Arthur Hebecker

D7-Brane Chaotic Inflation

Abstract We analyze string-theoretic large-field inflation in the regime of spontaneously-broken supergravity with conventional moduli stabilization by fluxes and non-perturbative effects. The main ingredient is a shift-symmetric Kahler potential, supplemented by flux-induced shift symmetry breaking in the superpotential. The central technical observation is that all these features are present for D7-brane position moduli in Type IIB orientifolds, potentially allowing for a realization of the axion monodromy proposal in a string theory compactification. Furthermore, our model is explicit enough to address issues of control and moduli stabilization quantitatively. On the one hand, in the large complex structure regime the D7-brane position moduli inherit a shift symmetry from their mirror-dual Type IIA Wilson lines. On the other hand, the Type IIB flux superpotential generically breaks this shift symmetry and allows, by appealing to the large flux discretuum, to tune the relevant coefficients to be small. The shift-symmetric direction in D7-brane moduli space can then play the role of the inflaton: While the D7-brane circles a certain trajectory on the Calabi–Yau many times, the corresponding F -term energy density grows only very slowly, thanks to the above-mentioned tuning of the flux. To be successful our model requires that the dilaton, all complex structure moduli and all D7-brane moduli except the inflaton are fixed at leading order by fluxes. Then the large-field inflationary trajectory can be realized in a regime where Kahler, complex structure and other brane moduli are stabilized in a conventional manner, as we demonstrate using the example of the Large Volume Scenario.

Kähler corrections for the volume modulus of flux compactifications

Abstract No-scale models arise in many compactifications of string theory and supergravity, the most prominent recent example being type IIB flux compactifications. Focussing on the case where the no-scale field is a single unstabilized volume modulus (radion), we analyse the general form of supergravity loop corrections that affect the no-scale structure of the Kahler potential. These corrections contribute to the 4d scalar potential of the radion in a way that is similar to the Casimir effect. We discuss the interplay of this loop effect with string-theoretic α ′ corrections and its possible role in the stabilization of the radion.

Finite temperature effective potential to order g4, λ2 and the electroweak phase transition

The standard model effective potential is calculated at finite temperature to order g4, λ2 and a complete zero temperature renormalization is performed. In comparison with lower order calculations the strength of the first order phase transition has increased dramatically. This effect can be traced back to infrared contributions from typical non-Abelian diagrams and to the infrared behaviour of the scalar sector close to the critical temperature. Several quantities, e.g. surface tension, latent heat and field expectation value are analysed for an SU(2)-Higgs model and for the full standard model in detail. An explicit formula enabling further analytic or numerical study is presented.

Gauge unification in highly anisotropic string compactifications

Abstract It is well known that heterotic string compactifications have, in spite of their conceptual simplicity and aesthetic appeal, a serious problem with precision gauge coupling unification in the perturbative regime of string theory. Using both a duality-based and a field-theoretic definition of the boundary of the perturbative regime, we reevaluate the situation in a quantitative manner. We conclude that the simplest and most promising situations are those where some of the compactification radii are exceptionally large, corresponding to highly anisotropic orbifold models. Thus, one is led to consider constructions which are known to the effective field-theorist as higher-dimensional or orbifold grand unified theories (orbifold GUTs). In particular, if the discrete symmetry used to break the GUT group acts freely, a non-local breaking in the larger compact dimensions can be realized, leading to a precise gauge coupling unification as expected on the basis of the MSSM particle spectrum. Furthermore, a somewhat more model dependent but nevertheless very promising scenario arises if the GUT breaking is restricted to certain singular points within the manifold spanned by the larger compactification radii.

D7‐brane moduli space in axion monodromy and fluxbrane inflation

We analyze the quantum-corrected moduli space of D7-brane position moduli with special emphasis on inationary model building. D7-brane deformation moduli are key players in two recently proposed inationary scenarios: The rst, D7-brane chaotic ination, is a variant of axion monodromy ination which allows for an eective

