Quantum corrections to inflation: the importance of RG-running and choosing the optimal RG-scale

We demonstrate the importance of correctly implementing RG-running and choosing the RG-scale when calculating quantum corrections to inflaton dynamics. We show that such corrections are negligible for single-field inflation, in the sense of not altering the viable region in the $n_s-r$ plane, when imposing Planck constraints on $A_s$. Surprisingly, this also applies, in a nontrivial way, for an inflaton coupled to additional spectator degrees of freedom. The result relies on choosing the renormalisation scale (pseudo-)optimally, thereby avoiding unphysical large logarithmic corrections to the Friedmann equations and large running of the couplings. We find that the viable range of parameters of the potential is altered relative to the classical limit, and we find an upper limit of $g\simeq 10^{-4}$ on the value of the inflaton-spectator portal coupling still allowing for inflation. And an upper limit of $g\simeq 10^{-5}$ for inflation to correctly reproduce the scalar amplitude of fluctuations $A_s$.