Nissan Itzhaki

Closed strings as imaginary D-branes

Abstract Sen has recently drawn attention to an exact time-dependent boundary conformal field theory with the space–time interpretation of brane creation and annihilation. An interesting limit of this BCFT is formally equivalent to an array of D-branes located in imaginary time. This raises the question: what is the meaning of D-branes in imaginary time? The answer we propose is that D-branes in imaginary time define purely closed string backgrounds. In particular we prove that the disk scattering amplitude of m closed strings off an arbitrary configuration of imaginary branes is equivalent to a sphere amplitude with m +1 closed string insertions. The extra puncture is a specific closed string state, generically normalizable, that depends on the details of the brane configuration. We study in some detail the special case of the array of imaginary D-branes related to Sens BCFT and comment on its space–time interpretation. We point out that a certain limit of our set-up allows to study classical black hole creation and suggests a relation between Choptuiks critical behavior and a phase-transition a la Gregory–Laflamme. We speculate that open string field theory on imaginary D-branes is dual to string theory on the corresponding closed string background.

Observables of String Field Theory

We study gauge invariant operators of open string field theory and find a precise correspondence with on-shell closed strings. We provide a detailed proof of the gauge invariance of the operators and a heuristic interpretation of their correlation functions in terms of on-shell scattering amplitudes of closed strings. We also comment on the implications of these operators to vacuum string field theory.

I-brane dynamics

We study the dynamics near a 1+1 dimensional intersection of two orthogonal stacks of fivebranes in type IIB string theory, using an open string description valid at weak coupling, and a closed string description valid at strong coupling. The weak coupling description suggests that this system is invariant under eight supercharges with a particular chirality in 1+1 dimensions, and its spectrum contains chiral fermions localized at the intersection. The closed string description leads to a rather different picture — a three dimensional Poincare invariant theory with a gap and sixteen supercharges. We show that this dramatic change in the behavior of the system is partly due to anomaly inflow. Taking it into account leads to a coherent picture, both when the fivebranes in each stack are coincident and when they are separated.

On the BCFT description of holes in the c = 1 matrix model

Abstract We propose a boundary conformal field theory (BCFT) description of hole states in the c =1 matrix model.

Large- N harmonic oscillator as a string theory

We propose a duality between the large-N gauged harmonic oscillator and a novel string theory in two dimensions.

Non-supersymmetric deformations of non-critical superstrings

We study certain supersymmetry breaking deformations of linear dilaton backgrounds in different dimensions. In some cases, the deformed theory has bulk closed strings tachyons. In other cases there are no bulk tachyons, but there are localized tachyons. The real time condensation of these localized tachyons is described by an exactly solvable worldsheet CFT. We also find some stable, non-supersymmetric backgrounds.

A comment on technical naturalness and the cosmological constant

We propose a model of dynamical relaxation of the cosmological constant. Technical naturalness of the model and the present value of the vacuum energy density imply an upper bound on the supersymmetry breaking scale and the reheating temperature at the TeV scale.