We present simple explicit formulae for the change of the $\textit{g}$-function, boundary state, boundary spectrum and structure constants between the endpoints of short boundary RG flows at next-to-leading order. The formulae are derived using open string field theory and tested on integrable RG flows between conformal boundary states in unitary Virasoro minimal models.

United States

Jaroslav Scheinpflug

Switzerland

Czechia

Martin Schnabl

France

Jakub Vošmera

Observables of boundary RG flows from string field theory

We construct fully gauge-invariant kinetic terms for open and closed string field theories on a target space with boundary. This is realized by promoting the gauge parameters at the boundary to extra dynamical modes describing boundary degrees of freedom. Having a gauge-invariant classical action, we also construct the corresponding BV master action and show that the master equation is obeyed thanks to a peculiar nilpotent structure of the BV kinetic operator. From this general construction we explicitly derive free actions for massless and massive modes of open and closed strings propagating on a target with a boundary, including linear-dilaton backgrounds.

Italy

Carlo Maccaferri

Alberto Ruffino

Gauge-invariant action for free string field theory with boundary

We study topological defect lines in the symmetric-product orbifolds Sym ($\textit{X}$) for a generic seed CFT $\textit{X}$. We focus on the defects which preserve the maximum of the $\textit{S}$ symmetry. When $\textit{X}$ is taken to describe the free theory of four fermions and four bosons on a 𝕋 , we construct holographically dual backgrounds describing propagation of tensionless closed strings in the presence of spacetime defects wrapping the conformal boundary. We find a precise match between the spectra of local on-shell closed-string vertex operators in the bulk and the spectra of non-local disorder fields in the spacetime theory.

United Kingdom

Bob Knighton

Vit Sriprachyakul

Topological defects and tensionless holography

We discuss the construction of boundary contributions to free string field theory actions in the context of the bosonic string. We show that it is generally possible to obtain a well-defined variational principle by adding a simple boundary term (which only depends on the value of the bulk fields at the boundary) to the original bulk action. However, it is in general not possible to do this in a gauge-invariant way unless suitable boundary degrees of freedom are added. We explicitly construct such boundary contributions for the massless level of both the open and the closed SFT, as well as for the tensionless limit of the full string field theory. Our results reproduce linearized general relativity with the Gibbons-Hawking-York term and provide similar gauge-invariant actions for the infinite tower of massless higher-spin gauge theories for all Regge trajectories. By writing down a gauge-invariant action for the first massive level of the open string, we provide evidence that an analogous construction should be possible for the full tensile string field theory.

Riccardo Poletti

Boundary modes in string field theory

We consider computing the on-shell disk action of open-closed string field theory as a gauge-invariant way of capturing the shift in D-brane tension that is induced by a deformation of the bulk CFT. We study the effect of bulk matter deformations (both marginal and relevant) on a wide range of boundary conditions in a number of CFTs up to subleading (two-loop) order in perturbation theory. In all analyzed examples, we find that the shift in the g-function of the matter boundary state is always accompanied by a boundary-independent shift in the string coupling constant, whose leading behaviour is universally proportional to the sphere two-point function of the deforming bulk operator.

Bulk-induced D-brane deformations and the string coupling constant

We use the new nilpotent formulation of open-closed string field theory to explore the limit where the number N of identical D-branes of the starting background is large. By reformulating the theory in terms of the ’t Hooft coupling λ := κN, where κ is the string coupling constant, we explicitly see that at large N only genus zero vertices with arbitrary number of boundaries survive. After discussing the homotopy structure of the obtained large N open-closed theory we discuss the possibility of integrating out the open string sector with a quantum but planar homotopy transfer. As a result we end up with a classical closed string field theory, described by a weak L  ∞ -algebra containing a tree-level tadpole which, to first order in λ, is given by the initial boundary state. We discuss the possibility of removing the tadpole with a closed string vacuum shift solution, to end up with a new classical closed string background, where the initial D-branes have been turned into pure closed-string backreaction.

Open-closed string field theory in the large N limit

In this note we revisit the homotopy-algebraic structure of oriented bosonic open-closed string field theory and we give a new compact formulation in terms of a single cyclic open-closed co-derivation which defines a single nilpotent structure describing the consistency of generic open-closed off-shell amplitudes with arbitrary number of boundaries and at arbitrary genus.

The nilpotent structure of open-closed string field theory

We consider deformations of D-brane systems induced by a change in the closed string background in the framework of bosonic open-closed string field theory, where it is possible to unambiguously tame infrared divergences originating from both open and closed string degenerations. A closed string classical solution induces a tadpole for the open strings which shifts the open string vacuum and generates a cosmological constant composed of two terms: one which is directly related to the closed string solution and the other which depends on the open string vacuum shift. We show that only the sum of these two terms is invariant under closed SFT gauge transformations and therefore is an observable. We conjecture that this observable is universally proportional to the shift in the world-sheet disk partition function between the starting D-brane in undeformed background and the final D-brane in deformed background, which typically includes also a change in the string coupling constant. We test the conjecture by considering a perturbative closed string solution describing deformations of a Narain compactification and, from the SFT cosmological constant, we reproduce the expected shift in the g-function of various D-branes living in the compactification. In doing this we are also able to identify a surprising change in the string coupling constant at second order in the deformation.

The classical cosmological constant of open-closed string field theory

String theory on AdS3 × S3 × 𝕋4 with minimal k = 1 NS-NS flux can be described in terms of a free field worldsheet theory in the hybrid formalism. We construct various D-branes of this string theory and calculate their associated cylinder amplitudes. We find that these amplitudes match with the cylinder correlators of certain boundary states of the dual symmetric orbifold CFT Sym(𝕋4), thus suggesting a direct correspondence between these boundary conditions. We also show that the disk amplitudes of these D-branes localise to those points in the worldsheet moduli space where the worldsheet disk holomorphically covers the spacetime disk.

Matthias Gaberdiel

D-branes in AdS3 × S3 × 𝕋4 at k = 1 and their holographic duals

This is the first of a series of three papers on open string field theories based on Witten star product deformed with a gauge invariant open/closed coupling. This de- formation is a tree-level tadpole which destabilizes the initial perturbative vacuum. We discuss the existence of vacuum-shift solutions which cancel the tadpole and represent a new configuration where the initial D-brane system has adapted to the change in the closed string background. As an example we consider the bulk deformation which changes the compactification radius and, to first order in the deformation, we reproduce the shift in the mass of the open string KK modes from the new kinetic operator after the vacuum shift. We also discuss the possibility of taming closed string degenerations with the open string propagator in the simplest amplitude corresponding to two closed strings off a disk.