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{\bf EPS-HEP99} \\
{\bf Abstract \# 000} \\
{\bf Parallel sessions: 7} \\
{\bf Plenary sessions:  7,8}
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\abstractheader{ {\bf
\boldmath A Search for Leptoquark Bosons and 
Lepton Flavor Violation in $e^+ p$ collisions at HERA   } \\ \\
H1 Collaboration, \\
DESY, Notkestr.85, D-22607 Hamburg, Germany
}

\abstractbody{
A search for new bosons possessing couplings to lepton-quark pairs is
performed in the H1 experiment at HERA using 1994$\rightarrow$1997 data
corresponding to an integrated luminosity of $37 $pb$^{-1}$.
First generation leptoquarks (LQs) are searched in very high $Q^2$ 
neutral (NC) and charged (CC) current-like data samples 
which are compared to Standard Model (SM) expectations from
deep-inelastic scattering (DIS).
A deviation in the $Q^2$ spectrum previously observed in the
1994$\rightarrow$1996 dataset at $Q^2 \ge 15000 $\, GeV$^2$ remains,
though
with less significance.
This deviation corresponded to a clustering in the invariant mass spectrum
at $M \simeq 200 $\, GeV which is not observed with the 1997 dataset
alone.
The NC DIS data is used to constrain the Yukawa couplings of first
generation scalar and vector LQs in the Buchm\"uller-R\"uckl-Wyler
effective
model, taking into account $s$- and $u$-channel LQ contributions
as well as the interference with SM boson exchange.
Scalar LQs are excluded for masses up to $275 $\, GeV for a coupling
of electromagnetic strength ($\lambda = 0.3$) 
For any LQ type, constraints are established for $\lambda \le 1.0$ for
masses up to $400 $\, GeV.
The NC and CC DIS data is combined to constrain $\lambda$ for arbitrary
branching ratios in a generic model.
Exclusion limits are derived which considerably improve earlier LQs search
results from HERA and extend beyond the coupling-mass domain covered so
far 
at other colliders for a small branching ratio to $e^+ q$.
For a branching ratio to $e^+ u$ pair as small as $10 \%$, leptoquark
masses up to $260 $\, GeV are ruled out by the present analysis for a 
coupling $\lambda = 0.3$.
Leptoquarks possessing couplings to mixed fermion generations,
which could lead to signals of lepton flavor violation (LFV),
are searched in events with a high transverse momentum $\mu$ or $\tau$.
No $\mu + X$ or $\tau + X$ event candidate is found that is compatible
with the specific hypothesis of LFV due to a leptoquark.
Constraints are set on the Yukawa coupling involving the $\mu$ and $\tau$
lepton for a mass range extending beyond the domain presently covered at
other colliders and by low energy experiments.
}

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