentum due to the scattered neutrino. In those events, a hadronic jet can have low multiplicity and mimick the tau signature. This background contribution is calculated using the DJANGO~\cite{DJANGO} program. 
% \par
% Other background process is related to the pair-produced $\tau$ leptons, where the tau signature is real but the missing transverse momentum is low and should be aligned with the $\tau$ pair direction in the transverse plane. The contribution from lepton pair production, dominated by two--photon processes, is calculated with the  GRAPE~\cite{GRAPE} generator. 
% \par
% Events with genuinely no missing transverse momentum may contribute to the searched topology is a energy measurement fluctuation induce significant $P_T$. This is the case for the neutral current ($NC$) and multi--jet photoproduction ($\gamma p$) processes. In the $NC$ events the scattered electron can also mimick a narrow hadronic jet,  while for the $\gamma p$ events one of the jets in the final state may have low multiplicity and fake the $\tau$ candidate.The background contribution from NC deep inel
% Multi--jet events with photon virtualities $Q^2<4$~GeV$^2$ are generated with the PYTHIA Monte Carlo program~\cite{PYTHIA}. Both generators rely on first order QCD matrix elements
% and use leading--log parton showers
% and string fragmentation~\cite{JETSET74}.
% Both light and heavy quark flavours are generated. 
% The GRV LO (GRV--G LO) parton densities~\cite{SFGRVGLO} in the proton
% (photon) are used. The production of $W$ bosons and their hadronic decay are simulated using the EPVEC generator. This contribution is negligible in the present analysis.
% \par
% All generated events are passed through the full GEANT~\cite{Brun:1987ma} based simulation of the H1 apparatus and are reconstructed using the same program chain as for the data. 
% 
% \par

%======================================
\section{Experimental Conditions}
%======================================

At HERA electrons or positrons with an energy $E_e$ of
27.6~\gev~collide with protons at an energy of 920 GeV, giving a
centre--of--mass energy of~$\sqrt{s} = 319$~\gev. Up to 1997 the
proton energy was 820~\gev, giving $\sqrt{s} = 301$~\gev. The analysis
is based on HERA~I data set recorded by the H1 experiment between 1996
and 2000, which corresponds to an integrated luminosity of
108~pb$^{-1}$.

\par

A detailed description of the H1 detector can be found in
\cite{H1detector}.  Only the components essential for this analysis
are described here. The right handed Cartesian coordinate system used
in the following has its origin at the nominal primary $ep$
interaction vertex. The proton direction defines the $z$ axis.  The
polar angle $\theta$ and transverse momenta $P_T$ are defined with
respect to this axis.

\par

The inner tracking system contains the central ($25^\circ < \theta <
155^\circ$) and forward ($7^\circ < \theta < 25^\circ$) drift
chambers. It is used to determine the position of the interaction
vertex and to measure the trajectories of charged particles.  Particle
transverse momenta are determined from the curvature of the
trajectories in a solenoidal magnetic field of 1.15 Tesla.

\par

Hadronic final state particles as well as electrons and photons are
absorbed in the highly segmented liquid argon (LAr)
calorimeter~\cite{h1cal} ($ 4^\circ < \theta < 154^\circ$), which is 5
to 8 hadronic interaction lengths deep depending on the polar angle.
The LAr also includes an electromagnetic section which is 20 to 30
radiation lengths deep. Electromagnetic shower energies are measured
with a precision of $\sigma (E) / E = 12\% / \sqrt{E/\mathrm{GeV}}
\oplus 1\%$, hadronic shower energies with $\sigma (E) / E = 50\% /
\sqrt{E/\mathrm{GeV}} \oplus 2\%$, as determined in test beam
measurements~\cite{h1testbeam}.  In the backward region ($153^\circ <
\theta < 178^\circ$), the LAr is complemented by a lead--scintillating
fibre spaghetti calorimeter~(SPACAL)~\cite{h1spacal}. In the forward
region ($0.6^\circ < \theta < 3.5^\circ$) the LAr is complemented by a
sandwich calorimeter constructed from copper plates and silicon
counters (PLUG)~\cite{h1plug}.

\par

The calorimeter is contained within a superconducting coil and an iron
return yoke, instrumented with streamer tubes, which is used as a muon
detector and covers the range $ 4^\circ < \theta < 171^\circ$. Tracks
of penetrating particles, such as muons, are reconstructed from their
hit pattern in the streamer tubes and are detected with an efficiency
of above 90\%.  The instrumented iron also serves as a backing
calorimeter to measure the energies of hadrons that are not fully
absorbed in the LAr.

\par

The events studied in this analysis are triggered by requiring large
missing transverse momentum measured in the calorimeter. At the
trigger level the missing transverse momentum is identified using the
vector sum of LAr ``trigger towers'', which are groups of trigger
regions with a projective geometry pointing to the nominal interaction
vertex. The trigger efficiency is 50\% (85\%) for events with a
missing transverse momentum above 12~GeV (25~GeV).


