The Zero-Mass Limit Problem for a Relativistic Spinless Particle in an   Electromagnetic Field
  It is shown that mass-parameter-dependent solutions of the imaginary-time magnetic relativitstic Schr\"odinger equations converge as functionals of L\'evy processes represented by stochastic integrals of stationary Poisson point processes if mass-parameter goes to zero. 
Takashi Ichinose and Taro Murayama
math.PR