A generalized Nieh-Yan coupling in metric-affine gravity can restore the consistency of Palatini inflation with CMB observations and produce testable tensor-to-scalar ratios.
Minimal but non-minimal inflation and electroweak symmetry breaking
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abstract
We consider the most minimal scale invariant extension of the standard model that allows for successful radiative electroweak symmetry breaking and inflation. The framework involves an extra scalar singlet, that plays the r\^ole of the inflaton, and is compatibile with current experimental bounds owing to the non-minimal coupling of the latter to gravity. This inflationary scenario predicts a very low tensor-to-scalar ratio $r \approx 10^{-3}$, typical of Higgs-inflation models, but in contrast yields a scalar spectral index $n_s \simeq 0.97$ which departs from the Starobinsky limit. We briefly discuss the collider phenomenology of the framework.
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Inflation with Nieh-Yan-like terms in metric-affine gravity
A generalized Nieh-Yan coupling in metric-affine gravity can restore the consistency of Palatini inflation with CMB observations and produce testable tensor-to-scalar ratios.