Bach V., Froehlich J., Sigal I.M.
Renormalization Group Analysis
of Spectral Problems in Quantum Field Theory
(974K, Postscript)

ABSTRACT.  In this paper we present a self-contained and detailed exposition
of the new renormalization group technique proposed in
\cite{BachFroehlichSigal1995,BachFroehlichSigal1997a}.
Its main feature is that the renormalization group transformation
acts directly on a space of operators rather than on objects 
such as a propagator, the partition function, or
correlation functions.
We apply this renormalization transformation to a Hamiltonian
describing the physics of an atom
interacting with the quantized electromagnetic field, and we prove
that excited atomic states turn into resonances when the
coupling between electrons and field is nonvanishing.