% The trigger conditions for interactions leading to high transverse energy in the final state are mainly based on liquid argon calorimeter signals.
% Events in the leptonic channel are triggered by their calorimetric missing transverse momentum. The trigger efficiency is 50\% (85\%) for events with a missing transverse momentum above 12~GeV (25~GeV). 
% Events containing an electron with an energy of at least 10 GeV are triggered via the energy deposition in the electromagnetic calorimeter with an efficiency larger than 95\%. Events with muons may also be triggered by a set of triggers based on signals consistent with a minimum ionising particle in the muon system in coincidence with tracks in the inner tracking system. 
% In the hadronic channel, the triggering of events with three or more high $P_T$ jets is based on the scalar sum of the transverse energy deposited in the liquid argon calorimeter.

%======================================
\section{Search for  Events with Missing Transverse Momentum and Isolated Tau Leptons}
%======================================

This search aims to complement the previous observation of events with
electrons or muons and significant $P_{T}^{miss}$. Due to the
difficult tau lepton identification, more powerful background
supression is required.

\par

The identification of events with missing transverse momentum is based
on the following observables that quantify the momentum imbalance in
the event:

\begin{itemize}
\item $P^{\rm calo}_{T}$, the net transverse momentum measured from
  all energy deposits recorded in the calorimeters.
  
\item $P^{\rm miss}_{T}$, the total missing transverse momentum
  reconstructed from all observed particles (electrons, muons and
  hadrons). $P^{\rm miss}_{T}$ differs most from $P^{\rm calo}_{T}$ in
  the case of events with muons, since they deposit little energy in
  the calorimeter.
    
\item $E-P_z=\sum_{i} E_i(1-\cos \theta_i)$, where $E_i$ and
  $\theta_i$ denote the energy and polar angle of each particle in the
  event detected in the main detector. 
  %with $\theta_e<176^\circ$). 
  For an event where only momentum in the proton direction is undetected,
  $E-P_z = 2 E_e=55~\gev$ ($E_e=27.5~\gev$ is the electron beam
  energy). This quantity gives a measure of the longitudinal momentum
  balance.
\end{itemize}


The hadronic final state (HFS) is measured by combining calorimeter
energy deposits with low momentum tracks. Identified isolated
electrons or muons are excluded from the hadronic final state. The
calibration of the hadronic energy scale is made by comparing the
transverse momentum of the precisely measured scattered electron to
that of the HFS in a large NC event sample. Particles from the
reconstructed HFS are combined into jets using an inclusive $k_T$
algorithm with a minimum $P_T$ of 4 GeV.

\par

The tau candidate forms a narrow hadronic jet associated to one
isolated charged track measured in the inner tracker.  The present
analysis restricts the search for hadronic tau decays to the
``one-prong'' decays, which make up 50\% of the total tau decay width.
The size of the hadronic jet is estimated using the jet radius
$R_{jet}$, defined as the energy weighted average distance in the
$\eta-\phi$ plane between the jet axis and the hadrons making up the
jet:

$$
R_{jet} = \frac{1}{E_{jet}}\sum_i E_i \sqrt{
  \Delta\eta(jet,i)^2+\Delta\phi(jet,i)^2 }
$$

where the sum runs over all hadrons that belong to the candidate
hadronic jet. The isolation of the tau candidate is measured by the
distance in $\eta-\phi$ plane to the closest hadronic jet ($D_{\rm
  jet}$) or to the closest track ($D_{\rm track}$). The remaining
hadronic system after excluding the tau jet candidate, is denoted by
$X$ hereafter.

\par
The selection of events with isolated tau candidates and missing
transverse momentum is perfomed in three steps:

\begin{itemize}
  
\item {\bf $ P_T^{\rm miss}$ preselection} (table~\ref{cuts1}).\\
  The candidate events are selected if the calorimetric transverse
  momentum is greater then $12~\gev$. The missing transverse momentum,
  $P_{T}^{miss}$ is also required to be above $12~\gev$ and the
  longitudinal momentum imbalance $E-P_z < 45~\gev$.  In order to
  further supress the NC and $\gamma p$ processes, a topological
  cut is applied on the acoplanarity between the transverse momentum
  measured in the central calorimeter and that measured by the forward
  PLUG calorimeter $\Delta \phi \left( Calo, \mbox{PLUG} \right)$ as a
  function of $P_T^{\mathrm calo}$, as described in table~\ref{cuts1}.
  The cut is harsher at lower $P_T^{\rm calo}$ where the background is
  higher, while at $P_T^{\rm calo}>25~\gev$ the cut has no effect.
  Additionally, at least one jet with $P_T > 7~\gev$ is required.  
%  The
%  system X is defined at this stage by excluding the highest $P_T$ jet
%  from the hadronic final state.
  The system $X$ is defined at this stage as the hadronic final
  state excluding the highest $P_T$ jet.

