%================================================================ % LaTeX file with prefered layout for H1 paper drafts % use: dvips -D600 file-name %================================================================ \documentclass[12pt]{article} \usepackage{epsfig} \usepackage{amsmath} \usepackage{hhline} \usepackage{amssymb} \usepackage{times} \usepackage{cite} \usepackage{multirow} \renewcommand{\topfraction}{1.0} \renewcommand{\bottomfraction}{1.0} \renewcommand{\textfraction}{0.0} \newlength{\dinwidth} \newlength{\dinmargin} \setlength{\dinwidth}{21.0cm} \textheight23.5cm \textwidth16.0cm \setlength{\dinmargin}{\dinwidth} \setlength{\unitlength}{1mm} \addtolength{\dinmargin}{-\textwidth} \setlength{\dinmargin}{0.5\dinmargin} \oddsidemargin -1.0in \addtolength{\oddsidemargin}{\dinmargin} \setlength{\evensidemargin}{\oddsidemargin} \setlength{\marginparwidth}{0.9\dinmargin} \marginparsep 8pt \marginparpush 5pt \topmargin -42pt \headheight 12pt \headsep 30pt \footskip 24pt \parskip 3mm plus 2mm minus 2mm %===============================title page============================= \begin{document} % The rest \newcommand{\pom}{{I\!\!P}} \newcommand{\reg}{{I\!\!R}} \newcommand{\slowpi}{\pi_{\mathit{slow}}} %\newcommand{\gevsq}{\mathrm{GeV}^2} \newcommand{\fiidiii}{F_2^{D(3)}} \newcommand{\fiidiiiarg}{\fiidiii\,(\beta,\,Q^2,\,x)} \newcommand{\n}{1.19\pm 0.06 (stat.) \pm0.07 (syst.)} \newcommand{\nz}{1.30\pm 0.08 (stat.)^{+0.08}_{-0.14} (syst.)} \newcommand{\fiidiiiful}{F_2^{D(4)}\,(\beta,\,Q^2,\,x,\,t)} \newcommand{\fiipom}{\tilde F_2^D} \newcommand{\ALPHA}{1.10\pm0.03 (stat.) \pm0.04 (syst.)} \newcommand{\ALPHAZ}{1.15\pm0.04 (stat.)^{+0.04}_{-0.07} (syst.)} \newcommand{\fiipomarg}{\fiipom\,(\beta,\,Q^2)} \newcommand{\pomflux}{f_{\pom / p}} \newcommand{\nxpom}{1.19\pm 0.06 (stat.) \pm0.07 (syst.)} \newcommand {\gapprox} {\raisebox{-0.7ex}{$\stackrel {\textstyle>}{\sim}$}} \newcommand {\lapprox} {\raisebox{-0.7ex}{$\stackrel {\textstyle<}{\sim}$}} \def\gsim{\,\lower.25ex\hbox{$\scriptstyle\sim$}\kern-1.30ex% \raise 0.55ex\hbox{$\scriptstyle >$}\,} \def\lsim{\,\lower.25ex\hbox{$\scriptstyle\sim$}\kern-1.30ex% \raise 0.55ex\hbox{$\scriptstyle <$}\,} \newcommand{\pomfluxarg}{f_{\pom / p}\,(x_\pom)} \newcommand{\dsf}{\mbox{$F_2^{D(3)}$}} \newcommand{\dsfva}{\mbox{$F_2^{D(3)}(\beta,Q^2,x_{I\!\!P})$}} \newcommand{\dsfvb}{\mbox{$F_2^{D(3)}(\beta,Q^2,x)$}} \newcommand{\dsfpom}{$F_2^{I\!\!P}$} \newcommand{\gap}{\stackrel{>}{\sim}} \newcommand{\lap}{\stackrel{<}{\sim}} \newcommand{\fem}{$F_2^{em}$} \newcommand{\tsnmp}{$\tilde{\sigma}_{NC}(e^{\mp})$} \newcommand{\tsnm}{$\tilde{\sigma}_{NC}(e^-)$} \newcommand{\tsnp}{$\tilde{\sigma}_{NC}(e^+)$} \newcommand{\st}{$\star$} \newcommand{\sst}{$\star \star$} \newcommand{\ssst}{$\star \star \star$} \newcommand{\sssst}{$\star \star \star \star$} \newcommand{\tw}{\theta_W} \newcommand{\sw}{\sin{\theta_W}} \newcommand{\cw}{\cos{\theta_W}} \newcommand{\sww}{\sin^2{\theta_W}} \newcommand{\cww}{\cos^2{\theta_W}} \newcommand{\trm}{m_{\perp}} \newcommand{\trp}{p_{\perp}} \newcommand{\trmm}{m_{\perp}^2} \newcommand{\trpp}{p_{\perp}^2} \newcommand{\alp}{\alpha_s} \newcommand{\alps}{\alpha_s} \newcommand{\sqrts}{$\sqrt{s}$} \newcommand{\LO}{$O(\alpha_s^0)$} \newcommand{\Oa}{$O(\alpha_s)$} \newcommand{\Oaa}{$O(\alpha_s^2)$} \newcommand{\PT}{p_{\perp}} \newcommand{\JPSI}{J/\psi} \newcommand{\sh}{\hat{s}} %\newcommand{\th}{\hat{t}} \newcommand{\uh}{\hat{u}} \newcommand{\MP}{m_{J/\psi}} %\newcommand{\PO}{\mbox{l}\!\mbox{P}} \newcommand{\PO}{I\!\!P} \newcommand{\xbj}{x} \newcommand{\xpom}{x_{\PO}} \newcommand{\ttbs}{\char'134} \newcommand{\xpomlo}{3\times10^{-4}} \newcommand{\xpomup}{0.05} \newcommand{\dgr}{^\circ} \newcommand{\pbarnt}{\,\mbox{{\rm pb$^{-1}$}}} \newcommand{\gev}{\,\mbox{GeV}} \newcommand{\WBoson}{\mbox{$W$}} \newcommand{\fbarn}{\,\mbox{{\rm fb}}} \newcommand{\fbarnt}{\,\mbox{{\rm fb$^{-1}$}}} % % Some useful tex commands % \newcommand{\qsq}{\ensuremath{Q^2} } \newcommand{\gevsq}{\ensuremath{\mathrm{GeV}^2} } \newcommand{\et}{\ensuremath{E_t^*} } \newcommand{\rap}{\ensuremath{\eta^*} } \newcommand{\gp}{\ensuremath{\gamma^*}p } \newcommand{\dsiget}{\ensuremath{{\rm d}\sigma_{ep}/{\rm d}E_t^*} } \newcommand{\dsigrap}{\ensuremath{{\rm d}\sigma_{ep}/{\rm d}\eta^*} } % Journal macro \def\Journal#1#2#3#4{{#1} {\bf #2} (#3) #4} \def\NCA{\em Nuovo Cimento} \def\NIM{\em Nucl. Instrum. Methods} \def\NIMA{{\em Nucl. Instrum. Methods} {\bf A}} \def\NPB{{\em Nucl. Phys.} {\bf B}} \def\PLB{{\em Phys. Lett.} {\bf B}} \def\PRL{\em Phys. Rev. Lett.} \def\PRD{{\em Phys. Rev.} {\bf D}} \def\ZPC{{\em Z. Phys.} {\bf C}} \def\EJC{{\em Eur. Phys. J.} {\bf C}} \def\CPC{\em Comp. Phys. Commun.} \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 %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 \epsfig{file=/h1/www/images/H1logo_bw_small.epsi,width=2.cm} \\[.2em] \hline \multicolumn{4}{l}{{\bf XXII International Symposium on Lepton-Photon Interactions at High Energy}, June~30,~2005,~Uppsala} \\ & Abstract: & {\bf 417} &\\ & Session: & {\bf Electroweak physics and beyond} &\\ \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. 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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,'' Nucl.\ Instrum.\ Meth.\ A {\bf 336} (1993) 499. %%CITATION = NUIMA,A336,499;%% \bibitem{h1spacal} %\cite{Appuhn:1996na} %\bibitem{Appuhn:1996na} R.~D.~Appuhn {\it et al.} [H1 SPACAL Group], %``The H1 lead/scintillating-fibre calorimeter,'' 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.fig5a.ps, bbllx=11,bblly=142,bburx=493,bbury=639,clip=, height=7.2cm}}} \put(77,-3){\mbox{\epsfig{file = H1prelim-04-061.fig5b.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.fig6.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}