Particle Production During Inflation: Observational Constraints and
  Signatures
  In a variety of inflation models the motion of the inflaton may trigger the
production of some non-inflaton particles during inflation, for example via
parametric resonance or a phase transition. Particle production during
inflation leads to observables in the cosmological fluctuations, such as
features in the primordial power spectrum and also nongaussianities. Here we
focus on a prototype scenario with inflaton, \phi, and iso-inflaton, \chi,
fields interacting during inflation via the coupling g^2 (\phi-\phi_0)^2\chi^2.
Since several previous investigations have hinted at the presence of localized
"glitches" in the observed primordial power spectrum, which are inconsistent
with the simplest power-law model, it is interesting to determine the extent to
which such anomalies can be explained by this simple and well-motivated model.
Our prototype scenario predicts a bump-like feature in the primordial power
spectrum, rather than an oscillatory "ringing" pattern as has previously been
assumed. We discuss the observational constraints on such features. We find
that bumps with amplitude as large as 10% of the usual scale invariant
fluctuations from inflation are allowed on scales relevant for Cosmic Microwave
Background experiments. Our results imply an upper limit on the coupling g^2
which is crucial for assessing the detectability of the nongaussianity produced
by inflationary particle production. We also discuss more complicated features
that result from superposing multiple instances of particle production.
Finally, we point to a number of microscopic realizations of this scenario in
string theory and supersymmetry and discuss the implications of our constraints
for the popular brane/axion monodromy inflation models.
