  We discuss a number of experimental constraints on Weinberg's Higgs portal
model. In this framework, the standard model (SM) particle spectrum is extended
to include one complex scalar field S and one Dirac fermion \psi. These new
fields are singlets under the SM gauge group and are charged under a global
U(1) symmetry. Breaking of this U(1) symmetry results in a massless Goldstone
boson \alpha and a massive CP-even scalar r, and splits the Dirac fermion into
two new mass-eigenstates \psi_\pm, corresponding to Majorana fermions. The
interest on such a minimal SM extension is twofold. On the one hand, if the
Goldstone bosons are in thermal equilibrium with SM particles until the era of
muon annihilation their contribution to the effective number of neutrino
species can explain the hints from cosmological observations of extra
relativistic degrees of freedom at the epoch of last scattering. On the other
hand, the lightest Majorana fermion \psi_- provides a plausible dark matter
candidate. Mixing of r with the Higgs doublet \phi is characterized by the mass
of hidden scalar m_h and the mixing angle \theta. We constrain this parameter
space using a variety of experimental data, including heavy meson decays with
missing energy, the invisible Higgs width, and direct dark matter searches. We
show that different experimental results compress the allowed parameter space
in complementary ways, covering a large range of \psi_- masses (5 GeV \alt m_-
\alt 100 GeV). Though current results narrow the parameter space significantly
(for the mass range of interest, \theta \alt 10^{-3} to 10^{-4}), there is
still room for discovery (\alpha decoupling at the muon annihilation era
requires \theta \agt 10^{-5} to 10^{-4}). In the near future, measurements from
ATLAS, CMS, LHCb, NA62, XENON1T, LUX, and CDMSlite will probe nearly the full
parameter space.
