Higgs-Dilaton cosmology: Are there extra relativistic species?
  Recent analyses of cosmological data suggest the presence of an extra
relativistic component beyond the Standard Model content. The Higgs-Dilaton
cosmological model predicts the existence of a massless particle -the dilaton-
associated with the spontaneous symmetry breaking of scale invariance and
undetectable by any accelerator experiment. Its ultrarelativistic character
makes it a suitable candidate for contributing to the effective number of light
degrees of freedom in the Universe. In this Letter we analyze the dilaton
production at the (p)reheating stage right after inflation and conclude that no
extra relativistic degrees of freedom beyond those already present in the
Standard Model are expected within the simplest Higgs-Dilaton scenario. The
elusive dilaton remains thus essentially undetectable by any particle physics
experiment or cosmological observation.
