The splitting processes of bremsstrahlung and pair production in a medium are
coherent over large distances in the very high energy limit, which leads to a
suppression known as the Landau-Pomeranchuk-Migdal (LPM) effect. We continue
study of the case when the coherence lengths of two consecutive splitting
processes overlap (which is important for understanding corrections to standard
treatments of the LPM effect in QCD), avoiding soft-emission approximations. In
this particular paper, we show (i) how the "instantaneous" interactions of
Light-Cone Perturbation Theory must be included in the calculation to account
for effects of longitudinally-polarized gauge bosons in intermediate states,
and (ii) how to compute virtual corrections to LPM emission rates, which will
be necessary in order to make infrared-safe calculations of the characteristics
of in-medium QCD showering of high-energy partons. In order to develop these
topics in as simple a context as possible, we will focus in the current paper
not on QCD but on large-Nf​ QED, where Nf​ is the number of electron
flavors.Comment: 43 pages + appendices for 89 pages total; 43 figures. Difference from
v2: Overall sign of eqs. (F30,F33,F39,F42) fixed; correction to eq. (H14);
final results of paper are unchange
