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\begin{titlepage}
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%%\begin{flushleft}
%%{\bf  Draft 1.00\hspace{5mm} \today}\\
%%{\bf  Editor : E. Perez }\\
%%{\bf  Referee : B. Heinemann} \\
%%\end{flushleft}
%%To be Submitted to the 30th International Conference on
%%High-Energy Physics ICHEP00, \\ Osaka, Japan, July 2000
%

\begin{flushleft}
Submitted to the 30th International Conference on High-Energy 
Physics ICHEP2000,\\
Osaka, Japan, July 2000
\end{flushleft}

\vspace{4cm}

\begin{center}
\begin{Large}
\bf{A Search for Leptoquark Bosons 
    in {\boldmath $e^- p$} Collisions at HERA \\ }

\vspace{2cm}

H1 Collaboration

\end{Large}
\end{center}

\vspace{2cm}

\begin{abstract}
\noindent

A search for scalar and vector leptoquarks (LQs) coupling to first generation
fermions is performed in the H1 experiment at the $ep$ collider HERA.
The analysis uses all available $e^- p$ data collected in 1998 and 
1999 at a centre of mass energy of $\sim 320 \GeV$, corresponding
to an integrated luminosity of $\sim 14.4 \picob^{-1}$.
No evidence for the direct production of such particles is
found in the high $Q^2$ neutral current deep-inelastic scattering data sample,
and constraints on LQs are obtained.
For a Yukawa coupling of electromagnetic strength
LQs are excluded for masses up to $\sim 290 \GeV$.
This analysis complements the LQs searches performed previously 
using data collected while HERA was operating with positrons
instead of electrons.
\end{abstract}

\vfill

\begin{flushleft}
\textbf{Abstract: 954\\
    Parallel Session: 11\\
    Plenary Talk: 7}
\end{flushleft}

\end{titlepage}


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

The $ep$ collider HERA offers the unique possibility to search for
$s$-channel resonant production of new particles which couple to
lepton-parton pairs.
Examples are leptoquark (LQ) colour triplet bosons which appear naturally
in various unifying theories beyond the Standard Model (SM).
At HERA, leptoquarks could be singly produced by the fusion of the $27.5 \GeV$
initial state lepton with a quark of the $920 \GeV$ incoming proton,
with masses up to the kinematic limit of
$\sqrt{s_{ep}} \simeq 320 \GeV$.

This analysis presents a search for LQs using $e^- p$ data collected
in 1998 and 1999. 
Collisions between {\it{electrons}} and protons provide a
high sensitivity to LQs coupling to $e^-$ and a {\it{valence quark}}
(i.e. LQs with fermion number $F=2$)
while the production of such LQs is largely suppressed in
$e^+ p$ collisions where an {\it{antiquark}} should participate
to the fusion.
Thus this analysis complements the searches for LQs in $e^+ p$
data published recently~\cite{H1LQ99}.

The search is restricted to the neutral
current (NC) decay mode of the LQ which leads to final states
similar to those of deep-inelastic scattering (DIS) at very high
squared momentum transfer $Q^2$.
The luminosity amounts to $14.41 \picob^{-1}$,
an increase in statistics of a factor $\sim 35$ compared to
previous LQs searches~\cite{H1LQ94} in $e^- p$ collisions.


%=======================================================================
\section{Leptoquark Phenomenology}
%=======================================================================

