Supersymmetric Extra Dimensions Research Articles

Light axiodilatons: matter couplings, weak-scale completions and long-distance tests of gravity

We consider the physical implications of very light axiodilatons motivated by a novel mechanism to substantially reduce the vacuum energy proposed in 2110.10352. We address the two main problems concerning the light axiodilaton that appears in the low-energy limit, namely that the axion has a very low decay constant fa ∼ eV (as read from its kinetic term) and that the dilaton is subject to bounds that are relevant to tests of GR once ρ vac ≲ 10-80 Mp 4. We show that eV scale axion decay constants need not be a problem by showing how supersymmetric extra dimensions provide a sample unitarization for axion physics above eV scales for which non-anomalous matter/axiodilaton couplings can really have gravitational strength, showing how naive EFT reasoning can mistakenly overestimates axion interactions at eV. When axions really do couple strongly at eV scales we identify the dimensionless interaction in the UV completion that is also O(1), and how axion energy-loss bounds map onto known extra-dimensional constraints. We find a broad new class of exact exterior solutions to the vacuum axiodilaton equations and knowledge of axiodilaton-matter couplings also allows us to numerically search for interior solutions that match to known exterior solutions that can evade solar-system tests. We find no examples that do so, but also identify potential new candidate mechanisms for reducing the effective dilaton-matter coupling to gravitating objects without also undermining the underlying suppression of ρ vac.

Fine-tuning with brane-localized flux in 6D supergravity

There are claims in the literature that the cosmological constant problem could be solved in a braneworld model with two large (micron-sized) supersymmetric extra dimensions. The mechanism relies on two basic ingredients: First, the cosmological constant only curves the compact bulk geometry into a rugby shape while the 4D curvature stays flat. Second, a brane-localized flux term is introduced in order to circumvent Weinberg's fine-tuning argument, which otherwise enters here through a backdoor via the flux quantization condition. In this paper, we show that the latter mechanism does not work in the way it was designed: The only localized flux coupling that guarantees a flat on-brane geometry is one which preserves the scale invariance of the bulk theory. Consequently, Weinberg's argument applies, making a fine-tuning necessary again. The only remaining window of opportunity lies within scale invariance breaking brane couplings, for which the tuning could be avoided. Whether the corresponding 4D curvature could be kept under control and in agreement with the observed value will be answered in our companion paper [arXiv:1512.03800].

Technically natural cosmological constant from supersymmetric 6D brane backreaction

Abstract We provide an explicit example of a higher-dimensional model describing a non-supersymmetric spectrum of 4D particles of mass M, whose 4D geometry – including loop effects – has a curvature that is of order R ∼ m K K 4 / M p 2 , where m K K is the extra-dimensional Kaluza–Klein scale and Mp is the 4D Planck constant. m K K is stabilized and can in particular satisfy m K K ≪ M . The system consists of a (5+1)-dimensional model with a flux-stabilized supersymmetric bulk coupled to non-supersymmetric matter localized on a (3+1)-dimensional positive-tension brane. We use recent techniques for calculating how extra dimensions respond to changes in brane properties to show (at the classical level) that the extra-dimensional volume adjusts to ensure that the low-energy 4D geometry is exactly flat, independent of the value of the brane tensions. Its mechanism for doing so is the transfer of stabilizing flux between the bulk and the branes. The UV completion of the model can arise at scales much larger than M, allowing the calculation of quantum effects like the zero-point energy of very massive particles in the vacuum. We find that brane-localized loops do not affect the 4D curvature at all, but bulk loops can. These can be estimated on general grounds and we show that supersymmetry dictates that they generate curvatures that are generically of order m K K 4 / M p 2 . For realistic applications this points to a world with two supersymmetric extra dimensions, with supersymmetry in the bulk broken at the sub-eV KK scale – as proposed in hep-th/0304256 – requiring a 6D gravity scale somewhat higher than 10 TeV. Ordinary Standard Model particles are brane-localized and not at all supersymmetric (implying in particular no superpartners or the MSSM). We discuss how the model evades various no-go theorems that would naively exclude it, and briefly outline several striking observational implications for tests of gravity and at the LHC.

Über-naturalness: unexpectedly light scalars from supersymmetric extra dimensions

Standard lore asserts that quantum effects generically forbid the occurrence of light (non-pseudo-Goldstone) scalars having masses smaller than the Kaluza Klein scale, M_KK, in extra-dimensional models, or the gravitino mass, M_3/2, in supersymmetric situations. We argue that a hidden assumption underlies this lore: that the scale of gravitational physics, M_g, (e.g. the string scale, M_s, in string theory) is of order the Planck mass, M_p = 10^18 GeV. We explore sensitivity to this assumption using the spectrum of masses arising within the specific framework of large-volume string compactifications, for which the ultraviolet completion at the gravity scale is explicitly known to be a Type IIB string theory. In such models the separation between M_g and M_p is parameterized by the (large) size of the extra dimensional volume, V (in string units), according to M_p: M_g: M_KK: M_3/2 = 1: V^{-1/2}: V^{-2/3}: V^{-1}. We find that the generic size of quantum corrections to masses is of the order of M_KK M_3/2 / M_p ~ M_p / V^{5/3}. The mass of the lighest modulus (corresponding to the extra-dimensional volume) which at the classical level is M_V ~ M_p/V^{3/2} << M_3/2 << M_KK is thus stable against quantum corrections. This is possible because the couplings of this modulus to other forms of matter in the low-energy theory are generically weaker than gravitational strength (something that is also usually thought not to occur according to standard lore). We discuss some phenomenological and cosmological implications of this observation.

Supersymmetric extra dimensions: Gravitino effects in selectron pair production

We examine the phenomenological consequences of a supersymmetric bulk in the scenario of large extra dimensions. We assume that supersymmetry is realized in the bulk and study the interactions of the resulting bulk gravitino Kaluza-Klein (KK) tower of states, with supersymmetry breaking on the brane inducing a light mass for the zero-mode gravitino. We derive the 4D effective theory, including the couplings of the bulk gravitino KK states to fermions and their scalar superpartners. The virtual exchange of the gravitino KK states in selectron pair production in polarized ${e}^{+}{e}^{\ensuremath{-}}$ collisions is then examined. We find that the leading order operator for this exchange is dimension 6, in contrast with that of bulk graviton KK exchange which induces a dimension 8 operator at lowest order. The resulting kinematic distributions for selectron production are dramatically altered from those in $D=4$ supersymmetric scenarios and can lead to an enormous sensitivity to the fundamental higher-dimensional Planck scale, of order $(20--25)\ifmmode\times\else\texttimes\fi{}\sqrt{s}.$

4D constructions of supersymmetric extra dimensions and gaugino mediation

We present 4D gauge theories which at low energies coincide with higher dimensional supersymmetric (SUSY) gauge theories on a transverse lattice. We show that in the simplest case of pure 5D SUSY Yang-Mills there is an enhancement of SUSY in the continuum limit without fine-tuning. This result no longer holds in the presence of matter fields, in which case fine-tuning is necessary to ensure higher dimensional Lorentz invariance and supersymmetry. We use this construction to generate 4D models which mimic gaugino mediation of SUSY breaking. The way supersymmetry breaking is mediated in these models to the MSSM is by assuming that the physical gauginos are a mixture of a number of gauge eigenstate gauginos: one of these couples to the SUSY breaking sector, while another couples to the MSSM matter fields. The lattice can be as coarse as just two gauge groups while still obtaining the characteristic gaugino-mediated soft breaking terms.