Review on physics of Higgs-R 2 inflation.

Higgs inflation, identifying the Standard Model Higgs boson as the inflaton, causes a unitarity issue during the preheating epoch after inflation in its simplest form, calling for a UV completion. Higgs-R 2 inflation serves as a natural candidate for the UV completion of Higgs inflation, providing a consistent framework to describe the inflationary dynamics till the end of the reheating. Higgs-R 2 inflation contains a new scalar degree of freedom, the scalaron, which unitarizes the theory in the same way that the Higgs boson unitarizes the longitudinal electroweak gauge boson scatterings in the Standard Model. In this short review, we will summarize the physics of Higgs-R 2 inflation. A particular emphasis is put on its particle physics aspects, such as the unitarity structure, frame-independence, and the renormalization group evolution.Unitarity violation of Higgs inflationHiggs inflation identifies the unique elementary scalar field in the SM, the Higgs boson, as the inflaton, and is a simple possibility to explore. However, the simplest version of Higgs inflation, in which the Higgs boson has a non-minimal coupling, causes a unitarity issue during the preheating epoch. We therefore need a UV completion of Higgs inflation, and this motivates to consider Higgs-R 2 inflation.