arXiv subjects

Stefan Forste

Publications and source records attributed to Stefan Forste.

At least 19 recordsLinked to original sources

arXiv2025

Grand Canonical vs Canonical Krylov Complexity in Double-Scaled Complex SYK Model

We consider the complex SYK model in the double-scaling limit. We obtain the transfer matrix for the grand canonical ensemble and symmetrize it. In the (n,Q)- basis of chord states, the grand canonical transfer matrix is block diagonal, where each block is the canonical transfer matrix for the respective charge sector. We therefore conclude that the Krylov complexity for the grand canonical ensemble is given by the sum of the complexities in the charge sectors weighted by a probability function that depends on the chemical potential. Finally, we compute the Krylov complexity analytically in the limit of early and late time in the charge sector and numerically for both canonical and grand canonical ensemble.

hep-th↗

arXiv2025

Measurement and Teleportation in the cSYK/JT Correspondence

We consider two complex SYK models entangled in a thermofield double (TFD) state and investigate the effect of one-sided projective measurements. As measurement operator we choose single site charge operators. Performing a measurement results in a non-zero $U(1)$ charge. The entropy curve differs from the previously studied SYK model due to a thermodynamic phase transition that takes place after a certain charge is reached. We also match our results to a dual bulk description. Finally, a teleportation protocol is provided to support the notion of a traversable wormhole being formed.

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arXiv2024

Half-Wormholes in a Supersymmetric SYK Model

We identify half-wormhole contributions to the non averaged $\mathscr{N} =1$ supersymmetric SYK model in which time has been reduced to a point. As in previously studied examples, the inclusion of half-wormholes restores factorisation in the large $N$ limit. Wormholes as well as half-wormholes break supersymmetry.

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arXiv2024

Ensemble Averages of $\mathbb{Z}_2$ Orbifold Classes of Narain CFTs

In this work we study families of $\mathbb{Z}_2$ orbifolds of toroidal conformal field theories based on both factorizable and non-factorizable target space tori. For these classes of theories, we analyze their moduli spaces, and compute their partition functions. Building on previous work, we express the calculated partition functions in terms of suitable Siegel-Narain theta functions that allow us to determine their ensemble averages. We express the derived averaged partition functions of the studied families of conformal field theories in a manifest modular invariant finite sum of products of real analytic Eisenstein series. We speculate on a tentative holographic three-dimensional dual bulk interpretations for the considered $\mathbb{Z}_2$ orbifold classes of ensembles of conformal field theories.

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arXiv2022

Lyapunov exponents in $\mathbf{{\cal N}=2}$ supersymmetric Jackiw-Teitelboim gravity

We study $\mathcal{N}=2$ supersymmetric Jackiw-Teitelboim (JT) gravity at finite temperature coupled to matter. The matter fields are related to superconformal primaries by AdS/CFT duality. Due to broken super reparametrisation invariance in the SCFT dual, there are corrections to superconformal correlators. These are generated by the exchange of super-Schwarzian modes which is dual to the exchange of 2D supergravity modes. We compute corrections to four-point functions for superconformal primaries and analyse the behaviour of out-of-time-ordered correlators. In particular, four-point functions of two pairs of primaries with mutually vanishing two-point functions are considered. By decomposing the corresponding supermultiplet into its components, we find different Lyapunov exponents. The value of the Lyapunov exponents depends on whether the correction is due to graviton, gravitini or graviphoton exchange. If mutual two-point functions do not vanish all components grow with maximal Lyapunov exponent.

hep-th↗

arXiv2021

Deformations of JT Gravity via Topological Gravity and Applications

We study the duality between JT gravity and the double-scaled matrix model including their respective deformations. For these deformed theories we relate the thermal partition function to the generating function of topological gravity correlators that are determined as solutions to the KdV hierarchy. We specialise to those deformations of JT gravity coupled to a gas of defects, which conforms with known results in the literature. We express the (asymptotic) thermal partition functions in a low temperature limit, in which non-perturbative corrections are suppressed and the thermal partition function becomes exact. In this limit we demonstrate that there is a Hawking-Page phase transition between connected and disconnected surfaces for this instance of JT gravity with a transition temperature affected by the presence of defects. Furthermore, the calculated spectral form factors show the qualitative behaviour expected for a Hawking-Page phase transition. The considered deformations cause the ramp to be shifted along the real time axis. Finally, we comment on recent results related to conical Weil-Petersson volumes and the analytic continuation to two-dimensional de Sitter space.

