Abstract:We study closed string axions in type IIB orientifold compactifications. We show that for natural values of the background fluxes the moduli stabilisation mechanism of the LARGE Volume Scenario (LVS) gives rise to an axiverse characterised by the presence of a QCD axion plus many light axion-like particles whose masses are logarithmically hierarchical. We study the phenomenological features of the LVS axiverse, deriving the masses of the axions and their couplings to matter and gauge fields. We also determine when closed string axions can solve the strong CP problem, and analyse the first explicit examples of semi-realistic models with stable moduli and a QCD axion candidate which is not eaten by an anomalous Abelian gauge boson. We discuss the impact of the choice of inflationary scenario on the LVS axiverse, and summarise the astrophysical, cosmological and experimental constraints upon it. Moreover, we show how models can be constructed with additional light axion-like particles that could explain some intriguing astrophysical anomalies, and could be searched for in the next generation of axion helioscopes and lightshining-through-a-wall experiments.

We introduce a simple string model of inflation, in which the inflaton field can take trans-Planckian values while driving a period of slow-roll inflation. This leads naturally to a realisation of large field inflation, inasmuch as the inflationary epoch is well described by the single-field scalar potential V = V 0 3 − 4e −φ/ √ 3 . Remarkably, for a broad class of vacua all adjustable parameters enter only through the overall coefficient V 0 , and in particular do not enter into the slow-roll parameters. Consequently these are determined purely by the number of e -foldings, N e , and so are not independent: ε ≃ 3 2 η 2 . This implies similar relations among observables like the primordial scalar-to-tensor amplitude, r, and the scalar spectral tilt, n s : r ≃ 6(n s − 1) 2 . N e is itself more model-dependent since it depends partly on the post-inflationary reheat history. In a simple reheating scenario a reheating temperature of T rh ≃ 10 9 GeV gives N e ≃ 58, corresponding to n s ≃ 0.970 and r ≃ 0.005, within reach of future observations. The model is an example of a class that arises naturally in the context of type IIB string compactifications with large-volume moduli stabilisation, and takes advantage of the generic existence there of Kähler moduli whose dominant appearance in the scalar potential arises from string loop corrections to the Kähler potential. The inflaton field is a combination of Kähler moduli of a K3-fibered Calabi-Yau manifold. We believe there are likely to be a great number of models in this class -'high-fibre models' -in which the inflaton starts off far enough up the fibre to produce observably large primordial gravity waves.

We study the behaviour of the string loop corrections to the N = 1 4D supergravity Kähler potential that occur in flux compactifications of IIB string theory on general Calabi-Yau three-folds. We give a low energy interpretation for the conjecture of Berg, Haack and Pajer for the form of the loop corrections to the Kähler potential. We check the consistency of this interpretation in several examples. We show that for arbitrary Calabi-Yaus, the leading contribution of these corrections to the scalar potential is always vanishing, giving an "extended no-scale structure". This result holds as long as the corrections are homogeneous functions of degree −2 in the 2-cycle volumes. We use the Coleman-Weinberg potential to motivate this cancellation from the viewpoint of low-energy field theory. Finally we give a simple formula for the 1-loop correction to the scalar potential in terms of the tree-level Kähler metric and the conjectured correction to the Kähler potential. We illustrate our ideas with several examples. A companion paper will use these results in the study of Kähler moduli stabilisation.implies that the Kähler potential can receive corrections to all orders in the α ′ and g s expansions. The presence of no-scale structure in the Kähler potential makes understanding the Kähler corrections particularly pressing, as it is the corrections that give rise to the leading perturbative terms in the scalar potential.Mirror symmetry and the underlying N = 2 structure was used to extract the leading order α ′ corrections [4]. Explicit string amplitude calculations to determine the loop corrections to K are not available for general fluxed Calabi-Yau compactifications, and only simple unfluxed toroidal orientifold cases have been used for concrete computations [5].Despite the difficulty of explicitly computing loop corrections in general Calabi-Yau flux backgrounds, given their importance it is necessary to try and go as far as possible. In this respect we observe that there is an easier and a harder part to computing the form of loop corrections. The easier part involves the parametric scaling of moduli that control the loop expansion -in IIB these are the dilaton, which controls the string coupling, and the Kähler moduli, which controls the gauge coupling on D7 branes. The harder part involves the actual coefficients of the loop expansion, which depend on the complex structure moduli and would require a explicit string computation. This article focuses entirely on the 'easier' part; however as the Kähler moduli are unstabilised at tree-level, such knowledge is very important for moduli stabilisation.Recently, Berg, Haack and Pajer (BHP) [6] gave arguments for the general functional dependence of the leading order loop corrections to K on the Kähler moduli. By comparing with the toroidal orientifold calculations and the standard transformations required to go from the string frame, where string amplitudes are computed, to the physical Einstein frame that enters the supergravity action, they conjectured the parametric ...

We study the topological conditions for general Calabi-Yaus to get a nonsupersymmetric AdS exponentially large volume minimum of the scalar potential in flux compactifications of IIB string theory. We show that negative Euler number and the existence of at least one blow-up mode resolving point-like singularities are necessary and sufficient conditions for moduli stabilisation with exponentially large volumes. We also analyse the general effects of string loop corrections on this scenario. While the combination of α ′ and nonperturbative corrections are sufficient to stabilise blow-up modes and the overall volume, quantum corrections are needed to stabilise other directions transverse to the overall volume. This allows exponentially large volume minima to be realised for fibration Calabi-Yaus, with the various moduli of the fibration all being stabilised at exponentially large values. String loop corrections may also play a role in stabilising 4-cycles which support chiral matter and cannot enter directly into the non-perturbative superpotential. We illustrate these ideas by studying the scalar potential for various Calabi-Yau three-folds including K3 fibrations and briefly discuss the potential phenomenological and cosmological implications of our results.