  We studied angular distributions of x rays emitted in resonant recombination
of highly charged iron and krypton ions, resolving dielectronic, trielectronic,
and quadruelectronic channels. A tunable electron beam drove these processes,
inducing x rays registered by two detectors mounted along and perpendicular to
the beam axis. The measured emission asymmetries comprehensively benchmarked
full-order atomic calculations. We conclude that accurate polarization
diagnostics of hot plasmas can only be obtained under the premise of inclusion
of higher-order processes that were neglected in earlier work.
