2018/11/30 by Soubhik Kumar, Raman Sundrum · 2 citations
Physics and Astronomy · #hep-ph #astro-ph.CO #gr-qc
paper · pdf · doi:10.1007/jhep04(2019)120
published as JHEP04(2019)120 · v1: 30+10 pages, 6 figures; v2: discussion added at the end of subsection 4.3, reference added, conclusions unchanged, journal version
arxiv created 2019/04/25 · arxiv updated 2019/04/26
The observed low-energy values of the SU(3)× SU(2)× U(1) gauge couplings, extrapolated via the minimal Standard Model Renormalization Group evolution, hint at the exciting possibility of a Grand Unified Theory (GUT) at MU ∼ 1014 GeV --- a scale, however, too high to probe directly via collider searches. Fortunately, since the Hubble scale H can be as high as 5 × 1013 GeV ∼ MU during the inflationary era, such GUT scale states can be cosmologically produced at that time and leave direct on-shell signatures such as their masses and spins, via primordial non-Gaussianity (NG). We explore this possibility in one of its simplest realizations given by the extra-dimensional framework of orbifold GUTs, in which proton decay can be straightforwardly suppressed to be within the stringent bounds. Here, along with the massive GUT states there must also be H-mass spin-2 Kaluza-Klein (KK) gravitons, collectively giving rise to striking NG signatures. In our set-up we localize the inflaton on one of the boundaries of an extra dimension. The inflationary vacuum energy can readily lead to formation of a horizon in the bulk, where the KK modes then form a continuum above a mass gap of ∼ O(H). We find that the optimal case for observable NG signals is when the extra dimension is stabilized close to the onset of this horizon, ensuring a discrete KK spectrum such that the lightest KK modes can be cosmologically produced without significant Boltzmann suppressions. Although we mostly focus on the case where there is no higher-dimensional cosmological constant, we also obtain considerable holographic insights from the AdS5/CFT4 correspondence when such a cosmological constant is included.