%  and a set of topological filters are applied in order to reject non $ep$ events due to cosmic or halo muons. 

\par

After these cuts, 4142 events are selected, in good agreement with the SM
prediction of 4100$\pm$851. Figure~\ref{step1a} presents
the global event variables $E-P_z$, $P_T^{\rm calo}$ and the $P_T$ of
the highest $P_T$ jet. Figure~\ref{step1b} presents the jet radius of
this jet and the transverse momentum $P_T^X$ of the remaining  hadronic
system. At this stage, for all distributions, the data are in good
agreement with the Monte Carlo simulation.

\item {\bf $\tau+P_T^{\mathrm miss}$ preselection} (table~\ref{cuts2}).\\
  In the remaining data sample, events containing tau candidates are
  selected.  The tau candidate selection is based on the typical
  signature of the hadronic one prong tau decay: one charged track
  associated to a narrow ("pencil-like") hadronic deposit in the
  calorimeter. The tau lepton candidates are selected from the
  reconstructed hadronic jets.  The candidate hadronic jet is required
  to have a $P_T > 7~\gev$ and to be in the polar angle range
  $20^{\circ} < \theta < 120^{\circ}$. The jet is required to contain
  only one charged track and to be isolated from other jets or charged tracks:     $D_{jet}>1$ and $D_{track}>1$.
  \par
  After this selection step, 26 events are selected in the data, well in agreement with the expectation of $24.2\pm 4.4$. 
  The distributions of the jet radius $R_{jet}$ of the selected jet and of the     remaining hadronic transverse momentum $P_T^X$  at this
  stage are presented in figure~\ref{step2}. Good agreement is
  observed between the data and the Monte Carlo simulation.
 
% 
% To discriminate between $\tau$-leptons and the QCD jets, a jet radius $R_{jet}$  is calculated using the calorimetric deposits associated to the jet:
% $$
% R_{jet} = \frac{1}{E_{jet}}\sum_i E_i \sqrt{\Delta\phi(jet,i)^2 + \Delta\eta(jet,i)^2} 
% $$
  
\item {\bf $\tau+P_T^{\mathrm miss}$ final selection} (table~\ref{cuts3}).\\
  Narrow "pencil-like" jets are selected by the requirement
  $R_{jet}<0.12$. The acoplanarity $\Delta \phi (jet,X)$ between the tau           candidate momentum
  and the momentum of the remaining measured particles in the event is
  required to be below $170^{\circ}$. In order to further supress the
  mainly low $P_{T}$ background, further cuts $P_T^{calo} > 20$ GeV
  and $P_{track} > 5$ GeV are applied.

\end{itemize}

\par

After the final selection, five events are observed in the
data sample compared to a SM expectation of $5.8 \pm 1.3$. 
The $P_T^{X}$ spectrum after the final selection is shown in figure~\ref{ptxfinal}. The data
events are in the very low $P_T^{X}$ region. In the
region at large hadronic momentum $P_T^{X} > 25$ GeV no events are
observed.  Table~\ref{results} summarizes the results of the search.

\par

This preliminary analysis completes the picture of the searches for
events with isolated leptons and missing transverse momentum at HERA.
The status after HERA I data taking is summarised in
table~\ref{heraisolep}. While H1 reports events with electrons and
muons in the high $P_T^{X}$ region in excess to the SM prediction,
ZEUS observes good agreement with the Standard Model. In turn, ZEUS
observes two tau events at large
$P_T^{X}$ for $0.20\pm0.05$ expected. The new H1 preliminary search for $\tau+P_T^{\rm miss}$
events presented here reveals no candidate at large $P_T^{X}$.  In the
tau channel, the H1 analysis has a $W$ detection efficiency which is
roughly two times that of the ZEUS analysis in the high-$P_T^{X}$
region. The background suppression of the ZEUS analysis is roughly two
times stronger than that of the H1 analysis in the same region.


