Superstring-inspired particle cosmology: inflation, neutrino masses, leptogenesis, dark matter & the SUSY scale

Abstract

We develop a string-inspired model for particle cosmology, based on a flipped SU(5)×U(1) gauge group formulated in a no-scale supergravity framework. The model realizes Starobinsky-like inflation, which we assume to be followed by strong reheating, with the GUT symmetry being broken subsequently by a light 'flaton' field whose decay generates a second stage of reheating. We discuss the production of gravitinos and the non-thermal contribution made by their decays to the density of cold dark matter, which is assumed to be provided by the lightest neutralino. We also discuss the masses of light and heavy neutrinos and leptogenesis. As discussed previously [1], a key rôle is played by a superpotential coupling between the inflaton, matter and GUT Higgs fields, called λ 6 . We scan over possible values of λ 6 , exploring the correlations between the possible values of observables. We emphasize that the release of entropy during the GUT transition allows large regions of supersymmetry-breaking parameter space that would otherwise lead to severe overdensity of dark matter. Furthermore, we find that the Big Bang nucleosynthesis lower limit on the reheating temperature of ∼ 1 MeV restricts the supersymmetry-breaking scale to a range O(10) TeV that is consistent with the absence of supersymmetric particles at the LHC.

Proton decay: flipped vs. unflipped SU(5)

“…As discussed in detail in refs. [49][50][51][52], the λ 6 coupling in this model controls i) inflaton decays and reheating; ii) the gravitino production rate and therefore the non-thermal abundance of the LSP; iii) neutrino masses; and iv) the baryon asymmetry of the Universe via leptogenesis [82]. In particular, we showed in refs.…”

“…It is therefore important to assemble a kit of diagnostic tools that the upcoming experiments can use to discriminate between the flipped and unflipped SU(5) GUT models. 1 This issue has been discussed previously [68][69][70][71][72][73][74], and the purpose of this paper is to update the available diagnostic kit in the framework of the unified cosmological framework that we have proposed previously [49][50][51][52], stressing the connection between the flavour structure of nucleon decay operators and the pattern of mixing between neutrinos and their mass ordering.…”

“…Nevertheless, the proton decay issue has added to the motivations for considering the supersymmetric flipped SU(5) GUT [37][38][39][40][41][42][43], in which an economical missing-partner mechanism [40,[44][45][46] suppresses dimension-5 proton decay. This model is also of interest because it can easily be accommodated within string theory [41-43, 47, 48], and a unified cosmological scenario for inflation, dark matter, neutrino masses and baryogenesis has been constructed [49][50][51][52] in the combined framework of flipped SU(5) and string-motivated [53] no-scale supergravity [54][55][56].…”

“…We identify two primary proton decay signatures of the no-scale flipped SU(5) model [49][50][51][52] that may also cast light on the mass-ordering of light neutrinos. One signature is the ratio Γ(p → π 0 µ + )/Γ(p → π 0 e + ), and the other is Γ(p → π +ν )/Γ(p → π 0 e + ).…”

“…In the no-scale flipped SU(5) × U(1) GUT model [37][38][39][40][41][42][43][49][50][51][52], the three generations of the minimal supersymmetric extension of the Standard Model (MSSM) matter fields are embedded, together with three right-handed singlet neutrino chiral superfields, into three sets of 10,5, and 1 representations of SU(5), which we denote by F i ,f i and c i , respectively, where i = 1, 2, 3 is the generation index. In units of 1/ √ 40, the U(1) charges of the F i ,f i and c i are +1, −3, and +5, respectively.…”