Effective potential at finite temperature in a constant magnetic field   I: Ring diagrams in a scalar theory
  We study symmetry restoration at finite temperature in the theory of a charged scalar field interacting with a constant, external magnetic field. We compute the finite temperature effective potential including the contribution from ring diagrams. We show that in the weak field case, the presence of the field produces a stronger first order phase transition and that the temperature for the onset of the transition is lower, as compared to the case without magnetic field. 
Alejandro Ayala (ICN-UNAM), Angel S\'anchez (ICN-UNAM), Gabriella   Piccinelli (CTA-UNAM), Sarira Sahu (ICN-UNAM)