% \subsection{Data quality}
% 
% Data and Monte Carlo samples used for the analysis are requested, as
% very first step, to pass some quality criteria on noise level, HV on
% (for CJC1,CJC2,CIP,COP,ToF,LAr,Lumi detectors), on the difference
% between the actual primary central z vertex position and the
% run-period averaged one, on the difference between the actual CJC T0
% timing and the run-period averaged one, and against non ep events
% (e.g. beam gas events).
% 
% Additionally, the request of some  triggers on the data and MC
% photoproduction sample is applied.
% 
% For all other MCs the pseudo charged current trigger weights are applied.
% Additonal weights are applied, to the data for L4,L5 efficiencies, and
% to the MC for generator level cuts. 
% This is summarized in table \ref{dq}.
%  
% \begin{table}[hb]
% \begin{center}
% \begin{tabular}{|c c c|}
% \hline
% Requirement & DATA & MC\\
% \hline
% $L1 ~ 66 \| L1 ~ 67 \| (L1 ~ 71 \&\& L2 ~ 15) \| (L1 ~ 75 \&\& L2 ~
% 11) \| L1 ~ 77$ & yes & no (except for $\gamma$p sample)\\
% good or medium quality & yes & no \\
% Noisy runs & yes & no\\
% HV on  & yes & no \\
% $z_{vtx} - \overline{z_{vtx}}< 35.$ cm & yes & yes \\
% $CJC ~ T0 - \overline{CJC ~ T0} < 25$ clockbins & yes & no\\
% non ep finders IBG,IBGFM,IBGAM & yes & yes \\
% \hline
% \end{tabular}
% \caption{Data quality requirements for data and MC samples (yes or no
% meaning they have been applied to the given sample or not)}
% \label{dq}
% \end{center}
% \end{table}
% 
% \newpage
%  
% \section{Data samples}
% 
% The analysis uses data and Monte Carlo samples processed with the
% H1 object oriented software, from release 2.5.8. 
% 
% All data from 1996 to 2000, corresponding to a luminosity of 108
% pb$^{-1}$  have been used.
% 
% Concerning the Monte Carlo samples, the FCNC single top production is
% simulated using the generator ANOTOP. 
% Standard Model W production is generated using the generator EPVEC, and the sample used corresponds
% to a luminosity of 100068 pb$^{-1}$.
% Charged Current, Neutral Current and Photoproduction samples are
% generated using respectively DJANGOH1.2, RAPGAP and PYTHIA generators,
% for luminosities of about 134000, 2037, and 2000 pb$^{-1}$ (composed
% of two samples, one with $P_T^{HAT}>3$ GeV and one with $P_T^{HAT}>10$
% GeV). 
% The NC and Photoproduction sample have also additional cuts at the
% generator level, to make the sample production reasonably fast (``MAP
% selection'').
% $\gamma \gamma$ processes have also been taken in consideration, with
% final states $e^+e^-$,$\mu^+\mu^-$ and $\tau^+\tau^-$, and with
% luminosity respectively 30000, 50000, and 100000 pb$^{-1}$.
% 
% 

\clearpage
\newpage

%======================================
\section{Observation of Tau Pairs in Elastic $\gamma\gamma$--processes.}
%======================================

As a supplementary investigation of the ability of the H1 detector to
detect tau leptons, a preliminary search for elastic tau--pair
production in $ep$ collisions is performed.  Events with semi-leptonic
tau decays are selected by requiring one identified electron or muon
and one hadronic tau candidate (1-prong or 3-prong) with $P_T>2$~GeV.
The hadronic tau signature is verified by a neural network algorithm,
based mainly on hadronic cluster shape and trained differently for
one- or three-prong candidates.  In order to select elastic events,
events with additional energy deposits or tracks that cannot be
interpreted as being due to the scattered electron are rejected.  
\par
%Nocut is applied on the charges of the two tau leptons. 
 With this selection, $15$ events are observed with opposite charges of the two
tau leptons, compatible with an expectation of $17.6 \pm 3.9$ events,
dominated by the tau--pair process. A selected data event is presented
in figure~\ref{event}. In the event sample with likesign charges,
dominated by elastic NC scattering, $1$ event is observed for an
expectation of $1.9 \pm 1.2$.

\par

The distribution of the visible transverse momentum of the
hadronically decaying tau lepton, reconstructed by the hadronic
cluster, is shown in figure \ref{taupairpt}. The results of the
preliminary search for elastic tau--pair production are in agreement
with the expectation and demonstrate the ability of the H1 detector to
detect tau leptons.

%======================================
\section{Conclusions}
%======================================

The search for isolated tau leptons presented here completes the
analyses of isolated leptons and missing $P_T$ performed by the
experiments H1 and ZEUS experiments using the data collected during
HERA~I running period. In a data sample corresponding to an integrated
luminosity of 108~pb$^{-1}$, five data candidates are selected in the
final analysis for an expectation of $5.8\pm1.4$.  No selected data
event has a hadronic system with large transverse momentum
$P_T^X>25~\gev$, a region where 0.5 events are expected.  A
preliminary search for tau pairs produced in elastic photon--photon
collisions has also been performed in the semileptonic decay mode.
After a neural network based identification of hadronic tau decay, 15
events with opposite charges of the two tau leptons are selected in
the data for an expectation of $17.6$, dominated by the tau--pair
production process.


