Relative energy dependence of the Bethe-Salpeter amplitude in   two-dimensional massless QED
  The exact $q\bar{q}$ Bethe-Salpeter bound state amplitude is investigated in the space of relative energy $E$ for fixed value of relative position. By means of approximate analysis it is shown to possess singularities in $E$ whenever one of the quarks reaches the energy threshold for the creation of certain number of Schwinger bosons. These results are afterwards confirmed by numerical plots. 
Tomasz Radozycki
hep-th