Condensates and quasiparticles in inflationary cosmology: mass
  generation and decay widths
  During de Sitter inflation massless particles of minimally coupled scalar
fields acquire a mass and a decay width thereby becoming \emph{quasiparticles}.
For bare massless particles non-perturbative infrared radiative corrections
lead to a self-consistent generation of mass, for a quartic self interaction $M
\propto \lambda^{1/4} H$, and for a cubic self-interaction the mass is induced
by the formation of a non-perturbative \emph{condensate} leading to $M \propto
\lambda^{1/3} H^{2/3}$. These radiatively generated masses restore de Sitter
invariance and result in anomalous scaling dimensions of superhorizon
fluctuations. We introduce a generalization of the non-perturbative
Wigner-Weisskopf method to obtain the time evolution of quantum states that
include the self-consistent generation of mass and regulate the infrared
behavior. The infrared divergences are manifest as poles in $\Delta=M^2/3H^2$
in the single particle self-energies, leading to a re-arrangement of the
perturbative series non-analytic in the couplings. A set of simple rules that
yield the leading order infrared contributions to the decay width are obtained
and implemented. The lack of kinematic thresholds entail that all particle
states acquire a decay width, dominated by the emission and absorption of
superhorizon quanta $\propto (\lambda/H)^{4/3}\,[H/k_{ph}(\eta)]^6 ;
\lambda\,[H/k_{ph}(\eta)]^6 $ for cubic and quartic couplings respectively to
leading order in $M/H$. The decay of single particle quantum states hastens as
their wavevectors cross the Hubble radius and their width is related to the
highly squeezed limit of the bi- or tri-spectrum of scalar fluctuations
respectively.
