
We examine the impact of a nonminimal coupling of the inflaton to the Ricci scalar, 1/2 xi R phi(2), on the inflationary predictions. Such a nonminimal 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 proportional to phi(2), using the latest combined 2015 analysis of Planck and the BICEP2/Keck Array. We find that the presence of a coupling xi is favored at a significance of 99% C.L., assuming that nature has chosen the potential V proportional to phi(2) to generate the primordial perturbations and a number of efoldings 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 crosscorrelation polarization spectra from the BICEP2/Keck Array and Planck, a value of r = 0.038(0.030)(+0.039) is predicted in this particular nonminimally coupled scenario. Future cosmological observations may therefore test these values of r and verify or falsify the nonminimally coupled model explored here.