The phenomenology of LQs at HERA is presented in detail
in~\cite{H1LQ99}. At HERA, LQs can be resonantly produced in
%
% --------------- FIGURE 1 : Diagrammes DIS + LQ, s/u  ---------------
 \begin{figure}[h]
  \begin{center}
   \epsfxsize=0.7\textwidth
   \epsffile{H1prelim-00-061.fig1.eps}
   \caption[]{ \label{fig:diagdislq}
      {\small Diagrams 
              (a) $s$-channel resonant production and
              (b) $u$-channel exchange of a leptoquark with
              fermion number $F = 2$.
              Diagrams involving a $ F = 0$ leptoquark are
              obtained from (a) and (b) by exchanging $q$ and
              $\bar{q}$. }}
   \end{center}
  \end{figure}
% --------------------------------------------------------------------
%
the $s$-channel or exchanged in the $u$-channel between the
incoming lepton and a quark coming from the proton. This is
illustrated in Fig.~\ref{fig:diagdislq}, where $\lambda$ denotes
the unknown Yukawa coupling of the LQ to the $eq$ pair.
Both these processes interfere with NC DIS.
In contrast to~\cite{H1LQ99}
where both contributions and their interference with DIS 
were carefully taken into account
in order to probe LQs masses close to or above the kinematic limit,
we only consider here the resonant process.
% in which a LQ is produced at a mass $M =\sqrt{s_{ep} x}$
% where $x$ is the momentum fraction of the proton carried by the struck quark.
% The case of extreme LQ masses $M_{LQ}$ and high coupling values,
% where the convolution of the Breit-Wigner distribution of finite width
% $\Gamma_{LQ} \propto \lambda^2 M_{LQ}$ characterizing the LQ resonance
% with the steeply falling density in the proton of the incoming quark 
% leads to a strong distorsion of the LQ mass peak towards lower values,
% is not adressed here.
% 
% We restrict to LQ masses $M_{LQ}$ and coupling values small enough, such that
% convoluting the Breit-Wigner distribution of finite width
% $\Gamma_{LQ} \propto \lambda^2 M_{LQ}$ characterizing the LQ resonance
% with the density in the proton of the incoming quark does not lead
% 
This approach is valid as long as the LQ mass $M_{LQ}$ is not too high
and its width $\Gamma \propto \lambda^2 M_{LQ}$
is small enough for the narrow-width approximation (NWA)
to hold.
In that case, the LQ mass spectrum is not distorted by the convolution
of the Breit-Wigner distribution characterizing the LQ resonance
with the density in the proton of the incoming quark~\cite{H1LQ99}.
For $\sqrt{s_{ep}} = 320 \GeV$, this is the case for LQ masses
up to $\sim 290 \GeV$ when the strength of the Yukawa coupling 
is of the order of $\alpha_{EM}$.

In the $s$-channel, a LQ is produced at a mass $M =\sqrt{s_{ep} x}$
where $x$ is the momentum fraction of the proton carried by the struck quark.
Scalar LQs produced in the $s$-channel decay isotropically in their rest
frame leading to a flat ${\rm d} \sigma\,/\,{\rm d}y \;$ spectrum where
$y= Q^2 / s_{ep}x = \frac{1}{2}\left(1+\cos{\theta^*}\right)$ is the
Bjorken scattering variable in DIS and $\theta^*$ is the decay polar
angle of the lepton relative to the incident proton in the LQ centre of
mass frame.
In contrast, events resulting from the production and decay of
vector LQs would be distributed according to
${\rm d} \sigma\,/\, {\rm d} y \propto \, (1-y)^2$.
These $y$ spectra (or in other words the specific angular distributions
of the decay products) from scalar or vector LQ production are markedly
different from the ${\rm d} \sigma\,/\, {\rm d} y \propto \,y^{-2}$
distribution expected at fixed $x$ for the dominant $t$-channel
photon exchange in neutral current DIS events.
Hence, a LQ signal in the NC-like channel will be statistically most prominent
at high $y$.



%=======================================================================
\section{Event Selection and Comparison with SM Expectations}
%=======================================================================

The search for first generation LQs relies essentially on an
inclusive NC selection requiring an identified positron at high transverse
energy.
Selected events must have been
accepted by a LAr trigger and survived a set of
filters to reject non $ep$ colliding background, and
a primary vertex is required in the range $\mid z - \bar{z} \mid < 35\cm$.

%............................
\subsection{Event Selection}
\label{eventselec}
%............................

The selection of NC-like events is identical to that presented
in~\cite{H1LQ99}. It mainly requires an isolated electron in
the LAr with $E_{T,e} > 15 \GeV$ and an energy-momentum balance.
A ``loose" requirement of a jet in the LAr, with $P_{T,jet} > 7 \GeV$,
allows to suppress non-DIS contamination as $\gamma \gamma$
or QED Compton processes.
The kinematic variables $Q^2$, $M$ and $y$ are determined using
the measurement of the scattered electron energy and angle,
the subscript $e$ being used in the following to label
the corresponding reconstructed variables.