hep-th↗

arXiv2020

Supersymmetric Black Holes and the SJT/nSCFT$_1\!$ Correspondence

We consider 1/4 BPS black hole solutions of ${\cal N}=2$ gauged supergravity in $AdS_4$. The near horizon geometry is $AdS_2 \times S^2$ and supersymmetry is enhanced. In the first part of the paper we choose a moment map, which allows the embedding of this supergravity solution into a sugra theory with a hypermultiplet. We then perform the s-wave reduction of this theory at the horizon and determine the dilaton multiplet, which couples to both metric and gravitino fluctuations. In the second part we work with Euclidean axial $\mathcal{N}=(2,2)$ JT supergravity and show how to add gauged matter in form of covariantly twisted chiral and anti-chiral multiplets. We demonstrate how to reduce the on-shell action to boundary superspace. We compare both theories and calculate the fourpoint function by integrating out gravitons, gravitini and photons for the s-wave setting and by use of the Super-Schwarzian modes in the JT theory.

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arXiv2018

Yukawa couplings from magnetized D-brane models on non-factorisable tori

We compute Yukawa couplings in type IIB string theory compactified on a non factorisable six-torus in the presence of D9 branes and fluxes. The setting studied in detail, is obtained by T-dualising an intersecting brane configuration of type IIA theory compactified on a torus generated by the SO(12) root lattice. Particular deformations of such torus are taken into account and provide moduli dependent couplings. Agreement with the type IIA result is found in a non trivial way. The classical type IIB calculation gives also information on a factor accessible only by quantum computations on the type IIA side.

hep-th↗

arXiv2018

Yukawa couplings for intersecting D-branes on non-factorisable tori

We compute Yukawa couplings in type IIa string theory compactified on a six-torus in the presence of intersecting D6-branes. The six-torus is generated by an SO(12) root lattice. Yukawa couplings are expressed as sums over worldsheet instantons. Our result extends known expressions to a non-factorisable torus. As an aside we also fill in some details for the factorisable torus and non-coprime intersection numbers.

hep-th↗

arXiv2018

Towards the Holographic Dual of N = 2 SYK

The gravitational part of the holographic dual to the SYK model has been conjectured to be Jackiw-Teitelboim (JT) gravity. In this paper we construct an AdS2 background in N = (2,2) JT gravity and show that the gravitational dynamics are - as in the N = 0 and N = 1 cases - fully captured by the extrinsic curvature as an effective boundary action. This boundary term is given by the super-Schwarzian of the N = 2 SYK model, thereby providing further evidence of the JT/SYK duality. The chirality of this SYK model is reproduced by the inherent chirality of axial N = (2,2) supergravity.

hep-th↗

arXiv2017

Nearly AdS2 Sugra and the Super-Schwarzian

In nearly AdS2 gravity the Einstein-Hilbert term is supplemented by the Jackiw-Teitelboim action. Integrating out the bulk metric gives rise to the Schwarzian action for the boundary curve. In the present note, we show how the extension to supergravity leads to the super-Schwarzian action for the superspace boundary.

hep-th↗

arXiv2016

Tuning the Cosmological Constant, Broken Scale Invariance, Unitarity

We study gravity coupled to a cosmological constant and a scale but not conformally invariant sector. In Minkowski vacuum, scale invariance is spontaneously broken. We consider small fluctuations around the Minkowski vacuum. At the linearised level we find that the trace of metric perturbations receives a positive or negative mass squared contribution. However, only for the Fierz-Pauli combination the theory is free of ghosts. The mass term for the trace of metric perturbations can be cancelled by explicitly breaking scale invariance. This reintroduces fine-tuning. Models based on four form field strength show similarities with explicit scale symmetry breaking due to quantisation conditions.