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\bibitem{SPIRA}
%\cite{Diener:2002if}
%\bibitem{Diener:2002if}
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%%CITATION = HEP-PH 0203269;%%
%\cite{Nason:1999xs}
%\bibitem{Nason:1999xs}
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%\cite{Spira:1999ja}
%\bibitem{Spira:1999ja}
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%%CITATION = HEP-PH 9905469;%%

\bibitem{PRIVSPI}
%\cite{Diener:2003df}
%\bibitem{Diener:2003df}
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hep-ex/0302040.
%%CITATION = HEP-EX 0302040;%%

%\bibitem{LPAIR}
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% W.~Buchm\"uller and G.~Ingelman (Editors),
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% J.A.M.~Vermaseren, Nucl. Phys. B229 (1983) 347.
\bibitem{GRAPE}
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Comput.\ Phys.\ Commun.  {\bf 136} (2001) 126 [hep-ph/0012029].
%%CITATION = HEP-PH 0012029;%%
%\cite{Abe:2000cv}



\bibitem{DJANGO}
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\bibitem{RAPGAP} 
%\cite{Jung:1993gf}
%\bibitem{Jung:1993gf}
H.~Jung,
%``Hard diffractive scattering in high-energy e p collisions and the Monte Carlo generation RAPGAP,''
Comput.\ Phys.\ Commun.\  {\bf 86} (1995) 147; 
%%CITATION = CPHCB,86,147;%%
RAPGAP program manual (1998) unpublished [http://www-h1.desy.de/$\sim$jung/RAPGAP.html].

\bibitem{PYTHIA}
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\bibitem{JETSET74}
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\bibitem{SFGRVGLO}
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%\cite{Brun:1987ma}
\bibitem{Brun:1987ma}
GEANT3; R.~Brun  {\it et al.}, 
%, F.~Bruyant, M.~Maire, A.~C.~McPherson and P.~Zanarini,
%``Geant3,''
 CERN-DD/EE/84-1.
%\href{http://www.slac.stanford.edu/spires/find/hep/www?r=cern-dd\%2Fee\%2F84-1}{SPIRES entry}

ANOTOP

\bibitem{H1detector}
%H1 Collaboration, I. Abt et al., Nucl. Instr. and Meth.
%{\bf A386} (1997) 310 and 348.
%\bibitem{Abt:hi}
I.~Abt {\it et al.}  [H1 Collaboration],
%``The H1 Detector At Hera,''
Nucl.\ Instrum.\ Meth.\ A {\bf 386} (1997) 310 and 348.
%%CITATION = NUIMA,A386,310;%%
%I.~Abt {\it et al.}  [H1 Collaboration],
%``The H1 Detector At Hera,''
%Nucl.\ Instrum.\ Meth.\ A {\bf 386} (1997) 348.
%%CITATION = NUIMA,A386,310;%%

\bibitem{h1cal}
%H1 Calorimeter Group, B. Andrieu et al.,
%Nucl. Instr. and Meth. {\bf A336} (1993) 460.
%%\cite{Andrieu:1993kh}
%\bibitem{Andrieu:1993kh}
B.~Andrieu {\it et al.}  [H1 Calorimeter Group],
%``The H1 liquid argon calorimeter system,''
Nucl.\ Instrum.\ Meth.\ A {\bf 336} (1993) 460.
%%CITATION = NUIMA,A336,460;%%
%
\bibitem{h1testbeam}
%\cite{Andrieu:1993xn}
%\bibitem{Andrieu:1993xn}
B.~Andrieu {\it et al.}  [H1 Calorimeter Group],
%``Electron / pion separation with the H1 LAr calorimeters,''
Nucl.\ Instrum.\ Meth.\ A {\bf 344} (1994) 492; \\
%%CITATION = NUIMA,A344,492;%%
%\cite{Andrieu:1994yn}
%\bibitem{Andrieu:1994yn}
B.~Andrieu {\it et al.}  [H1 Calorimeter Group],
%%``Beam tests and calibration of the H1 liquid argon calorimeter with electrons,''
Nucl.\ Instrum.\ Meth.\ A {\bf 350} (1994) 57; \\
%%CITATION = NUIMA,A350,57;%%
%\cite{Andrieu:1993tz}
%\bibitem{Andrieu:1993tz}
B.~Andrieu {\it et al.}  [H1 Calorimeter Group],
%``Results from pion calibration runs for the H1 liquid argon calorimeter and comparisons with simulations,''
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%%CITATION = NUIMA,A336,499;%%

\bibitem{h1spacal}
%\cite{Appuhn:1996na}
%\bibitem{Appuhn:1996na}
R.~D.~Appuhn {\it et al.}  [H1 SPACAL Group],
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Nucl.\ Instrum.\ Meth.\ A {\bf 386} (1997) 397.
%%CITATION = NUIMA,A386,397;%%


\bibitem{h1plug}
W.~Hildesheim {\it et al.},
%{\it The Plug Calorimeter Users Guide},
H1 Internal Note, {\bf H1-IN-372} (08/1994).

