Back to overview schedule of all parallel sessions

 

 Angus, Heterotic Cosmic String Vacua and Generalized Half-flat Manifolds, pdf

In heterotic string theory the presence of only NS-NS flux restricts the possibilities for moduli stabilization via flux compactification on a Calabi-Yau manifold. One promising avenue is then to consider more general manifolds with SU(3) structure, in which the intrinsic torsion plays the role of an additional flux component. This includes the so-called half-flat manifolds, which have previously been shown to allow 4d domain wall solutions, in which the transverse direction and six compact dimensions together admit a G2 structure. In this talk I will introduce an extension of this construction that leads to cosmic string vacua, and I will explore its relationship with generalized half-flat manifolds where the 2+6 dimensional transverse space admits a Spin(7) structure. 

 

Cicoli, Soft-terms in Sequestered dS Models, pdf

 

Ciupke, Higher Derivative Supergravity and Moduli Stabilization, pdf

We review the ghost-free four-derivative terms for chiral superfields in N = 1 supersymmetry and supergravity. These terms induce cubic polynomial equations of motion for the chiral auxiliary fields and correct the scalar potential. We discuss the different solutions and argue that only one of them is consistent with the principles of effective field theory. Special attention is paid to the corrections along flat directions which can be stabilized or destabilized by the higher-derivative terms. We then compute these higher-derivative terms explicitly for the type IIB string compactified on a Calabi-Yau orientifold from the leading order $R^4$ corrections in ten dimensions for the respective N = 1 Kähler moduli sector. We prove that provided the overall sign of the higher-derivative correction is negative and the Euler number of the threefold is positive this new correction together with the respective  corrections to the Kahler potential stabilizes all four-cycle volumes in a model-independent way.

 

Constantin, A Heterotic QCD Axion, pdf

We show that a KSVZ axion with a decay constant in the phenomenologically allowed range can be obtained in certain E8xE8 heterotic string models. These models have an enhanced symmetry locus in the moduli space, and a non-universal, Kahler moduli dependent Fayet-Iliopoulos term which vanishes at this locus. Close to this locus the Fayet-Iliopoulos term is small and can lead to an axion decay constant significantly lower than the string scale. In this way, the no-go arguments of Svrcek and Witten, which are based on a universal, dilaton-dependent Fayet-Iliopoulos term, can be avoided. The relevant axion originates from phases of bundle moduli which correspond to deformations away from the enhanced symmetry locus. We construct an explicit example, based on a heterotic line bundle standard model, with all the required ingredients.

 

Junghans, Large-Field Inflation with Multiple Axions and the Weak Gravity Conjecture, pdf

We discuss the implications of the weak gravity conjecture (WGC) for general models of large-field inflation with a large number of axions $N$. We first show that, from the bottom-up perspective, such models admit a variety of different regimes for the enhancement of the effective axion decay constant, depending on the amount of alignment and the number of instanton terms that contribute to the scalar potential. This includes regimes of no enhancement, power-law enhancement and exponential enhancement with respect to $N$. As special cases, we recover the Pythagorean enhancement of $N$-flation, the $N$ and $N^{3/2}$ enhancements derived by Bachlechner, Long and McAllister and the exponential enhancement by Choi, Kim and Yun. We then analyze which top-down constraints are put on such models from the requirement of consistency with quantum gravity. In particular, the strong version of the WGC implies that the enhancement of the effective axion decay constant must not grow parametrically with $N$ for $N \gg 1$. On the other hand, recent works proposed that axions might be able to  violate the conjecture under certain circumstances. Our general expression for the enhancement allows us to translate this possibility into a condition on the number of instantons that couple to the axions. We argue that, at large $N$, any model consistent with quantum gravity must either be able to violate the WGC bound or have an enormous, possibly even exponentially large, number of unsuppressed instanton terms in the scalar potential.

 

Kappl, Natural Inflation and Low Scale Supersymmetry, pdf

Natural (axionic) inflation provides a well-motivated and predictive scheme for the description of the early universe. It leads to sizeable primordial tensor modes and thus a high mass scale of the inflationary potential. Naively this seems to be at odds with low (TeV) scale supersymmetry, especially when embedded in superstring theory. I will outline that low scale supersymmetry is compatible with natural (high scale) inflation. The mechanism requires the presence of two axions that are provided through the moduli of string theory.

