The H1 collaboration reported the observation of events with high-p_t isolated leptons
and missing transverse momentum. This characteristics turned out to be an
atypical kinematic property when compared to the Standard Model (SM) calculations which
are dominated by direct production of W-bosons.
The lepton and the missing transverse momentum are likely to be associated with a
W-boson decaying
into the lepton and a neutrino. The large transverse momentum of the hadronic
system, however, may indicate the
presence of a further mechanism beyond Standard Model W production.
A possible origin of these events is production of top quarks which
predominantly decay into a b-quark jet and a W-boson.
Within the SM, top quark production is negligible at HERA.
However, in several extensions of the SM, the top quark
is predicted to have significant flavour changing neutral
current (FCNC) interactions, which could lead to a sizeable
single top production cross-section at HERA.
In the SM neutral current interactions
preserve the involved flavours in leading order.
Therefore flavour changing neutral current processes
only arise via higher order radiative corrections and are
highly suppressed. Their low SM rates make FCNC processes
an ideal place to look for new physics at high energy colliders.
Due to the large top quark mass close to the electroweak symmetry
breaking scale, deviations from the SM might be observed first in
the top sector. FCNC interactions involving the
top quark have thus received considerable interest since the discovery
of the top at the Fermilab Tevatron.
Such interactions are present in many extensions of the SM like
models with an extended Higgs sector,
supersymmetry, dynamical breaking of the electroweak symmetry,
or of an additional symmetry.
At HERA, FCNC interactions
coupling a top quark to a light quark (u or c)
and a gauge boson would lead to single top production via
----------------------
e | e / /
| b / /
| gamma top decay: / / nu
| t / W /
| ---------------------------
q(u or c) | lepton
----------------------
t
The crossed process e+ e- --> t \bar{q} or \bar{t} q is actively searched for at the
LEP collider since its centre-of-mass energy is above the top production threshold.