Does Current Data Prefer a Non-minimally Coupled Inflaton?
  We examine the impact of a non-minimal coupling of the inflaton to the Ricci
scalar, $\frac12 \xi R\phi^2$, on the inflationary predictions. Such a
non-minimal coupling is expected to be present in the inflaton Lagrangian on
fairly general grounds. As a case study, we focus on the simplest inflationary
model governed by the potential $V\propto \phi^2$, using the latest combined
2015 analysis of Planck and BICEP2/Keck Array. We find that the presence of a
coupling $\xi$ is favoured at a significance of $99\%$ CL, assuming that nature
has chosen the potential $V\propto \phi^2$ to generate the primordial
perturbations and a number of e-foldings $N=60$. Within the context of the same
scenario, we find that the value of $\xi$ is different from zero at the
$2\sigma$ level. When considering the cross-correlation polarization spectra
from BICEP2/Keck Array and Planck, a value of $r=0.038_{-0.030}^{+0.039}$ is
predicted in this particular non-minimally coupled scenario. Future
cosmological observations may therefore test these values of $r$ and verify or
falsify the non-minimally coupled model explored here.
