arXiv subjects

Chrysoula Markou

Publications and source records attributed to Chrysoula Markou.

14 recordsLinked to original sources

arXiv2026

Closed string trajectories from a new "tiling"

Based on an efficient technology for the excavation of entire open string trajectories of physical states, we propose an algorithmic method of constructing the largely unknown closed string trajectories. Due to combinatorial complexity, the "double copy" of open strings is limited in efficiency as a means of building closed string trajectories. We bypass this technical difficulty by employing one open string as the fundamental seed and then dressing it by a suitable selection of generators of a symplectic algebra that acts on both the left and the right sector. By applying Howe duality, the dressed seeds amount to closed string trajectory candidates, finding the physical subset of which is possible by solving systems of equations involving Diophantine-like recursion relations, which we also illustrate with examples.

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arXiv2026

Ultraviolet completion of Starobinsky inflation

We construct an $N=1$ supergravity action whose bosonic part contains an arbitrary function of the scalar curvature, the so-called $F(R)$ gravity. As in $R+R^2$ supergravity, it can be described in terms of two chiral superfields of no-scale supergravity: one contains the scalaron which plays the role of the Starobinsky inflaton and the other contains the goldstone fermion of spontaneously broken supersymmetry during the inflation plateau. Its (complex) scalar component acquires a non-tachyonic mass in the presence of the string dilaton and can be set to zero, together with the pseudoscalar partner of the scalaron, so that the scalar potential is reduced to the one of $F(R)$ gravity. In a perturbative expansion in powers of $R$, one obtains a small deformation of the Starobinsky cosmological model that solves the problem of initial conditions within the validity of the effective field theory, below the scale of tower of states predicted by the swampland distance conjecture. We also show that a particular example of an underlying microscopic theory with such properties is provided by a four-dimensional heterotic string model containing the Standard Model of particle physics.

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arXiv2026

An introduction to string states and their interactions

The subject matter of these lecture notes is the open bosonic critical string, its perturbative spectrum and interactions. We begin with a brief review of classical string propagation, quantization, as well as the level-by-level construction of physical string states. We then review a new, covariant and efficient technology of constructing entire trajectories of infinitely many physical states deeper in the string spectrum. Finally, elements of the calculation of string scattering amplitudes, including aspects of the application of the technology for the efficient calculation of tree-level amplitudes of deeper trajectories, are also covered. The material is based on three invited lectures delivered by the author at the 2024 Modave Summer School in Mathematical Physics.

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arXiv2025

On the deep string spectrum

These proceedings are based on the author's invited talk reviewing the original published work [1,2] of the author with collaborators. The subject matter is a new, covariant and efficient technology of constructing entire trajectories of physical string states deeper inside the string spectrum than the leading Regge. The key observation behind the technology is that the lowering operators of a symplectic algebra appear in the Virasoro constraints which impose physicality of states in the open bosonic string. This algebra commutes with the spacetime Lorentz algebra, (of the little group) of which all string states are irreducible representations. Employing then the so-called Howe duality of representation theory, one may relate the irreducible representations of the two algebras via a bijection. The spectrum thus splits into two parts: trajectories that are lowest weight states of the symplectic algebra and their infinitely many clones. The latter can then be reached by suitably dressing the former with the raising operators of the symplectic algebra. The technology is nontrivially extended to the open superstring, where the relevant Howe dual is an orthosymplectic algebra.

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arXiv2024

On the deep superstring spectrum

We propose a covariant method of constructing entire trajectories of physical states in superstring theory in the critical dimension. It is inspired by a recently developed covariant technology of excavating bosonic string trajectories, that is facilitated by the observation that the Virasoro constraints can be written as linear combinations of lowering operators of a bigger algebra, namely a symplectic algebra, which is Howe dual to the spacetime Lorentz algebra. For superstrings, it is the orthosymplectic algebra that appears instead, with its lowest weight states forming the simplest class of physical trajectories in the NS sector. To construct the simplest class in the R sector, the lowest weight states need to be supplemented with other states, which we determine. Deeper trajectories are then constructed by acting with suitable combinations of the raising operators of the orthosymplectic algebra, which we illustrate with several examples.

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arXiv2023

An excursion into the string spectrum

We propose a covariant technique to excavate physical bosonic string states by entire trajectories rather than individually. The approach is based on Howe duality: the string's spacetime Lorentz algebra commutes with a certain inductive limit of $sp(\bullet)$, with the Virasoro constraints forming a subalgebra of the Howe dual algebra $sp(\bullet)$. There are then infinitely many simple trajectories of states, which are lowest-weight representations of $sp(\bullet)$ and hence of the Virasoro algebra. Deeper trajectories are recurrences of the simple ones and can be probed by suitable trajectory-shifting operators built out of the Howe dual algebra generators. We illustrate the formalism with a number of subleading trajectories and compute a sample of tree-level amplitudes.

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arXiv2023

A stringy massive double copy

We derive a massive double copy construction within string theory. To this end, we use massive vectors of the open string spectrum that appear in compactifications to four dimensions and construct massive spin-2 tensors as closed string states, thereby mimicking the structure of the massless graviton. We then compute three-point amplitudes for the scattering of massless and massive spin-2 closed string states and reveal the double copy structure of the latter. With these results being finite in the string scale, we are further able to reproduce the cubic Lagrangian of ghost-free bimetric theory around flat spacetime for bulk massive spin-2 states originating in products of vectors of extended brane supersymmetry.