The considered phase space is restricted to the kinematic range
$Q^2_e > 2500 \GeV^2$ and $0.1 < y_e < 0.9$.
The resolution in $M_e$ degrades with decreasing $y_e$
($ \delta M_e / M_e \propto 1/ y_e$) and so the low
$y$ domain is excluded.
Excluding the high $y$ values avoids the region where migrations effects due
to QED radiation in the initial state are largest for the $e$-method,
and also suppresses down to a negligible level the photoproduction background
where e.g. a jet has been misidentified as an electron.
In the kinematic range considered, the NC trigger
efficiency exceeds $98\%$ and is consistent with $100\%$ to within
experimental error.

Applying our NC selection criteria,
696 NC DIS candidates are observed in the considered kinematic
range.
% in good agreement
% with the expectation of $675.7 \pm 50.6$ events from 
% standard NC DIS.

The distribution of these events in the $y_e - M_e$ kinematic
plane is shown in Fig.~\ref{fig:scatter}.
%
% --------------- FIGURE 1: Scatter plot -------------------------
%
\begin{figure}[htb]
   \begin{center}
     \epsfxsize=0.6\textwidth
     \epsffile{H1prelim-00-061.fig2.eps}
      \caption
         {\small \label{fig:scatter}
           Kinematics in the $y_e -M_e$ plane of the selected NC DIS
           candidates. Three isocurves at $Q^2 = 2500, 15000$ 
           and $30000 \GeV^2$ are also shown as full lines. }
 \end{center}
\end{figure}
%----------------------------------------------------------------------
In this plane, events coming from a resonantly produced 
scalar LQ decaying
to $eq$ would accumulate in $M_e$ around the LQ mass, and
be flatly distributed in $y$.



%..................................
\subsection{Comparison with NC DIS}
%..................................

The SM prediction for DIS is obtained using the event generator
DJANGO~\cite{DJANGO} which includes QED first order radiative
corrections and a modelling of the emission of QCD radiation 
to all orders according to the Color Dipole Model as 
implemented in ARIADNE~\cite{ARIADNE}.
The parton densities in the proton used throughout are taken
from the MRST~\cite{MRST} parametrization which includes 
constraints from HERA data up to $Q^2 = 5000 \GeV^2$.

The systematic errors propagated on the mean SM expectations include:~
%
\begin{itemize}
 \item the uncertainty on the integrated luminosity ($\pm 2.2 \%$);
  \item the uncertainty on the absolute calibration of the calorimeters
        for electromagnetic energies, ranging between $\pm 0.7\%$ in the
        central LAr wheels to $\pm 3\%$ in the forward region
        of the LAr calorimeter;
  \item the uncertainty on the calibration of the calorimeters
        for hadronic showers of $\pm 2\%$;
  \item the uncertainty on the partons densities in the proton 
        which lead to a $7\%$ theoretical uncertainty on the predicted
        NC DIS cross-section.
\end{itemize}

Applying the requirements mentioned in section~\ref{eventselec}, 
$675.7 \pm 50.6$ events are
expected from standard NC DIS, in good agreement with
the number of observed candidates.
Fig.~\ref{fig:ctrlpl} shows the distributions of the transverse
energy of the electron $E_{T,e}$, its polar angle $\theta_e$,
the missing transverse momentum $P_{T,miss}$ and 
the transverse momentum $P_{T,jet}$ of the highest $P_T$ jet,
for the 696 observed candidates and for the NC DIS expectation.
%
% --------------- FIGURE 2: Control plot -------------------------
%
\begin{figure}[htb]
   \begin{center}
     \epsfxsize=0.75\textwidth
     \epsffile{H1prelim-00-061.fig3.eps}
      \caption
         {\small \label{fig:ctrlpl}
           Distributions of (a) the transverse energy and (b)
           the polar angle of the electron; (c) the missing transverse 
           momentum and (d) the transverse momentum of the highest
           $P_T$ jet for the selected NC DIS candidates.
           Symbols correspond to data events and histograms
           to SM simulation. }
 \end{center}
\end{figure}
%----------------------------------------------------------------------
All distributions are seen to be well described by the NC DIS simulation.

