  In time-resolved photoemission experiments, more than one electron can be
emitted from the solid by a single ultra-short pulse. We theoretically
demonstrate how correlations between the momenta of outgoing electrons relate
to time-dependent two-particle correlations in the solid. This can extend the
scope of time- and angular-resolved photoemission spectroscopy to probe
superconducting and charge density fluctuations in systems without long-range
order, and to reveal their dynamics independent of the electronic gap and thus
unrestricted by the energy-time uncertainty. The proposal is illustrated for
superconductivity in a BCS model. An impulsive perturbation can quench the gap
on ultrafast timescales, while non-equilibrium pairing correlations persist
much longer, even when electron-electron scattering beyond mean-field theory is
taken into account. There is thus a clear distinction between a dephasing of
the Cooper pairs and the thermalization into the normal state. While a
measurement of the gap would be blind to such pairing correlations, they can be
revealed by the angular correlations in photoemission.

Strongly Correlated Electrons