hep-th↗

arXiv2013

General self-tuning solutions and no-go theorem

We consider brane world models with one extra dimension. In the bulk there is in addition to gravity a three form gauge potential or equivalently a scalar (by generalisation of electric magnetic duality). We find classical solutions for which the 4d effective cosmological constant is adjusted by choice of integration constants. No go theorems for such self-tuning mechanism are circumvented by unorthodox Lagrangians for the three form respectively the scalar. It is argued that the corresponding effective 4d theory always includes tachyonic Kaluza-Klein excitations or ghosts. Known no go theorems are extended to a general class of models with unorthodox Lagrangians.

hep-th↗

arXiv2011

Nontrival Cosmological Constant in Brane Worlds with Unorthodox Lagrangians

In self-tuning brane-world models with extra dimensions, large contributions to the cosmological constant are absorbed into the curvature of extra dimensions and consistent with flat 4d geometry. In models with conventional Lagrangians fine-tuning is needed nevertheless to ensure a finite effective Planck mass. Here, we consider a class of models with non conventional Lagrangian in which known problems can be avoided. Unfortunately these models are found to suffer from tachyonic instabilities. An attempt to cure these instabilities leads to the prediction of a positive cosmological constant, which in turn needs a fine-tuning to be consistent with observations.

hep-th↗

arXiv2011

Rigid D6-branes on T6/[Z(2)xZ(2M)xOmegaR] with discrete torsion

We give a complete classification of T6/[Z(2)xZ(2M)xOmegaR] orientifolds on factorisable tori and rigid D6-branes on them. The analysis includes the supersymmetry, RR tadpole cancellation and K-theory conditions and complete massless open and closed string spectrum (i.e. non-chiral as well as chiral) for fractional or rigid D6-branes for all inequivalent compactification lattices, without and with discrete torsion. We give examples for each orbifold background, which show that on Z(2)xZ(6) and Z(2)xZ(6') there exist completely rigid D6-branes despite the self-intersections of orbifold image cycles. This opens up a new avenue for improved Standard Model building. On the other hand, we show that Standard and GUT model building on the Z(2)xZ(4) background is ruled out by simple arguments.

hep-th↗

arXiv2010

Proton Hexality in Local Grand Unification

Proton hexality is a discrete symmetry that avoids the problem of too fast proton decay in the supersymmetric extension of the standard model. Unfortunately it is inconsistent with conventional grand unification. We show that proton hexality can be incorporated in the scheme of "Local Grand Unification" discussed in the framework of model building in (heterotic) string theory.

hep-ph↗

arXiv2008

Oddness from Rigidness

We revisit the problem of constructing type IIA orientifolds on T^6/(Z2 x Z2) which admit (non)-factorisable lattices. More concretely, we consider a (Z2 x Z2') orientifold with torsion, where D6-branes wrap rigid 3-cycles. We derive the model building rules and consistency conditions in the case where the compactification lattice is non-factorisable. We show that in this class of configurations, (semi) realistic models with an odd number of families can be easily constructed, in contrast to compactifications where the D6-branes wrap non-rigid cycles. We also show that an odd number of families can be obtained in the factorisable case, without the need of tilted tori. We illustrate the discussion by presenting three family Pati-Salam models with no chiral exotics in both factorisable and non-factorisable toroidal compactifications.

hep-th↗

arXiv2008

Scattering amplitudes in strongly coupled N=4 SYM from semiclassical strings in AdS

Very recently in arXiv:0705.0303 Alday and Maldacena gave a string theory prescription for computing (all) planar amplitudes in N=4 supersymmetric gauge theory at strong coupling using the AdS/CFT correspondence. These amplitudes are determined by a classical string solution and contain a universal exponential factor involving the action of the classical string. On the gauge theory side, expressions for perturbative amplitudes at strong coupling were previously proposed only for specific helicities of external particles -- the maximally helicity violating or MHV amplitudes. These follow from the exponential ansatz of Bern, Dixon and Smirnov for MHV amplitudes in N=4 SYM. In this paper we examine the amplitudes dependence on helicities and particle-types of external states. We consider the prefactor of string amplitudes and give arguments suggesting that the prefactor at strong coupling should be the same as the Yang-Mills tree-level amplitude for the same process. This implies that scattering amplitudes in N=4 SYM simplify dramatically in the strong coupling limit. It follows from our proposal that in this limit all (MHV and non-MHV) n-point amplitudes are given by the (known) tree-level Yang-Mills result times the helicity-independent (and particle-type-independent) universal exponential.

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