Gravitational Lensing of the CMB: a Feynman Diagram Approach
  We develop a Feynman diagram approach to calculating correlations of the
Cosmic Microwave Background (CMB) in the presence of distortions. As one
application, we focus on CMB distortions due to gravitational lensing by Large
Scale Structure (LSS). We study the Hu-Okamoto quadratic estimator for
extracting lensing from the CMB and derive the noise of the estimator up to
${\mathcal O}(\phi^4)$ in the lensing potential $\phi$. The previously noted
large ${\mathcal O}(\phi^4)$ term can be significantly reduced by a
reorganization of the $\phi$ expansion. Our approach makes it simple to obtain
expressions for quadratic estimators based on any CMB channel. We briefly
discuss other applications to cosmology of this diagrammatic approach, such as
distortions of the CMB due to patchy reionization, or due to Faraday rotation
from primordial axion fields.
