  Supersymmetric hybrid inflation is an exquisite framework to connect
inflationary cosmology to particle physics at the scale of grand unification.
Ending in a phase transition associated with spontaneous symmetry breaking, it
can naturally explain the generation of entropy, matter and dark matter.
Coupling F-term hybrid inflation to soft supersymmetry breaking distorts the
rotational invariance in the complex inflaton plane---an important fact, which
has been neglected in all previous studies. Based on the delta-N formalism, we
analyze the cosmological perturbations for the first time in the full two-field
model, also taking into account the fast-roll dynamics at and after the end of
inflation. As a consequence of the two-field nature of hybrid inflation, the
predictions for the primordial fluctuations depend not only on the parameters
of the Lagrangian, but are eventually fixed by the choice of the inflationary
trajectory. Recognizing hybrid inflation as a two-field model resolves two
shortcomings often times attributed to it: The fine-tuning problem of the
initial conditions is greatly relaxed and a spectral index in accordance with
the PLANCK data can be achieved in a large part of the parameter space without
the aid of supergravity corrections. Our analysis can be easily generalized to
other (including large-field) scenarios of inflation in which soft
supersymmetry breaking transforms an initially single-field model into a
multi-field model.
