Threshold expansion of Feynman diagrams within a configuration space technique

The near threshold expansion of generalized sunset-type (water melon) diagr ams with arbitrary masses is constructed by using a configuration space tec hnique. We present analytical expressions for the expansion of the spectral density near threshold and compare it with the exact expression obtained e arlier using the method of the Hankel transform. We formulate a generalized threshold expansion with partial resummation of the small mass corrections for the strongly asymmetric case where one particle in the intermediate st ate is much lighter than the others. (C) 2000 Elsevier Science B.V. All rig hts reserved.