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arXiv2022

Extracting Bigravity from String Theory

The origin of the graviton from string theory is well understood: it corresponds to a massless state in closed string spectra, whose low-energy effective action, as extracted from string scattering amplitudes, is that of Einstein-Hilbert. In this work, we explore the possibility of such a string-theoretic emergence of ghost-free bimetric theory, a recently proposed theory that involves two dynamical metrics, that around particular backgrounds propagates the graviton and a massive spin-2 field, which has been argued to be a viable dark matter candidate. By choosing to identify the latter with a massive spin-2 state of open string spectra, we compute tree-level three-point string scattering amplitudes that describe interactions of the massive spin-2 with itself and with the graviton. With the mass of the external legs depending on the string scale, we discover that extracting the corresponding low-energy effective actions in four spacetime dimensions is a subtle but consistent process and proceed to appropriately compare them with bimetric theory. Our findings consist in establishing that string and bimetric theory provide to lowest order the same set of two-derivative terms describing the interactions of the massive spin-2 with itself and with the graviton, albeit up to numerical coefficient discrepancies, a fact that we analyze and interpret. We conclude with a mention of future investigations.

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arXiv2021

Aspects of compactification on a linear dilaton background

We consider the most general Kaluza-Klein (KK) compactification on $S^1/\mathbb{Z}_2$ of a five dimensional ($5D$) graviton-dilaton system, with a non-vanishing dilaton background varying linearly along the fifth dimension. We show that this background produces a Higgs mechanism for the KK vector coming from the $5D$ metric, which becomes massive by absorbing the string frame radion. The $\mathcal{N}=2$ minimal supersymmetric extension of this model, recently built as the holographic dual of Little String Theory, is then re-investigated. An analogous mechanism can be considered for the $4D$ vector coming from the (universal) $5D$ Kalb-Ramond two-form. Packaging the two massive vectors into a spin-$3/2$ massive multiplet, it is shown that the massless spectrum arranges into a $\mathcal{N}=1$, $D=4$ supersymmetric theory. This projection is compatible with an orbifold which preserves half of the original supersymmetries already preserved by the background. The description of the partial breaking $\mathcal{N}=2\rightarrow\mathcal{N}=1$ in this framework, with only vector multiplets and no hypermultiplets, remains an interesting open question which deserves further investigation.

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arXiv2019

Gravity with antisymmetric components

This work proposes a new gravitational theory formulated in terms of the vierbein field. The vierbein contains components which can be shifted by local Lorentz transformations and therefore do not show up in the spacetime metric. These components are given dynamics and become physical in our setup. They enter the massless theory in the form of an antisymmetric tensor field which makes the action reminiscent of the bosonic sector of supergravity. We then demonstrate that both the metric and the antisymmetric tensor can be made massive by adding a potential term for the vierbein. The form of this mass potential is inspired by ghost-free massive gravity. We confirm the absence of additional and potentially pathological degrees of freedom in an ADM analysis. However, at the linearized level around maximally symmetric solutions, the fluctuation of the antisymmetric tensor has a tachyonic mass pole.

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arXiv2019

Nonlinear ${\cal N}=2$ Global Supersymmetry

We study the partial breaking of ${\cal N}=2$ global supersymmetry, using a novel formalism that allows for the off-shell nonlinear realization of the broken supersymmetry, extending previous results scattered in the literature. We focus on the Goldstone degrees of freedom of a massive ${\cal N}=1$ gravitino multiplet which are described by deformed ${\cal N}=2$ vector and single-tensor superfields satisfying nilpotent constraints. We derive the corresponding actions and study the interactions of the superfields involved, as well as constraints describing incomplete ${\cal N}=2$ matter multiplets of non-linear supersymmetry (vectors and single-tensors).

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arXiv2019

Vierbein interactions with antisymmetric components

In this work we propose a new gravitational setup formulated in terms of two interacting vierbein fields. The theory is the fully diffeomorphism and local Lorentz invariant extension of a previous construction which involved a fixed reference vierbein. Certain vierbein components can be shifted by local Lorentz transformations and do not enter the associated metric tensors. We parameterize these components by an antisymmetric tensor field and give them a kinetic term in the action, thereby promoting them to dynamical variables. In addition, the action contains two Einstein-Hilbert terms and an interaction potential whose form is inspired by ghost-free massive gravity and bimetric theory. The resulting theory describes the interactions of a massless spin-2, a massive spin-2 and an antisymmetric tensor field. It can be generalized to the case of multiple massive spin-2 fields and multiple antisymmetric tensor fields. The absence of additional and potentially pathological degrees of freedom is verified in an ADM analysis. However, the antisymmetric tensor fluctuation around the maximally symmetric background solution has a tachyonic mass pole.

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arXiv2017

The effective supergravity of Little String Theory

In this work we present the minimal supersymmetric extension of the five-dimensional dilaton-gravity theory that captures the main properties of the holographic dual of little string theory. It is described by a particular gauging of $\mathcal{N}=2$ supergravity coupled with one vector multiplet associated to the string dilaton, along the $U(1)$ subgroup of $SU(2)$ R-symmetry. The linear dilaton in the fifth coordinate solution of the equations of motion (with flat string frame metric) breaks half of the supersymmetries to $\mathcal{N}=1$ in four dimensions. The non-supersymmetric version of this model was found recently as a continuum limit of a discretised version of the so-called clockwork setup.

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arXiv2015

The coupling of Non-linear Supersymmetry to Supergravity

We study the coupling of non-linear supersymmetry to supergravity. The goldstino nilpotent superfield of global supersymmetry coupled to supergravity is described by a geometric action of the chiral curvature superfield R subject to the constraint (R-λ)^2=0 with an appropriate constant λ. This constraint can be found as the decoupling limit of the scalar partner of the goldstino in a class of f(R) supergravity theories.

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