  Motivated by a number of recent experimental studies we have revisited the
problem of the microscopic calculation of the quasiparticle self-energy and
many-body effective mass enhancement in a two-dimensional electron liquid. Our
systematic study is based on the many-body local fields theory and takes
advantage of the results of the most recent Diffusion Monte Carlo calculations
of the static charge and spin response of the electron liquid. We report
extensive calculations of both the real and imaginary parts of the
quasiparticle self-energy. We also present results for the many-body effective
mass enhancement and the renormalization constant over an extensive range of
electron density. In this respect we critically examine the relative merits of
the on-shell approximation versus the self-consistent solution of the Dyson
equation. We show that in the strongly-correlated regime a solution of the
Dyson equation proves necessary in order to obtain a well behaved effective
mass. The inclusion of both charge- and spin-density fluctuations beyond the
Random Phase Approximation is indeed crucial to get reasonable agreement with
recent measurements.
