All six events show missing transverse
momentum and therefore the signature of possible neutrino production. In
the figure the transverse momentum PtX of the hadronic final state is
shown depending on the transverse mass MTl-nu of the lepton-neutrino
system. The measured events are shown together with the uncertainty in
the two observables.
Also shown are standard model expectations which correspond to 500
times the measured integrated luminosity. The circles represent the predictions
of W-boson production. For the muon decay channel also inelastic muon production
from two-photon processes is shown (triangle symbols). A direct comparison
of the measured events with the expectations from the different production
mechanisms of the standard model shows that the electron event can be explained
by W-boson production. However, for the five muon events only 0.8+-0.2
are expected. Two of the muon events are in a region of the phase space
in which W-boson production is expected. The observables of the other 3
muon events are in a phase space region that is atypical for W-boson production.
The possibilities to explain these events in the frame work of the standard
model are therefore limited.
In the analysis of the data between 1994-96 an excess of neutral current
events at
very high Q2 was found. This excess was 3 standard deviations above the
expectation of the standard model calculation. With the data of 1997 the
total integrated luminosity was doubled.
Also in the new data a tendency for more
events at very high Q2 is found. The magnitude of the excess is not confirmed
by 1997 data.
In lepton-proton collisions, a natural area of searches for physics
beyond the Standard Model is the search for leptoquarks, that are bound
states between a lepton and a quark. No sign for the production of such
particles was found in the data.
The figure shows limits for the production
of leptoquarks from positron-d-quark and positrons-u-quark processes. The
limits reach mass values of the leptoquarks up to about mLQ = 250 GeV.
The reduced strength of the limits at mass values slightly above mLQ =
200 GeV reflects the excess events that were found in neutral current events
at high Q2. At small values of the branching ratio
beta(LQ -> positron quark) for the decay of the leptoquark, the H1 data
are significantly more sensitive compared to the data of other accelerators.
This demonstrates the discovery potential for new physics at HERA.
