First Search for Kaluza-Klein Gravitons and Radion Using Planck Data

Heavy moduli and Kaluza-Klein (KK) gravitons from extra dimensions may evade terrestrial probes but can be produced during inflation, generating primordial non-Gaussianity (NG) through unavoidable couplings to density perturbations. In a warped five-dimensional (5D) model, we compute the full radion- and KK-graviton-mediated bispectra and perform the first search for these signals using Planck 2018 temperature and polarization data. We find no significant evidence for NG, with the maximum significance being $1.8\sigma$ for $m_{\mathrm{K}\mathrm{K}}\approx 1.6H$. We also identify a 5D setup which naturally generates NG with $f_{\mathrm{N}\mathrm{L}}\sim 1-50$, within the reach of future surveys.