CALCULATION OF INFRARED-DIVERGENT FEYNMAN DIAGRAMS WITH ZERO MASS THRESHOLD

Two-loop vertex Feynman diagrams with infrared and collinear divergenc es are investigated by two independent methods. On the one hand, a met hod of calculating Feynman diagrams from their small momentum expansio n [1] extended to diagrams with zero mass thresholds [2] is applied. O n the other hand, a numerical method based on a two-fold integral repr esentation is used [3], [4]. The application of the latter method is p ossible by using lightcone coordinates in the parallel space. The nume rical data obtained with the two methods are in impressive agreement.