We discuss a new model of quintessence in which the quintessence field is identified with the extra component of a gauge field in a compactified five-dimensional theory. We show that the extremely tiny energy scale ϳ(3ϫ10 Ϫ3 eV) 4 needed to account for the present acceleration of the Universe can be naturally explained in terms of high energy scales such as the scale of grand unification.

We propose a mechanism for mediating supersymmetry breaking in Type I string constructions. in gaugino mediated models, the gauginos and Higgs doublets receive direct soft masses from the source brane, and flavour-changing neutral currents are naturally suppressed since the first and second family squarks and sleptons receive suppressed soft masses. However, unlike the gaugino mediated model, the third family squarks and sleptons receive unsuppressed soft masses, resulting in a very distinctive spectrum with heavier stops, sbottoms and staus.

We consider the problem of supersymmetry and electroweak breaking in a 5d theory compactified on an S 1 /Z 2 orbifold, where the extra dimension may be large or small. We consider the case of a supersymmetry breaking 4d brane located at one of the orbifold fixed points with the Standard Model gauge sector, third family and Higgs fields in the 5d bulk, and the first two families on a parallel 4d matter brane located at the other fixed point. We compute the Kaluza-Klein mass spectrum in this theory using a matrix technique which allows us to interpolate between large and small extra dimensions. We also consider the problem of electroweak symmetry breaking in this theory and localize the Yukawa couplings on the 4d matter brane spatially separated from the brane where supersymmetry is broken. We calculate the 1-loop effective potential using a zeta-function regularization technique, and find that the dominant top and stop contributions are separately finite. Using this result we find consistent electroweak symmetry breaking for a compactification scale 1/R ≈ 830 GeV and a lightest Higgs boson mass m h ≈ 170 GeV.

We discuss how localized twisted moduli in type I string theory can provide a string realization of brane world supersymmetry breaking models.

We make a first study of the phenomenological implications of twisted moduli in type I intersecting D5-brane models, focussing on the resulting predictions at the LHC using SOFTSUSY to estimate the Higgs and sparticle spectra. Twisted moduli can play an important role in giving a viable string realisation of sequestering in the limit where supersymmetry breaking comes entirely from the twisted moduli. We focus on a particular string inspired version of gaugino mediation in which the first two families are localised at the intersection between D5-branes, whereas the third family and Higgs doublets are allowed to move within the world-volume of one of the branes. The soft supersymmetry breaking third family sfermion mass terms are then in general non-degenerate with the first two families. We place constraints upon parameter space and predictions of flavour changing neutral current effects. Twisted moduli domination is studied and, as well as solving the most serious part of the SUSY flavour problem, is shown to be highly constrained. The constraints are weakened by switching on gravity-mediated contributions from the dilaton and untwisted T-moduli sectors. In the twisted moduli domination limit we predict a stopheavy MSSM spectrum and quasi-degenerate lightest neutralino and chargino states with wino-dominated mass eigenstates. A. Derivation of the SUSY breaking F-terms 28B. Phenomenological problems with "pure"gMSB 30 imental signatures at future colliders, and also non-standard gauge coupling unification at scales well below M X ∼ 2 × 10 16 GeV [5]. The Dirichlet branes are required for consistency, but provide a mechanism for localising matter and gauge fields (open string states) on a lower-dimensional slice of the full 10d spacetime in which gravity (closed strings) lives. These exciting new features have lead to a renaissance in string phenomenology, and renewed attempts to derive the (supersymmetric) Standard Model directly from a string theory compactification.Recent developments have also motivated the study of higher-dimensional field theories that may describe the low-energy limit of string theory. Many string-inspired ideas and techniques including branes, orbifolding and Kaluza-Klein towers are now commonly used in model-building [6]. These models are studied with effective field theory (EFT) techniques valid up to some ultraviolet (UV) cutoff scale Λ U V < M * , where the string-like features cannot be resolved. In particular, branes can be associated with orbifold fixed points rather than the string-theoretic picture of hypersurfaces where open strings end, and strings are treated as point-particles. Also Standard Model states can be allocated arbitrarily to different points/branes in the higher-dimensional space, and often orbifold symmetry assignments dictate whether a particular coupling is allowed or not [7].Many new models have been constructed to investigate quasi-realistic extensions of the (supersymmetric) Standard Model, where the extra-dimensional framework offers novel solutions to proble...