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\begin{titlepage}


\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 Search for Leptoquarks in {\boldmath{$ep$}} collisions at HERA }

\vspace{1.5cm}

H1 Collaboration

\end{Large}
\end{center}

\vspace{1.5cm}

\begin{abstract}
A search for scalar and vector leptoquarks coupling to first generation
fermions is performed in the H1 experiment at the $ep$ collider HERA.
%
No significant evidence for the direct production of such particles is
found in $\sim 37 \picob^{-1}$ of $e^+ p$ integrated luminosity collected 
at $\sqrt{s} = 300 \GeV$ from 1994 to 1997, although
a slight excess over the Deep Inelastic Scattering Standard Model expectation
is observed in the mass spectrum for masses $M \simeq 200 \GeV$.
%
No significant evidence for leptoquark production is found in 
$\sim 14.4 \picob^{-1}$ of $e^- p$ data collected at
$\sqrt{s} = 320 \GeV$ from 1998 to 1999.
Mass dependent constraints on the Yukawa coupling of leptoquarks 
are derived. 
% combining the H1 results from direct searches and from indirect searches
% performed in a contact interactions approach.
% through contact interactions. 
The constraints on leptoquarks with 
fermionic number $F=0$ ($F=2$) make use of the $e^+ p$ ($e^- p$) data sample. 
For leptoquarks masses far above the centre-of-mass energy constraints
are also derived from indirect searches 
performed in a contact interaction approach.
%
A preliminary analysis of $\sim 47.6 \picob^{-1}$ of new $e^+ p$ data collected 
in 1999 to 2000 reveals no evidence for direct leptoquark production.

\end{abstract}

\vfill

\begin{flushleft}
\textbf{Abstract: 953\\
    Parallel Session: 11\\
    Plenary Talk: 7}
\end{flushleft}


\end{titlepage}
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A search for leptoquarks in $ e^+ p \rightarrow e + X$ and
$ e^+ p \rightarrow \nu + X$ processes using 1994 to 1997 
data collected at $\sqrt{s} = 300 \GeV$ by H1 has been presented 
in ref.~\cite{H1LQ99}.
% An excess of events 
% over the Standard Model (SM) prediction from Deep Inelastic
% Scattering (DIS)
% had been previously observed~\cite{H1ZEUSLQ97} 
% by the H1 and ZEUS experiments at HERA at high masses and squared momentum 
% transfer $Q^2$ in the data taken between 1994 and 1996.
% For H1 this excess appeared
% concentrated at masses $M \simeq 200 \GeV$.
% --------------- FIGURE 1: Limits BRW -------------------------
%
\begin{figure}[htb]
   \begin{center}
%
     \epsfxsize=0.9\textwidth \epsffile{H1prelim-00-161.fig1a.eps} \\
     \vspace*{-1.0cm}

     \epsfxsize=0.9\textwidth \epsffile{H1prelim-00-161.fig1b.eps}
%
      \caption
      { \small  \label{fig:S12LSOLbrw}
                Exclusion limits at $95 \%$ CL on the Yukawa
                coupling $\lambda$ as a function of the LQ mass
                for (top) a scalar LQ with $F=0$ and 
                    (bottom) a scalar LQ with $F=2$ described by 
                the BRW model.
                Greyed and hatched domains are excluded. }
 \end{center}
\end{figure}
%----------------------------------------------------------------------
%
% With the inclusion of the 1997 data, 
% which lead to an increase in statistics by more than
% a factor of two, the analysis revealed that such an excess
% persisted in the H1 data though with decreased 
% significance~\cite{H1LQ99,H1F299} and constraints on the Yukawa 
% coupling of leptoquarks were derived~\cite{H1LQ99}.
%
%
Since no significant evidence for a LQ signal has been observed,
constraints on the Yukawa coupling of leptoquarks were derived,
which represent the best constraints available from H1 for direct
searches for leptoquarks carrying fermionic number $F=0$.
The figure~\ref{fig:S12LSOLbrw} (top) shows for example a summary of the
H1 constraints on the $\tilde{S}_{1/2,L}$ leptoquark 
described in the
Buchm\"uller-R\"uckl-Wyler model~\cite{Buchmuller97} and a comparison
to constraints set at LEP~\cite{LQLEP} and Tevatron~\cite{LQTEVATRON}.
Also plotted is the corresponding bound derived by H1 via a 
contact interaction (CI) analysis using 
% the same dataset~\cite{H1CI9497}.
all the available data 
%
\footnote{
New CI constraints
combining the information from all available
$e^+p$ and $e^-p$ data are presented for the first time
at this ICHEP2000 conference~\cite{CICHEP2000}.}
%
accumulated by H1 between 1994 and 2000~\cite{CICHEP2000}.
Also discussed in ref.~\cite{H1LQ99} were constraints set in "generic" 
models where an arbitrary branching ratio $\beta_e$ for the LQ to
decay into $e + X$ is
assumed, as well as direct and indirect constraints on 
LQ induced lepton flavour
violating processes.   

A search for leptoquarks in $ e^- p \rightarrow e + X$ processes has been 
performed using $\sim 14.4 \picob^{-1}$ of $e^- p$ data collected at 
$\sqrt{s} = 320 \GeV$ from 1998 to 1999.
No evidence for leptoquark production was found and constraints 
were derived~\cite{Raedel00,H1LQ00}.
The figure~\ref{fig:S12LSOLbrw} (bottom) shows for example a summary of the
H1 constraints on the $S_{0,L}$ leptoquark described in
the Buchm\"uller-R\"uckl-Wyler model and a comparison
to constraints set at LEP~\cite{LQLEP} and Tevatron~\cite{LQTEVATRON}.
Also plotted is the corresponding constraint derived by H1 via a 
CI analysis using 
% same dataset~\cite{H1CI9497}.
% 1994-1997 data~\cite{H1CI9497}.
all available data accumulated by H1 between 1994 and 2000~\cite{CICHEP2000}.


