Tribrid Inflation in Supergravity
  We propose a novel class of F-term hybrid inflation models in supergravity
(SUGRA) where the $\eta$-problem is resolved using either a Heisenberg symmetry
or a shift symmetry of the Kaehler potential. In addition to the inflaton and
the waterfall field, this class (referred to as tribrid inflation) contains a
third 'driving' field which contributes the large vacuum energy during
inflation by its F-term. In contrast to the ''standard'' hybrid scenario, it
has several attractive features due to the property of vanishing inflationary
superpotential (W_inf=0) during inflation. While the symmetries of the Kaehler
potential ensure a flat inflaton potential at tree-level, quantum corrections
induced by symmetry breaking terms in the superpotential generate a slope of
the potential and lead to a spectral tilt consistent with recent WMAP
observations.