The observed and expected $Q^2_e$ distributions are compared
in Fig.~\ref{fig:q2pl}a.
%
% --------------- FIGURE 3: Q2 figure -------------------------
%
\begin{figure}[htb]
   \begin{center}
     \epsfxsize=0.7\textwidth
     \epsffile{H1prelim-00-061.fig4.eps}
      \caption 
      { \small  \label{fig:q2pl}(a) $Q^2_e$ distribution of the 
                selected NC DIS candidate
                events for the data ($\bullet$ symbols) and for standard
                NC DIS expectation (histogram);
                (b) ratio of the observed and expected (from
                NC DIS) number of
                events as a function of $Q^2_e$;
                the lines above and below unity specify the
                $\pm 1\sigma$ levels determined using the combination
                of statistical and systematic errors of
                the DIS expectation. }
 \end{center}
\end{figure}
%----------------------------------------------------------------------
Also shown in Fig.~\ref{fig:q2pl}b is the ratio of the observed $Q^2_e$
distribution to the NC DIS expectation.
The errors resulting from the convolution of the
systematic errors and the statistical error of the Monte Carlo sample
are correlated for
different $Q^2_e$ bins and are indicated in Fig.~\ref{fig:q2pl}b
as lines above and below unity joining the $\pm 1\sigma$ errors
evaluated at the centre of each bin.
The NC DIS expectation agrees well with the data, with a slight deficit
observed for $Q^2_e > 10000 \GeV^2$ where 42 events are observed
while $49.26 \pm 5.18$ are expected from SM expectation, 
which is however not statistically significant.
For $Q^2_e > 15000 \GeV^2$, 18 events are observed, in good agreement
with the NC DIS prediction of $16.9 \pm 2.43$.


%=======================================================================
\section{Direct Search for Leptoquarks}
%=======================================================================

Starting from the NC DIS selection presented above,
the significance of a possible LQ signal over the NC DIS background
is enhanced by exploiting the specific angular distributions
for LQ induced processes.
%
The LEGO~\cite{LEGO} event generator has been used to simulate
events coming from the resonant production of  
scalar and vector LQs decaying
into $eq$, for various values of the LQ mass $M_{LQ}$.
For the LQ mass range considered here,
% where the NWA holds,
the kinematics of events coming from $eq \rightarrow LQ \rightarrow eq$
does not depend on the value of the Yukawa coupling $\lambda$,
provided that $\lambda$ remains of moderate strength.
These Monte-Carlo event samples have been used to optimize
mass dependent lower $ y_e > y_{cut}$ cuts which 
maximize the signal significance. 
This has been achieved by finding the best compromise between the
efficiency loss on the LQ signal and the important background reduction.
Thus, with increasing LQ mass, the $y_{cut}$ decreases together with the
NC DIS expectation. For scalar (vector) LQs, this $y_{cut}$ continuously
decreases from $\sim$0.45 ($\sim$0.35) at 75 GeV to $\sim$0.35 ($\sim$0.1) at 200 GeV, reaching 0.1 (0.1) at 290 GeV. The optimized
$y_{cut}$ is less stringent for vector than for scalar LQs due to the
softer $y$ distribution ($\propto (1-y)^2$) of events induced by
vector LQ production.

%=================================
\subsection{Mass Distributions}
%=================================


The comparison of the measured mass spectrum with SM predictions
is shown in Fig.~\ref{fig:dndmlq} for the NC DIS analysis.
The measured and expected NC spectra are seen before and after applying the
mass dependent $y_e$ cut relevant for 
the search of a scalar (Fig.~\ref{fig:dndmlq}a) 
and a vector (Fig.~\ref{fig:dndmlq}b) LQ.
After applying the $y_e$ cut optimized for scalar LQs, 
we observe 298 events in the mass range
$M_e > 62.5 \GeV$ while $289.8 \pm 21.8$ are expected.
For $M_e > 137.5 \GeV$ 21 candidate events are observed, which
is slightly below the SM prediction of $29.8 \pm 3.2$. This
is the reflection of the slight deficit observed above
in the $Q^2$ distribution, for intermediate $Q^2$ values.
% The $y_e$ cut optimized for vector LQs searches leads to
% a smaller reduction of the sample
The sample is less reduced when applying the $y_e$ cut optimized 
for vector LQs
searches, which leads to 514 candidates in good agreement with
the $495.2 \pm 37.2$ expected from SM.

