The infrared behaviour in Nelson's model of a quantum particle coupled   to a massless scalar field
  We prove that Nelson's massless scalar field model is infrared divergent in three dimensions. In particular, the Nelson Hamiltonian and the Hamiltonian obtained from Euclidean quantization are not unitarily equivalent. In contrast, for dimensions higher than three the Nelson Hamiltonian has a unique ground state in Fock space and the two Hamiltonians are unitarily equivalent. We also show that the Euclidean Hamiltonian has no spectral gap. 
Jozsef Lorinczi, Robert A. Minlos and Herbert Spohn