  In the study of the strongly correlated electrons, one of the challenging
core tasks is to develop the potential techniques for direct detection of the
many-body correlations of the strongly correlated electrons. $(\gamma, 2e)$
photoemission technique has been developed to investigate the two-body
correlations of the target correlated electrons. In this article, we will focus
on this technique for the correlated electrons near the Fermi energy. The
coincidence detection probability of the two emitted electrons in the $(\gamma,
2e)$ photoemission measurement is shown to be relevant to a two-body
Bethe-Salpeter wave function, which describes the dynamical two-body
correlations of the target correlated electrons. As the coincidence detection
probability involves an electron-electron interaction matrix element, the
arbitrary momentum and/or energy transfer due to this electron-electron
interaction makes the $(\gamma, 2e)$ photoemission technique fail to reveal the
inner-pair structure of the two-body Bethe-Salpeter wave function. However, the
center-of-mass momentum and energy of the two-body Bethe-Salpeter wave function
can be distinctly resolved. Thus, the $(\gamma, 2e)$ photoemission technique
can provide the center-of-mass physics of the two-body Bethe-Salpeter wave
function of the target correlated electrons. It would be one potential
technique to study the center-of-mass physics of the Cooper pairs in
superconductor.
