Threshold Multiparticle Amplitudes in Phi^4 Theories at Large N
  I review some recent work on the problem of multiparticle production in a phi^4-theory with an O(N) symmetry. Threshold amplitudes with fixed number of produced particles are exactly calculated at large-N to all loops and vanish on mass shell for 2->n when n>2 due to a dynamical symmetry. I consider an extension to the cases when the O(N) symmetry is softly broken by masses including spontaneous breaking of a remaining reflection symmetry. The exact solutions are obtained by the Gelfand--Dikii technique of finding the diagonal resolvent of the Schrodinger operator which emerges due to factorization at large N. I report also some new results on the diagonal resolvent for a general Poschl--Teller potential, which could be useful for calculations of multiparticle amplitudes in the standard model. 
Yu. Makeenko