Effective potential at finite temperature in a constant hypermagnetic   field: Ring diagrams in the Standard Model
  We study the symmetry breaking phenomenon in the standard model during the electroweak phase transition in the presence of a constant hypermagnetic field. We compute the finite temperature effective potential up to the contribution of ring diagrams in the weak field, high temperature limit and show that under these conditions, the phase transition becomes stronger first order. 
Angel Sanchez, Alejandro Ayala and Gabriella Piccinelli