Scale invariant extension of the Standard Model: a nightmare scenario in cosmology

Abstract Inflationary observables of a classically scale invariant model, in which the origin of the Planck mass and the electroweak scale including the right-handed neutrino mass is chiral symmetry breaking in a QCD-like hidden sector, are studied. Despite a three-field inflation the initial-value-dependence is strongly suppressed thanks to a river-valley like potential. The model predicts the tensor-to-scalar ratio r of cosmological perturbations smaller than that of the R 2 inflation, i.e., 0.0044 ≳ r ≳ 0.0017 for e-foldings between 50 and 60: the model will be consistent even with a null detection at LiteBird/CMB-S4. We find that the non-Gaussianity parameter f NL is O (10 -2 ), the same size as that of single-field inflation. The dark matter particles are the lightest Nambu-Goldstone bosons associated with chiral symmetry breaking, which are decay products of one of the inflatons and are heavier than 10 9 GeV with a strongly suppressed coupling with the standard model, implying that the dark matter will be unobservable in direct as well as indirect measurements.