\end{thebibliography}


%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%% Tables and figures
\renewcommand{\arraystretch}{1.2}  
\begin{table}[hhh]
\begin{center}
\begin{tabular}{|r l|}
\hline
\multicolumn{2}{|c|}{\bf $P_T^{miss}$ Preselection} \\
\hline \hline
%Variable & Requirement \\
%\hline
%\multicolumn{2}{|c|}{Data Quality (HV,triggers,vertex, timing)} \\
$P_T^{calo}$ & $>$ 12 GeV \\
$P_T^{miss}$ & $>$ 12 GeV \\
$\Delta \phi \left( Calo, \mbox{PLUG} \right)$ & $<$ $150^{\circ} + 30^{\circ}
\cdot \left( \frac{P_T^{calo} - 12}{13} \right)$ \\
$\Sigma (E-P_z)$ & $<$ 45 GeV \\
$N_{jets}$ & $\ge 0$ \\
$P_T^{jet}$ & $>$ 7 GeV \\
\hline
\end{tabular}
\caption{The preselection of events with missing
transverse momentum: $P_T^{calo}$ is the total calorimetric transverse momentum,
$P_T^{miss}$ is the total transverse momentum of all reconstructed particles,
$\Delta \phi \left( Calo, \mbox{PLUG} \right)$ is the acoplanarity between the momentum measured in the main calorimeter and the momentum measured in the forward PLUG calorimeter,
$\Sigma (E-P_z)$ sums the $E-P_z$ contributions of all measured particles and is 55.2GeV for events with fully measured final state. At least one jet is required with a transverse momentum above 7 GeV.
}
\label{cuts1}
\end{center}
\end{table}


\begin{table}[hhh]
\begin{center}
\begin{tabular}{|r l|}
\hline
\multicolumn{2}{|c|}{\bf $\tau+P_T^{miss}$ Preselection} \\
\hline 
\hline
% Variable & Requirement \\
% \hline
$P_T^{miss}$ Preselection & (see table~\ref{cuts1}) \\
$P_T^{jet}$ & $>$ 7 GeV \\
$\theta^{jet}$ & from 20$^\circ$ to 120$^\circ$  \\
$N_{tracks}$ & $=1$ \\
$P_T^{track}$ & $>$ 2 GeV \\
%$R^{\eta\varphi}_{e,\mu,jets}$ & $>$ 1.0 \\
$D_{jet, track}$ & $>$ 1.0 \\
\hline
\end{tabular}
\caption{ The preselection of events with significant missing transverse momentum and a single track jet, corresponding to a tau candidate. The jet polar angle $\theta^{jet}$  is required to be in the central region and to contain exactly one charged track with a transverse momentum above 2~GeV. The jet is required to be isolated by asking the minimum distance in the $\eta-\varphi$ plane 
%$R^{\eta\varphi}$ of electrons, muons or other jets in the event to be above 1.
to or other tracks or jets in the event to be above 1. 
}
\label{cuts2}
\end{center}
\end{table}

\begin{table}[hhh]
\begin{center}
\begin{tabular}{|c c|}
\hline
\multicolumn{2}{|c|}{\bf $\tau+P_T^{miss}$ Final Selection} \\
\hline
$P_T^{miss}$ Preselection & (see table~\ref{cuts1}) \\
$P_T^{calo}$          & $>$ 20 GeV \\
 & \\
$P_T^{jet}$ & $>$ 7 GeV \\
$\theta^{jet}$ & from 20$^\circ$ to 120$^\circ$  \\
$N_{tracks}$ & $=1$ \\
$P_T^{track}$ & $>$ 5 GeV \\
$R^{\eta\varphi}_{e,\mu,jets}$ & $>$ 1.0 \\
$\Delta \phi (jet,X)$ & $< 170^{\circ}$ \\
 &  \\
$R^{jet}$             & $<$ 0.12 GeV \\
\hline
\end{tabular}
\caption{Final selection of events with isolated tau candidates and missing transverse momentum. 
Narrow calorimetric deposits are identified using the variable $R^{jet}$ (jet size), defined by
$ R_{jet} = \sum_i \frac{E_i \Delta^i(\varphi,\eta)}{E^{jet}}$ ( $i$ runs
over all particles in the jet, $E_i$ is the  particle energy and  $\Delta^i(\varphi,\eta)$ the  distance in $\eta-\varphi$ plane to the jet axis). The hadronic final state
excluding the tau candidate jet is denoted by $X$. A significant acoplanarity requirement  $\Delta \phi (jet,X)< 170^{\circ}$ ensures further background rejection. 
}
\label{cuts3}
\end{center}
\end{table}



\begin{figure}[hhh]
\setlength{\unitlength}{1mm}
\begin{center}
\begin{picture}(150,150)(0,0)
\put(-2,80){\mbox{\epsfig{file = H1prelim-04-061.fig1a.ps,
                          bbllx=-2,bblly=1,bburx=577,bbury=574, angle=0, clip=, width=8.2cm}}}
\put(75,80){\mbox{\epsfig{file = H1prelim-04-061.fig1b.ps,
                          bbllx=-2,bblly=1,bburx=577,bbury=574, angle=0, clip=, width=8.2cm}}}
\put(37,0){\mbox{\epsfig{file = H1prelim-04-061.fig1c.ps,
                          bbllx=-2,bblly=1,bburx=577,bbury=574, angle=0, clip=, width=8.2cm}}}
\end{picture}
\end{center}
\caption{The distributions of  $\Sigma (E - P_z)$, $P_T^{calo}$ and the highest
$P_T^{jet}$ in the data compared to the Monte Carlo simulation for the $P_T^{miss}$ event preselection described in table \ref{cuts1}. The SM $W$ contribution is still negligible at this level.}
\label{step1a}
\end{figure}