 

Marsh, Universal eigenvalue clustering in type IIB flux vacua 

Random matrix theory (RMT) may provide a very powerful tool for statistical studies of the flux landscape, primarily since, for large systems, the eigenvalue density of random matrix ensembles quickly tend to ‘universal’ limits that are largely independent of the statistical input. Reversely, the failure to reach these limits indicate the presence of competing, possibly ‘stringy’, features that remain relevant for generic, multi-parameter compactifications. In this talk, I will show that the perhaps most prominent matrix ensemble in the flux landscape is not well described by random matrix theory for any compactification geometry, number of complex structure moduli, or choice of flux, but still exhibits distinctive and universal features of string theory origin. I will comment on the possible implications for the statistics of type IIB flux vacua.

 

Martín Lozano, Dark Matter Phenomenology of intersecting D6 branes with a Stückelberg portal, pdf

In this work we study the phenomenological aspects of Stückelberg portals where the mediator between the Standard Model and the dark matter (DM) is a massive Z' boson. This scenario is well motivated by intersecting D6-brane constructions and naturally lead to isospin violating interactions of DM particles with nuclei. We show that in these kind of models the ratios between the DM coupling to neutrons and protons for both, spin-independent (fn/fp) and spin-dependent (an/ap) interactions are generically different from 1 (i.e. different couplings to protons and neutrons) leading to a potentially measurable distinction from other popular portals. We also perform a scan over all the parameters including bounds from searches for dijet and dilepton resonances at the LHC as well as LUX bounds on the elastic scattering of DM off nucleons to determine the experimentally allowed values of fn/fp and an/ap. We also obtain interesting phenomenological consequences for direct and indirect detection signals.

 

Montero, Transplanckian axions and the weak gravity conjecture, pdf

The weak gravity conjecture (WGC) has often been argued to hinder the obtention of transplanckian axion field ranges in any quantum theory of gravity. The WGC demands the existence of instantons of small action which couple to the axion and make the effective range subplanckian. Although the WGC only applies to a complete theory of gravity, one can build and study similar instantons in an effective theory. We discuss quantum gravitational effects in Einstein theory coupled to axion scalars to analyze the viability of several proposals to achieve superplanckian axion periods and their possible application to large field inflation models. These effects can be argued to generically spoil transplanckian field ranges in single axion models and multi-axion models with lattice alignment (like the Kim-Nilles-Peloso model). Finally, theories with N axions can still achieve a moderately superplanckian periodicity with no higher harmonics in the axion potential. In this case some of the instantons required by the WGC are suppressed by an approximate discrete $Z_N$ gauge symmetry. Finally we discuss the realization of these instantons as euclidean D-branes in string compactifications.

 

Muia, Dark Radiation in Sequestered String Models, pdf

 

Nelson,The Geometry of Generations, pdf

We present an intriguing and precise interplay between algebraic geometry and the phenomenology of generations of particles. Using the electroweak sector of the MSSM as a testing ground, we compute the moduli space of vacua as an algebraic variety for multiple generations of Standard Model matter and Higgs doublets. The space is shown to have Calabi-Yau, Grassmannian, and toric signatures which sensitively depend on the number of generations of leptons, as well as inclusion of Majorana mass terms for right-handed neutrinos. We speculate as to why three generations is special.

 

Otsuka, Natural inflation with and without modulations in string theory, pdf

We propose a mechanism for the natural inflation with and without modulation in the framework of type IIB string theory on toroidal orientifold or orbifold. We explicitly construct the stabilization potential of complex structure, dilaton and K\"ahler moduli, where one of the imaginary component of complex structure moduli becomes light which is identified as the inflaton. The inflaton potential is generated by the gaugino-condensation term which receives the one-loop threshold corrections determined by the field value of complex structure moduli and the axion decay constant of inflaton is enhanced by the inverse of one-loop factor. We also find the threshold corrections can also induce the modulations to the original scalar potential for the natural inflation. Depending on these modulations, we can predict several sizes of tensor-to-scalar ratio as well as the other cosmological observables reported by Planck collaboration.

 

Regalado, D-brane moduli stabilisation and linear equivalence, pdf

Linear equivalence is a criterion to compare submanifolds in the same homology class. We discuss its relevance in D-brane models and, in particular, in the stabilisation of D-brane moduli.

