  The helium atom, with one nucleus and two electrons, is a prototypical system
to study quantum many-body dynamics. Doubly-excited states, or quantum states
in which both electrons are excited by one photon, are showcase scenarios of
electronic-correlation mediated effects. In this paper, the natural lifetimes
of the doubly-excited $^1$P$^o$ 2s$n$p Rydberg series and the $^1$S$^e$ 2p$^2$
dark state in helium in the 60 eV to 65 eV region are measured directly in the
time domain with extreme-ultraviolet/near-infrared noncollinear attosecond
four-wave-mixing (FWM) spectroscopy. The measured lifetimes are in agreement
with lifetimes deduced from spectral linewidths and theoretical predictions,
and the roles of specific decay mechanisms are considered. While complex
spectral line shapes in the form of Fano resonances are common in absorption
spectroscopy of autoionizing states, the background-free and thus homodyned
character of noncollinear FWM results exclusively in Lorentzian spectral
features in the absence of strong-field effects. The onset of strong-field
effects that would affect the extraction of accurate natural lifetimes in
helium by FWM is determined to be approximately 0.3 Rabi cycles. This study
provides a systematic understanding of the FWM parameters necessary to enable
accurate lifetime extractions, which can be utilized in more complex quantum
systems such as molecules in the future.
