  New field content beyond that of the Standard Model of particle physics can
alter the thermal history of electroweak symmetry breaking in the early
universe. In particular, the symmetry breaking may have occurred through a
sequence of successive phase transitions. We study the thermodynamics of such
scenario in a real triplet extension of the Standard Model, using
nonperturbative lattice simulations. Two-step electroweak phase transition is
found to occur in a narrow region of allowed parameter space with the second
transition always being first order. The first transition into the phase of
non-vanishing triplet vacuum expectation value is first order in a
non-negligible portion of the two-step parameter space. A comparison with
2-loop perturbative calculation is provided and significant discrepancies with
the nonperturbative results are identified.