 

Ruehle, Natural inflation and moduli stabilization in heterotic orbifolds, pdf

We discuss moduli stabilization combined with inflation in heterotic orbifold setups. A trans-Planckian inflaton field range is implemented via aligned natural inflation. We argue that alignment is rather generic since the relevant non-perturbative effects are all governed by modular functions and are thus related. The proposed mechanism is illustrated using two examples based on Z6-II orbifold models: one with a gaugino condensate and worldsheet instantons and one with worldsheet instantons only.

 

Rummel, de Sitter Vacua from a D-term Generated Racetrack Uplift, pdf

We propose an uplift mechanism using a structure of multi-Kähler moduli dependence in the F-term potential of type IIB string theory compactifications. This mechanism requires a D-term condition that fixes one modulus to be proportional to another modulus, resulting in a trivial D-term potential. De Sitter minima are realized along with an enhancement of the volume in the Large Volume Scenario and no additional suppression of the uplift term such as warping is required. In particular, we discuss explicit Calabi-Yau constructions of this mechanism.

 

Soler, Natural inflation and the weak gravity conjecture, pdf

I will discuss implications that the Weak Gravity Conjecture has on models of natural inflation and the resulting difficulties in obtaining large axionic field ranges. Possible ways to overcome these difficulties will also be addressed.

 

Sousa, Stability of supersymmetric moduli in Kähler uplifted dS vacua, pdf

We study the metastability of complex structure moduli at flux vacua in the large volume regime of type IIB compactifications. Using techniques from Random Matrix Theory, we argue that the fraction of vacua where the masses of complex structure and Kahler moduli are separated by a large hierarchy is exponentially suppressed. We also show that when the volume is not exponentially large, as in Kahler uplifted dS vacua, the complex structure sector can be destabilised by the effect of supersymmetry breaking in the Kahler moduli sector. We present a new type of vacua for the complex structure sector where the mass spectrum presents a finite gap, without imposing a large mass hierarchy between moduli sectors. At these vacua the complex structure sector is protected from the appearance of tachyonic instabilities even at non-exponential volumes. Finally, we construct an explicit example of these vacua in the Calabi-Yau $P_{[11169]}^4$.

 

Staessens, A new look at the axion window through mixing axions, pdf

The low energy effective action for multiple axions is characterized by kinetic mixings effects among the axions, such as a non-diagonal axion metric and/or Stückelberg couplings. In this talk, we investigate the physical implications of the mixing effects on the axion decay constants and discuss how they allow to surpass the standard bounds on the decay constants. We indicate how these scenarios can be naturally implemented in string theory and discuss some explicit examples in Type IIA with intersecting D6-branes and Type IIB with intersecting D7-branes. 

 

Sumitomo, Accidental Kähler Moduli Inflation, pdf

We study an Accidental Kähler Moduli Inflation in type IIB string theory, where the inflation occurs near the inflection point of a Kähler modulus. The racetrack structure helps to alleviate the known concern that the string-loop corrections may spoil the Kähler Moduli Inflation without the significant dilaton suppression or a special setup. Also, the hierarchy of gauge ranks required to separate stabilization and inflation dynamics is well relaxed. The relaxation is more significant when we use the D-term generated racetrack model.

 

Svanes, Heterotic Superpotentials and Moduli, pdf

Heterotic supergravity is a very good candidate for a grand unified theory of the universe. Due to the presence of bundles, the standard model fits naturally within a heterotic framework. However, less is known about the geometrical structures and moduli that appear in heterotic compactifications then perhaps its type II cousins. I will discuss some recent developments in this directions, showing how the Strominger system can be put in terms of an integrable structure which can be used to compute moduli. I will also consider the four-dimensional theory, and derive the same structure form the Gukov-Vafa-Witten superpotential.

 

Witkowski, Evading the Weak Gravity Conjecture with F-term Winding Inflation?, pdf

I will present a new model of large field inflation along a winding trajectory in the field space of two axionic fields, where the 'axions' originate from the complex structure moduli sector of a Calabi-Yau 3-fold at large complex structure. The winding trajectory arises from fixing one combination of axions by bulk fluxes and allows for a transplanckian effective field range. The inflaton potential arises from small 'instantonic' corrections to the geometry and realises natural inflation. By working in a regime of large complex structure for two complex structure moduli the inflaton potential can be made subdominant without severe tuning. Interestingly, this construction is a candidate for a string theory model of large field inflation which is consistent with the mild form of the weak gravity conjecture for axions.