Axion monodromy and the weak gravity conjecture

A bstractAxions with broken discrete shift symmetry (axion monodromy) have recently played a central role both in the discussion of inflation and the ‘relaxion’ approach to the hierarchy problem. We suggest a very minimalist way to constrain such models by the weak gravity conjecture for domain walls: while the electric side of the conjecture is always satisfied if the cosine-oscillations of the axion potential are sufficiently small, the magnetic side imposes a cutoff, Λ3 ∼ mf Mpl, independent of the height of these ‘wiggles’. We compare our approach with the recent related proposal by Ibanez, Montero, Uranga and Valenzuela. We also discuss the non-trivial question which version, if any, of the weak gravity conjecture for domain walls should hold. In particular, we show that string compactifications with branes of different dimensions wrapped on different cycles lead to a ‘geometric weak gravity conjecture’ relating volumes of cycles, norms of corresponding forms and the volume of the compact space. Imposing this ‘geometric conjecture’, e.g. on the basis of the more widely accepted weak gravity conjecture for particles, provides at least some support for the (electric and magnetic) conjecture for domain walls.

A parton model for diffractive processes in deep inelastic scattering

Abstract We demonstrate that the global properties of the “rapidity gap” events, observed at HERA, can be understood based on electron-gluon scattering as the underlying partonic process. Using the measured inclusive structure function F2 to determine the parameters of the parton model, the diffractive structure function F2d is predicted. The ratio of diffractive and inclusive cross sections, R d = Σ d Σ incl ⋍ 1 9 , is determined by the probability of the produced quark-antiquark pair to evolve into a colour singlet state. This colour singlet cluster may fragment into hadrons independently of the proton remnant, yielding the observed gap in rapidity.

Tuning and backreaction in F-term axion monodromy inflation

Abstract We continue the development of axion monodromy inflation, focusing in particular on the backreaction of complex structure moduli. In our setting, the shift symmetry comes from a partial large complex structure limit of the underlying type IIB orientifold or F-theory fourfold. The coefficient of the inflaton term in the superpotential has to be tuned small to avoid conflict with Kahler moduli stabilisation. To allow such a tuning, this coefficient necessarily depends on further complex structure moduli. At large values of the inflaton field, these moduli are then in danger of backreacting too strongly. To avoid this, further tunings are necessary. In weakly coupled type IIB theory at the orientifold point, implementing these tunings appears to be difficult if not impossible. However, fourfolds or models with mobile D7-branes provide enough structural freedom. We calculate the resulting inflaton potential and study the feasibility of the overall tuning given the limited freedom of the flux landscape. Our preliminary investigations suggest that, even imposing all tuning conditions, the remaining choice of flux vacua can still be large enough for such models to provide a promising path to large-field inflation in string theory.

Inflationary correlation functions without infrared divergences

Inflationary correlation functions are potentially affected by infrared divergences. For example, the two-point correlator of curvature perturbation at momentum k receives corrections ~ ln (kL), where L is the size of the region in which the measurement is performed. We define infrared-safe correlation functions which have no sensitivity to the size L of the box used for the observation. The conventional correlators with their familiar log-enhanced corrections (both from scalar and tensor long-wavelength modes) are easily recovered from our IR-safe correlation functions. Among other examples, we illustrate this by calculating the corrections to the non-Gaussianity parameter fNL coming from long-wavelength tensor modes. In our approach, the IR corrections automatically emerge in a resummed, all-orders form. For the scalar corrections, the resulting all-orders expression can be evaluated explicitly.

The Throat as a Randall-Sundrum model with Goldberger-Wise stabilization

Abstract An interesting feature of type IIB flux compactifications is the natural presence of strongly warped regions or ‘throats’. These regions allow for a 5d Randall–Sundrum model interpretation with a large hierarchy between the UV and IR brane. We show that, in the 5d description, the flux stabilization of this hierarchy (or, equivalently, of the brane-to-brane distance) can be understood as an implementation of the Goldberger–Wise mechanism. This mechanism relies on the non-trivial bulk profile of the so-called Goldberger–Wise scalar, which in addition has fixed expectation values at the boundaries and thereby stabilizes the size of the 5d interval. The Goldberger–Wise scalar is realized microscopically by the continuously varying flux of the Neveu–Schwarz 2-form potential B 2 on the S 2 cycle in the throat. Its back-reaction on the 5d geometry leads to a significant departure from a pure AdS 5 background. We also find that, for a wide range of parameters, the universal Kahler modulus of the 10d compactification plays the role of a UV-brane field in the equivalent 5d model. It governs the size of a large 4d curvature term localized at the UV brane. We hope that our simple 5d description of the stabilized throat will be useful in various phenomenological and cosmological applications and that refined versions of this construction will be able to account for all relevant details of the 10d model.