Seeing Higher-Dimensional Grand Unification In Primordial Non-Gaussianities. (arXiv:1811.11200v1 [hep-ph])
<a href="http://arxiv.org/find/hep-ph/1/au:+Kumar_S/0/1/0/all/0/1">Soubhik Kumar</a>, <a href="http://arxiv.org/find/hep-ph/1/au:+Sundrum_R/0/1/0/all/0/1">Raman Sundrum</a>
The observed low-energy values of the SU(3)x SU(2)x 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 M_U sim
10^14 GeV — a scale, however, too high to probe directly via collider
searches. Fortunately, since the Hubble scale H can be as high as 5 x 10^13 GeV
sim M_U 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 AdS_5/CFT_4 correspondence when such
a cosmological constant is included.
The observed low-energy values of the SU(3)x SU(2)x 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 M_U sim
10^14 GeV — a scale, however, too high to probe directly via collider
searches. Fortunately, since the Hubble scale H can be as high as 5 x 10^13 GeV
sim M_U 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 AdS_5/CFT_4 correspondence when such
a cosmological constant is included.
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