  We have directly detected millimeter wave (mm-wave) free space superradiant
emission from Rydberg states ($n \sim 30$) of barium atoms in a single shot. We
trigger the cooperative effects with a weak initial pulse and detect with
single-shot sensitivity and 20 ps time resolution, which allows measurement and
shot-by-shot analysis of the distribution of decay rates, time delays, and
time-dependent frequency shifts. Cooperative line shifts and decay rates are
observed that exceed values that would correspond to the Doppler width of 250
kHz by a factor of 20 and the spontaneous emission rate of 50 Hz by a factor of
$10^5$. The initial superradiant output pulse is followed by evolution of the
radiation-coupled many-body system toward complex long-lasting emission modes.
A comparison to a mean-field theory is presented which reproduces the
quantitative time-domain results, but fails to account for either the
frequency-domain observations or the long-lived features.