% ---------- FIGURE 4: dNdM  Scalar and Vector  ----------------
%
\begin{figure}[htb]
  \begin{center}
  \begin{tabular}{cc}
     \hspace*{-0.9cm}\mbox{\epsfxsize=0.55\textwidth
        \epsffile{H1prelim-00-061.fig5a.eps}}
   &
     \hspace*{-0.8cm}\mbox{\epsfxsize=0.55\textwidth
     \epsffile{H1prelim-00-061.fig5b.eps}}
  \end{tabular}
  \end{center}
%
 \caption[]{ \label{fig:dndmlq}
 {\small Mass spectra for NC DIS-like final states
         for data (symbols) and DIS expectation (histograms).
         The NC DIS-like comparison is shown before (open triangles,
         white histogram) and after (closed dots, hatched histogram)
         a $y$ cut designed to maximize the significance of an eventual
         (a) scalar and (b) vector LQ signal.
         The greyed boxes indicate the $\pm 1 \sigma$ band combining the
         statistical and systematic errors of the NC DIS
         expectation. }}
\end{figure}
%---------------------------------------------------------------------------


%=================================
\subsection{Constraints on LQs}
%=================================


Since no evidence for LQ production is observed,
the data is used to set constraints on LQs coupling
to first generation fermions.

For a given LQ mass, we use the numbers of observed and
expected events within a mass bin $\left[ M_{min}, M_{max} \right]$
of variable width, adapted to the expected mass resolution 
(typically $3-6 \GeV$, dominated by experimental effects) and
measured mass values for a given true LQ mass, and
which slides over the accessible mass range.
For example, only candidates with
$M_e \in \left[ 187 ; 206 \right] \GeV$
will be used to constrain
a $200 \GeV$ LQ undergoing a NC DIS-like decay.
In the mass domain where the NWA holds, $M_{min}$ and $M_{max}$
vary linearly with the LQ mass.
Typical detection efficiencies of scalar (vector) LQs including 
the optimized $y$ and mass
cuts are found to vary between $35 \%$ ($18 \%$)
at $100 \GeV$, $40 \%$ ($45 \%$) at $200 \GeV$ 
and $52 \%$ ($45 \%$) at 290 GeV.

Assuming Poisson distributions for the SM background expectation and
for the signal, an upper limit on the number of events coming 
from LQ production is then obtained using a standard Bayesian
prescription with a flat prior for the signal cross section.
The procedure which folds in the
statistical and systematic errors is described in detail
in~\cite{H1LQ94}.
This number 
can be translated into a limit on the product
$\sigma_{LQ}\beta_e$ of the LQ production cross section
$\sigma_{LQ}$ and the branching ratio $\beta_e$ for the
LQ to decay into $eq$. 
NLO QCD corrections to $\sigma_{LQ}$ were calculated in~\cite{SPIRA}
and are taken into account here. They lead to an enhancement of
the LQ cross section by $15 \% - 30 \%$ for LQ masses between
100 GeV and 290 GeV.
% Assuming a specific LQ model (e.g.~\cite{BRW}) in which
% the cross section $\sigma (eq \rightarrow LQ \rightarrow eq)$
% depends only on the Yukawa coupling besides $M_{LQ}$,
% upper limits on $\lambda$ are thus obtained as a function of the LQ mass.
% 
% 
In the NWA,
$\sigma_{LQ}$ scales with the square of the Yukawa coupling,
hence upper limits on $\lambda \sqrt{\beta_e}$ are obtained
as a function of the LQ mass.
Thus, assuming a specific LQ model (e.g.~\cite{BRW}) in
which $\beta_e$ is known, mass dependent upper limits
on $\lambda$ can be derived.
On the other hand, assuming a given value for the coupling $\lambda$,
limits on the branching ratio $\beta_e$ can be obtained as a function
of the LQ mass. These two approaches are addressed in the following.




%.............................................................
\subsubsection{Limits on the Yukawa coupling in the BRW model}
%.............................................................


% --------------- FIGURE 5: Limits BRW -------------------------
%
\begin{figure}[htb]
   \begin{center}
     \epsfxsize=0.8\textwidth
     \epsffile{H1prelim-00-061.fig6.eps}
      \caption
      { \small  \label{fig:brw}
                Exclusion limits at $95 \%$ CL on the Yukawa
                coupling $\lambda$ as a function of the LQ mass
                for (a) scalar and (b) vector LQs with
                fermion number $F=2$ described by the BRW model.
                Domains above the curves are excluded.
                The upper curve on each plot indicates the limit
                obtained from the (less suited for $F=2$ LQs)
                $e^+ p$ data~\cite{H1LQ99}.  }
 \end{center}
\end{figure}
%----------------------------------------------------------------------