%\newpage
\newpage

\begin{figure}[hhh]
\setlength{\unitlength}{1mm}
\begin{center}
\begin{picture}(150,75)(0,0)
\put(-2,0){\mbox{\epsfig{file = H1prelim-04-061.fig2a.ps,
                          bbllx=-2,bblly=1,bburx=577,bbury=574, angle=0, clip=, width=7.2cm}}}
\put(75,0){\mbox{\epsfig{file = H1prelim-04-061.fig2b.ps,
                          bbllx=-2,bblly=1,bburx=577,bbury=574, angle=0, clip=, width=7.2cm}}}
\end{picture}
\end{center}
\caption{The distributions of $R^{jet}$ and $P_T^X$ in the data compared with the Monte Carlo simulation for the $P_T^{miss}$ event preselection described in table \ref{cuts1}. The SM $W$ contribution is still negligible at this level.}
\label{step1b}
\end{figure}

\begin{figure}[hhh]
\setlength{\unitlength}{1mm}
\begin{center}
\begin{picture}(150,75)(0,0)
\put(-2,0){\mbox{\epsfig{file = H1prelim-04-061.fig3a.ps,
                          bbllx=-2,bblly=1,bburx=577,bbury=574, angle=0, clip=, width=7.2cm}}}
\put(75,0){\mbox{\epsfig{file = H1prelim-04-061.fig3b.ps,
                          bbllx=-2,bblly=1,bburx=577,bbury=574, angle=0, clip=, width=7.2cm}}}
\end{picture}
\end{center}
\caption{The distributions of $R^{jet}$ and $P_T^X$ in the data compared with the Monte Carlo simulation for the $\tau+P_T^{miss}$ event preselection described in table \ref{cuts2}. Note that the jet radius $R^{jet}$ of jets selected in the Standard Model $W$ Monte Carlo are narrow, as expected 
  from tau jets. A cut $R^{jet} < 0.12$ is applied later in the final
  selection.}
\label{step2}
\end{figure}

\newpage

\begin{figure}[hhh]
\setlength{\unitlength}{1mm}
\begin{center}
\epsfig{file = H1prelim-04-061.fig4.ps,
        bbllx=0,bblly=0,bburx=577,bbury=574, angle=0, clip=, width=7.2cm}
\end{center}
\caption{ The distribution of the hadronic transverse momentum $P_T^X$ in the data compared with the Monte Carlo simulation for the $\tau+P_T^{miss}$ final selection described in table \ref{cuts3}. No events are observed in the high $P_T^X$ region, where
events for the template signal process sing-le $top$ production are expected.}
\label{ptxfinal}
\end{figure}

\vspace{2cm}

\begin{table}[hhh]
\begin{center}
{\large H1 Preliminary}
\begin{tabular}{|r || c | c || c | c|}
\hline
H1 Data 96-00 & \multirow{2}{18mm}{Data} & All SM &
\multirow{2}{18mm}{SM W} & Single top \\
108 pb$^{-1}$ & & Processes & & Efficiency * BR \\
\hline
\hline
Full Sample          & 5 & 5.81 $\pm$ 1.36 & 0.87 $\pm$ 0.15 & 0.52 $\%$ \\
\hline
$P_T^{X}$ $>$ 25 GeV & 0 & 0.53 $\pm$ 0.10 & 0.26 $\pm$ 0.05 & 0.49 $\%$ \\
\hline
$P_T^{X}$ $>$ 40 GeV & 0 & 0.22 $\pm$ 0.05 & 0.12 $\pm$ 0.03 & 0.42 $\%$ \\
\hline
\end{tabular}
\caption{Observed and predicted number of events in the 1996-2000 dataset after the final selection.
Also shown is the the product of the efficiency and the branching fraction for the anomalous single top production detected in the channel $t\rightarrow b W (\rightarrow \tau \nu_\tau)$ (the present analysis is only sensitive to the one-prong hadronic tau decays)}.
%   The last column quotes the expected efficiency for an anomalous
%   single top production Monte Carlo, used as a template process with
%   high-$P_T$ isolated tau leptons in the final state.  }
\label{results}
\end{center}
\end{table}

