Non-canonical Higgs inflation

The large value of non-minimal coupling constant xi required to satisfy cosmic microwave background observations in Higgs inflation violates unitarity. In this work we study Higgs-inflation with non-canonical kinetic term of DBI form to find whether xi can be reduced. To study the inflationary dynamics, we transform the action to the Einstein frame, in which the Higgs is minimally coupled to gravity with a non-canonical kinetic term and modified potential. We choose the Higgs self coupling constant lambda = 0.14 for our analysis. We find that the value of xi can be reduced from 103-104 to O(10) to satisfy Planck constraints on amplitude of scalar power spectrum. However, this model produces a larger tensor-to-scalar ratio r, in comparison to the Higgs inflation with canonical kinetic term. We also find that, to satisfy joint constraints on scalar spectral index ns and tensor-to-scalar ratio r from Planck-2018 and bounds on r from Planck and BICEP3, the value of xi should be of the order of 104. Thus, the issue of unitarity violation remains even after considering Higgs inflation with non-canonical kinetic term.