% ---------- FIGURE 3: dNdM  Scalars  ----------------
%
\begin{figure}[htb]
  \begin{center}
  \begin{tabular}{cc}
     \hspace*{-0.9cm}\mbox{\epsfxsize=0.55\textwidth
        \epsffile{H1prelim-00-161.fig2a.eps}}
   &
     \hspace*{-0.8cm}\mbox{\epsfxsize=0.55\textwidth
     \epsffile{H1prelim-00-161.fig2b.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 squares,
         white histogram) and after (closed dots, hatched histogram)
         a $y$ cut designed to maximize the significance of an eventual
         scalar. The spectra are shown for (a) 1994 to 1997 data and
         (b) 1999 to 2000 $e+p$ data.
         The greyed boxes indicate the $\pm 1 \sigma$ error band combining the
         statistical and systematic errors of the NC DIS
         expectation. }}
\end{figure}
%---------------------------------------------------------------------------

Although no significant deviation from the Standard Model expectation
was found in the 94-97 $e^+ p$ data an excess of events was observed 
for masses around 200 GeV as can be seen in Fig.~\ref{fig:dndmlq}a,
where the mass distribution of the
observed high $Q^2$ DIS candidates is compared to the
SM expectation.
%
%
A search for leptoquarks in $ e^+ p \rightarrow e + X$ processes using 
$\sim 47.6 \picob^{-1}$ of new data collected in 1999 to 2000
reveals no evidence for direct leptoquark production as can be seen 
in Fig.~\ref{fig:dndmlq}b.
% , where the mass distribution of the 
% observed high $Q^2$ DIS candidates is compared to the
% SM expectation.
The excess seen in the mass spectrum from the 
previous analysis~\cite{H1LQ99} based on 1994 to 1997 data (Fig.~\ref{fig:dndmlq}a) is
not confirmed by the new data (Fig.~\ref{fig:dndmlq}b). 

% New CI constraints 
% combining the information from all available 
% $e^+p$ and $e^-p$ data are presented at this ICHEP2000 conference~\cite{CICHEP2000}.

\vspace*{1cm}

%=======================================================================
\section*{Acknowledgements}
We are grateful to the HERA machine group whose outstanding efforts
made this experiment possible.
We appreciate the immense effort of the engineers and technicians who
constructed and maintain the detector.
We thank the funding agencies for their financial support of the experiment.
We wish to thank the DESY directorate for the hospitality extended
to the non-DESY members of the Collaboration.

%=======================================================================
\vfill
\clearpage

\begin{thebibliography}{99}

\bibitem{H1LQ99} 
 H1 Collaboration, 
 C. Adloff et al., \Journal{\EJC}{11}{1999}{447};
 {\it Erratum published online (April 2000)}

\bibitem{H1ZEUSLQ97}
 H1 Collaboration, 
 C. Adloff et al., \Journal{\ZPC}{74}{1997}{191};
 ZEUS Collaboration, 
 J. Breitweg et al., \Journal{\ZPC}{74}{1997}{207}.

%% \bibitem{ZEUSLQ00}
%% ZEUS Collaboration,
%% J. Breitweg et al.), preprint DESY-00-023 (February 2000) 31pp. 

\bibitem{H1F299}
 H1 Collaboration, C. Adloff et al., \Journal{\EJC}{13}{2000}{609}.

\bibitem{Buchmuller97}
 W.~Buchm\"uller, R~R\"uckl and D~Wyler, \Journal{\PLB}{191}{1997}{442};
 {\it Erratum} \Journal{\PLB}{1999}{320}.

\bibitem{LQLEP} 
% To be checked...
 OPAL Collaboration,
 K. Ackerstaff et al.,
 \Journal{\EJC}{6}{1999}{1}.

% Cite relevant LEP paper(s)

\bibitem{LQTEVATRON}
 CDF and $D \emptyset$ Collaborations,
 ``Combined Limits on First Generation Leptoquarks from the CDF and $D\emptyset$
 Experiments'', hep-ex/9810015, Oct. 1998, 9pp.

\bibitem{CICHEP2000}
 H1 Collaboration, ``Search for Compositeness, Leptoquarks and Large Extra Dimensions
 in $eq$ Contact Interactions at HERA,
 Contributed paper \#951, 30th International Conference on High Energy Physics,
 ICHEP2000, Osaka, Japan (July 2000) 11pp.

\bibitem{H1CI9497}
 H1 Collaboration, C. Adloff et al., \Journal{\PLB}{479}{2000}{358}. 

\bibitem{Raedel00}
 G.~R\"adel , ``Search for New Phenomena at HERA'',
 Proceedings of the XXXVth Rencontres de Moriond on Electroweak Interactions
 and Unified Theories (March 2000) 8pp.

\bibitem{H1LQ00}
 H1 Collaboration, ``A Search for Leptoquark Bosons in $e^-p$ Collisions at HERA'',
 Contributed paper \#954, 30th International Conference on High Energy Physics,
 ICHEP2000, Osaka, Japan (July 2000) 10pp.


\end{thebibliography}


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