\begin{table}
\renewcommand{\arraystretch}{1.3}
\begin{center}
\begin{tabular}{|c|c|c|c|c|} \hline
\multicolumn{2}{|c|}{ } & Electron & Muon & Tau {\footnotesize(H1:108~pb$^{-1}$)} \\
\multicolumn{2}{|c|}{1994-2000 $e^\pm p$} & obs./exp. & obs./exp. & obs./exp. \\
\multicolumn{2}{|c|}{ } 
  & {\footnotesize ($W^\pm$ contrib.) } 
  & {\footnotesize ($W^\pm$ contrib.) }
  & {\footnotesize ($W^\pm$ contrib.) } \\ 
  \hline 
  & Full Sample &  11 / 11.54$\pm$1.50   
  & 8 / 2.94$\pm$0.50   & 5 / 5.81$\pm$1.36 \\ 
  &  & (71\%) & (86\%)   & (15\%)\\ 
  \cline{2-5} {\large \bf H1}  
  & $P_T^{X}>25\gev$ &  5 / 1.76$\pm$0.30 
  & 6 / 1.68$\pm$0.30  & 0 / 0.53$\pm$0.10 \\
  &  &  (82\%)  & (88\%) & (49\%) \\
  \cline{2-5} { $118.4$~pb$^{-1}$} 
  &  $P_T^{X}>40\gev$ &  3 / 0.66$\pm$0.13 &
  3 / 0.64$\pm$0.14  & 0 / 0.22$\pm$0.05 \\ 
   &   & (80\%) & (92\%) & (54\%) \\ 
 \hline \hline & Full Sample &  24 / $20.6~^{+1.7}_{-4.6}$ &
  12 / 11.9$^{+0.6}_{-0.7}$   & 3 / 0.40$^{+0.12}_{-0.13}$ \\
  &  & (17\%)& (16\%)  &  (49\%) \\
  \cline{2-5} {\large \bf ZEUS } &  $P_T^{X}>25\gev$  &  
  2 / 2.90$^{+0.59}_{-0.32}$   & 5 / 2.75$^{+0.21}_{-0.21}$  &
  2 / 0.20$^{+0.05}_{-0.05}$ \\  
  & &  (45\%)  &  (50\%)  &(49\%) \\  
  \cline{2-5} {$130.2$~pb$^{-1}$} &  $P_T^{X}>40\gev$  &
  0 / 0.94$^{+0.11}_{-0.10}$   & 0 / 0.95$^{+0.14}_{-0.10}$  &
  1 / 0.07$^{+0.02}_{-0.02}$ \\ 
   & &  (61\%)   & (61\%)  & (71\%) \\ 
  \hline
\end{tabular}
\end{center}
\caption{Summary of the results of searches for events with isolated
leptons, missing transverse momentum and large $P_T^{X}$ at HERA. 
The number of observed events is compared to the SM prediction. The $W^\pm$
component is given in parentheses in percent. The statistical and systematic
uncertainties added in quadrature are also indicated.}
\label{heraisolep}
\end{table}

\newpage

%\begin{figure}[hhh]
%\setlength{\unitlength}{1mm}
%\begin{center}
%\epsfig{file = H1prelim-04-061.fig5.ps,
%               bbllx=11,bblly=142,bburx=493,bbury=639, height=7.2cm}
%\end{center}
%\caption{The $\Sigma(E-P_z)$ distribution of the pre-selected $\tau$-pair candidate events in data compared with the Monte Carlo simulation.}
%\end{figure}

\begin{figure}[hhh]
\setlength{\unitlength}{1mm}
\begin{center}
\begin{picture}(150,75)(0,0)
\put(-2,-3){\mbox{\epsfig{file = H1prelim-04-061.fig6a.ps,
                          bbllx=11,bblly=142,bburx=493,bbury=639,clip=, height=7.2cm}}}
\put(77,-3){\mbox{\epsfig{file = H1prelim-04-061.fig6b.ps,
                          bbllx=11,bblly=142,bburx=493,bbury=639,clip=, height=7.2cm}}}
\end{picture}
\end{center}
\caption{Control Plots of the visible transverse momentum distributions of the 
  hadronically decaying tau lepton; on the left for opposite charges
  of the two decaying tau candidates, on the right for equally charged
  tau candidates. In $\gamma\gamma \rightarrow \tau^{+} \tau^{-}$
  processes only candidates of opposite charge are expected.}
\label{taupairpt}
\end{figure}


\begin{figure}[hhh]
\setlength{\unitlength}{1mm}
\begin{center}
\begin{picture}(150,150)(0,0)
\put(-40,150){\mbox{\epsfig{file = H1prelim-04-061.fig7.ps, angle=270, width=19cm }}}
\end{picture}
\end{center}
\caption{ Tau--pair candidate event with one tau lepton decaying 
  leptonically to a muon, and the other tau lepton decaying to three
  charged hadrons (3-prong topology). The scattered positron is also
  detected in the backward calorimeter.}
\label{event}
\end{figure}



\end{document}




