We propose a simple, well-motivated and robust alternative to a metastable de Sitter vacuum in string theory, consistent with current observations of dark energy and naturally satisfying conjectured swampland constraints. Inflation ends in a supersymmetric Minkowski minimum, with a flat direction that is protected by non-renormalisation theorems. At some scale non-perturbative effects kick in, inducing a runaway scalar potential. The tail of this runaway potential cannot sustain a late-time dominating, slow-roll quintessence. However, the potential always contains a dS maximum. If the modulus starts close to the hilltop, it remains frozen there by Hubble friction for much of the cosmological history, at first sourcing a tiny classical vacuum energy and then constituting a rolling quintessence, with observable consequences. So long as the modulus is localised away from the Standard Model in the extra dimensions, there are no observable fifth forces nor is there time-variation of fundamental constants, and the modulus mass is protected from radiative corrections. We revisit concrete string models based on heterotic orbifold compactifications, and show that their de Sitter extrema satisfy the recently refined dS Swampland Conjecture. arXiv:1810.08634v2 [hep-th]
Discovering a selection principle and the origin of flavor symmetries from an ultraviolet completion of particle physics is an interesting open task. As a step in this direction, we classify all possible flavor symmetries of 4D massless spectra emerging from supersymmetric Abelian orbifold compactifications, including roto-translations and non-factorizable tori, for generic moduli values. Although these symmetries are valid in all string theories, we focus on the E 8 ×E 8 heterotic string. We perform the widest known search of E 8 ×E 8 Abelian orbifold compactifications, yielding over 121,000 models with MSSM-like features. About 75.4% of these models exhibit flavor symmetries containing D 4 factors and only nearly 1.2% have ∆(54) factors. The remaining models are furnished with purely Abelian flavor symmetries. Our findings suggest that, should particle phenomenology arise from such a heterotic orbifold, it could accommodate only one of these flavor symmetries.
Aiming at improving our knowledge of the low-energy limit of heterotic orbifold compactifications, we determine at lowest order the Kähler potential of matter fields in the case where more than three bulk Kähler moduli appear. Interestingly, bulk matter fields couple to more than one Kähler modulus, a subtle difference with models with only three Kähler moduli that may provide a tool to address the question of moduli stabilization in these models.1 See e.g. [1,2,3,4] for details on heterotic orbifold compactifications in the bosonic formulation. 2 Yet, despite the great progress on coupling selection rules [13,14,15,16], there remain some details to clarify.
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