Phase Transitions in Twin Higgs Models
  We study twin Higgs models at non-zero temperature and discuss cosmological
phase transitions as well as their implications on electroweak baryogenesis and
gravitational waves. It is shown that the expectation value of the Higgs field
at the critical temperature of the electroweak phase transition is much smaller
than the critical temperature, which indicates two important facts: (i) the
electroweak phase transition cannot be analyzed perturbatively (ii) the
electroweak baryogenesis is hardly realized in the typical realizations of twin
Higgs models. We also analyze the phase transition associated with the global
symmetry breaking, through which the Standard Model Higgs is identified with
one of the pseudo-Nambu-Goldstone bosons in terms of its linear realization,
with and without supersymmetry. For this phase transition, we show that, only
in the supersymmetric case, there are still some parameter spaces, in which the
perturbative approach is validated and the phase transition is the first order.
We find that the stochastic gravitational wave background is generated through
this first order phase transition, but it is impossible to be detected by
DECIGO or BBO in the linear realization and the decoupling limit. The detection
of stochastic gravitational wave background with the feature of first order
phase transition, therefore, will give strong constraints on twin Higgs models.
