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\begin{document}

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\begin{titlepage}

\noindent
\begin{flushleft}
{{H1prelim-02-064, April 2002}} \hfill\vspace*{-2cm}\epsfig
{file=/h1/www/images/H1logo_bw_small.epsi,width=2.cm}
%{file=H1logo_bw_small.epsi,width=2.cm}
\end{flushleft}
\vspace*{2cm}
\begin{center}
  \begin{Large}                   
  {\bf A search for leptoquarks at HERA}

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

\begin{abstract} \noindent
A search for scalar and vector leptoquarks coupling to first generation
fermions is performed in the H1 experiment using data collected from 
1994 to 2000. No significant evidence for the direct production of 
such particles is found in a data sample with a large transverse momentum
final state electron or with large missing transverse momentum, 
and constraints on leptoquark models are established. For leptoquark 
couplings of electromagnetic strength, leptoquark masses up to 290 GeV
are ruled out.
\end{abstract}


\end{titlepage}

\pagenumbering{arabic}

\clearpage

% Mass spectra
%
\begin{figure}[htb] 
\begin{center}
\begin{picture}(50,140)
\put(-60,-80){\epsfig{figure=H1prelim-02-064.fig1.eps,bbllx=0pt,bblly=0pt,bburx=594pt,
bbury=842pt,width=18cm}}
\end{picture}
\end{center}
  \caption{Mass spectra for the events from (a) neutral current (NC)
    and (b) charged current (CC) deep inelastic scattering (DIS)
    selections, together with the corresponding DIS expectations (histograms).
    The grey bands indicate the $\pm1\sigma$ uncertainty due to the systematic
    errors on the NC and CC DIS expectations.}
\end{figure} 

% Limits within the BRW model
%
\begin{figure}[p] 
\begin{center}
\begin{picture}(50,200)
\put(-60,-5){\epsfig{figure=H1prelim-02-064.fig2.eps,width=17cm}}
\end{picture}
\end{center}
  \caption{Exclusion limits for the 7 $F=0$ leptoquarks (LQs) described by the
Buchm\"uller, R\"uckl and Wyler (BRW) model. The limits are expressed at
95\% CL on the Yukawa coupling $\lambda$ as a function of the leptoquark
mass for (a) scalar LQs and (b) vector LQs. Domains above the curves are
excluded. Constraints on LQs with masses above the HERA centre-of-mass
energy, obtained from a contact interaction (CI) analysis, are shown in the
rightmost part of each plot.}
\end{figure} 

% Comparison with other experiments
%
\begin{figure}[p] 
\begin{center}
\begin{picture}(50,200)
\put(-30,60){\epsfig{figure=H1prelim-02-064.fig3a.eps,width=10.5cm}}
\put(-30,-30){\epsfig{figure=H1prelim-02-064.fig3b.eps,width=10.5cm}}
\end{picture}
\end{center}
  \caption{Exclusion limits at 95\% CL on the Yukawa coupling $\lambda$ as a
function of the leptoquark (LQ) mass for (top) a scalar with $F=0$ and
(bottom) a scalar LQ with $F=2$ described by the BRW model. Shaded and
hatched domains are excluded.}
\end{figure} 

% Limits within a generic model
%
\begin{figure}[p] 
\begin{center}
\begin{picture}(50,180)
\put(-55,-10){\epsfig{figure=H1prelim-02-064.fig4.eps,width=15cm}}
\end{picture}
\end{center}
  \caption{(a) Mass dependent exclusion limits at 95\% CL on the branching
ratio $\beta_e$ of a scalar leptoquark (LQ) which couples to $e^+u$ (with
the quantum numbers of the $S_{1/2, L}$). (b) Domains ruled out by the
combination of the NC and CC analyses, for a vector LQ which couples to
$e^+d$ (with the quantum numbers of the $V_{0,L}$) and decaying only into
$eq$ and $\nu q$ for four values of the Yukawa coupling $\lambda$. The
regions on the left of the full curves are excluded at 95\% CL. For
$\lambda=0.05$ and 0.3, the part of the $\beta_e-M_{\rm LQ}$ ($\beta_\nu
-M_{\rm LQ}$) plane on the left of the dashed (dotted) curve is excluded by
the NC (CC) analysis. The branching ratios $\beta_e$ and $\beta_\nu$ are
shown on the left and right exes respectively. In (a) and (b), the hatched
region represents the domain excluded by the D0 experiment. The D0 bounds do
not depend on the value of the Yukawa coupling.}
\end{figure} 

% efficiencies and signal/background ratios
%
\begin{figure}[p]
\begin{center}
\begin{picture}(50,205)
\put(-55,137.5){\epsfig{figure=H1prelim-02-064.fig5a.eps,width=7.5cm}}
\put(-42.5,153){\bf (a)}
\put(25,137.5){\epsfig{figure=H1prelim-02-064.fig5b.eps,width=7.5cm}}
\put(36.5,153){\bf (b)}
\put(-55,65){\epsfig{figure=H1prelim-02-064.fig5c.eps,width=7.5cm}}
\put(-42.5,80){\bf (c)}
\put(25,65){\epsfig{figure=H1prelim-02-064.fig5d.eps,width=7.5cm}}
\put(36.5,80){\bf (d)}
\put(-55,-7.5){\epsfig{figure=H1prelim-02-064.fig5e.eps,width=7.5cm}}
\put(-42.5,7.5){\bf (e)}
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\put(36.5,7.5){\bf (f)}
\end{picture}
\end{center}
  \caption{Leptoquarks (LQs) event distribution at 200 GeV for (a) 
  NC-like scalar, (c) NC-like vector, and (e) CC-like vector LQs, and the
corresponding signal over the Standard Model background ratios (b), (d), and
(e).}
\end{figure} 

% scattered plots
%
\begin{figure}[p]
\begin{center}
\begin{picture}(50,205)
\put(-55,137.5){\epsfig{figure=H1prelim-02-064.fig6a.eps,width=6.5cm}}
\put(-4,185){\bf (a)}
\put(25,137.5){\epsfig{figure=H1prelim-02-064.fig6b.eps,width=6.5cm}}
\put(76,185){\bf (b)}
\put(-55,65){\epsfig{figure=H1prelim-02-064.fig6c.eps,width=6.5cm}}
\put(-4,110){\bf (c)}
\put(25,65){\epsfig{figure=H1prelim-02-064.fig6d.eps,width=6.5cm}}
\put(76,110){\bf (d)}
\put(-55,-7.5){\epsfig{figure=H1prelim-02-064.fig6e.eps,width=6.5cm}}
\put(-4,40){\bf (e)}
\end{picture}
\end{center}
  \caption{Data events selected in the NC (a) and CC (b) analyses and the
  corresponding Monte Carlo events at 200 GeV for (c) NC-like scalar, (d)
  CC-like vector LQ, and (e) NC-like vector LQs.}
\end{figure} 

\end{document}
