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{\bf Abstract Registration Number 578} \\
{\bf Parallel sessions: 10,1} \\
{\bf Plenary sessions: 12,1}
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\abstractheader{\bf Observation of Events with an Isolated High Energy Lepton and
\\ Missing Transverse Momentum at HERA \\
H1 Collaboration, DESY}

\abstractbody{
A search for
events with an imbalance in transverse momentum
and with isolated high energy leptons has been carried out
at the positron-proton collider HERA.
One event with an  
$e^-$ and five events with a  $\mu^{\pm}$ are found  together
 with evidence for 
undetected particles carrying transverse momentum.
Within the Standard Model the dominant origin of events with
  this kind of topology is the production 
of $W$ bosons with subsequent leptonic decay.
Three of the six events are within measurement errors 
found in a region of phase space likely to be populated by
 this process, while the remaining events show 
kinematic properties which are atypical of  all Standard Model 
processes considered.
}

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