The phenomenological model proposed by 
Buchm\"uller-R\"uckl-Wyler (BRW)~\cite{BRW}
describes
14 LQs, half of them having a fermion number $F=2$, i.e.
coupling to $e^- + q$. We focus on those LQs since
LQs with $F=0$ are better constrained using $e^+ p$ data~\cite{H1LQ99}.
In the BRW model the branchings $\beta_e$ are fixed and equal
to 1 or 0.5 depending on the LQ type.
In this framework, mass dependent upper limits on the Yukawa
coupling can then be set for each LQ type.
These limits are represented in Fig.~\ref{fig:brw}.
%
Those limits considerably extend the domain excluded by the
analysis of $e^+ p$ data~\cite{H1LQ99}, represented as
the most upper curve on each plot.
For Yukawa coupling of the electromagnetic strength ($\lambda = 0.3$)
LQ masses up to $290 \GeV$ are excluded at $95 \%$ confidence level (CL).
% For such LQ mass and coupling, the NWA is still valid within
% $\sim 30 \%$. Moreover it leads to conservative constraints
% since the LQ cross section is underestimated by NWA.
For such LQ mass and coupling, 
the NWA is still valid within $\sim 30 \%$ and
the interference of LQ production
with NC DIS remains below $1 \%$ of the LQ resonant production
cross section in the kinematic domain where the signal is
searched for, which justifies the approximation made in the
analysis.




%.............................................................
\subsubsection{Mass Dependent Limits on
            {\boldmath $\beta(LQ \rightarrow eq)$ }}
%.............................................................

 Moving away from the BRW model, we now consider  leptoquarks which undergo NC
DIS-like decays
with a branching ratio $\beta_e$ and do not make any
assumption on the other possible decay modes of the LQ.
For a fixed value of the Yukawa coupling $\lambda$
upper limits on $\beta_e$ are derived as shown in Fig.~\ref{fig:beta}a
and Fig.~\ref{fig:beta}b for a $F=2$ scalar LQ coupling to 
$e^- d$ and $e^- u$ respectively.
%
% --------------- FIGURE 6 : Limits beta -------------------------
%
\begin{figure}[htb]
   \begin{center}
    \begin{tabular}{l}
     \mbox{\epsfxsize=0.8\textwidth
        \epsffile{H1prelim-00-061.fig7a.eps}}
  \\ 
     \mbox{\epsfxsize=0.8\textwidth
     \epsffile{H1prelim-00-061.fig7b.eps}}
    \end{tabular}
      \caption
      { \small  \label{fig:beta}
         Mass dependent exclusion limits at $95 \%$ CL on the branching
         ratio $\beta_e$ of the LQ to decay into $eq$
         for scalar LQs with $F=2$ produced by (a) $e^- d$
         and (b) $e^- u$ fusion. The $D\emptyset$ limit is also
         shown as hatched region.  }
 \end{center}
\end{figure}
%----------------------------------------------------------------------
It can be seen that 
the domain excluded by this analysis extends considerably beyond the
region excluded by the $D\emptyset$ experiment at the TeVatron,
represented as the hatched domain on Fig.~\ref{fig:beta}.


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

Leptoquarks with fermion number $F=2$ have been searched
using the $e^- p$ data collected by H1 in 1998 and 1999.
No signal has been observed and constraints on such LQs have been
set, allowing to extend beyond the domains excluded by other
experiments. Moreover the constraints set here on $F=2$ LQs
are more stringent than those on $F=0$ LQs obtained in~\cite{H1LQ99}
from the higher statistics $e^+ p$ data sample, due to the
enhanced energy in the centre of mass of the $ep$ collision.



\begin{thebibliography}{99}

\bibitem{H1LQ99}
 H1 Collaboration, C.~Adloff {\it et al.}, Eur.Phys.J. C11 (1999) 447.
\vspace{-2mm}

\bibitem{H1LQ94}
 H1 Collaboration, T.~Ahmed {\it et al.}, Z.~Phys.~C64 (1994) 545.
\vspace{-2mm}

\bibitem{DJANGO}
 DJANGO~6.2;
 G.A.~Schuler and H.~Spiesberger,
 Proc. of the Workshop Physics at HERA,
 W.~Buchm\"uller and G.~Ingelman (Editors),
 (October 1991, DESY-Hamburg) Vol. 3 p. 1419.
\vspace{-2mm}

\bibitem{ARIADNE}
 ARIADNE 4.08;
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