Dimitri V. Nanopoulos

D Branes from Liouville Strings

We develop quantization aspects of our Liouville approach to noncritical strings, proposing a path-integral formulation of a second quantization of string theory, that incorporates naturally the couplings of string sources to background fields. Such couplings are characteristic of macroscopic string solutions and/or D-brane theories. Resummation over world-sheet genera in the presence of stringy (σ-model) soliton backgrounds, and recoil effects associated with logarithmic operators on the world sheet, play a crucial role in inducing such sources as well-defined renormalization-group counterterms. Using our Liouville renormalization group approach, we derive the appropriate second-order equation of motion for the D brane. We discuss within this approach the appearance of open strings, whose ends carry nontrivial Chan–Paton-like quantum numbers related to the W∞ charges of two-dimensional string black holes.

ON ELEVATING FREE-FERMION Z2×Z2 ORBIFOLDS MODELS TO COMPACTIFICATIONS OF F THEORY

We study the elliptic fibrations of some Calabi–Yau threefolds, including the Z2×Z2 orbifold with (h1,1,h2,1)=(27, 3), which is equivalent to the common framework of realistic free-fermion models, as well as related orbifold models with (h1,1,h2,1)=(51, 3) and (31, 7). However, two related puzzles arise when one considers the (h1,1,h2,1)=(27, 3) model as an F theory compactification to six dimensions. The condition for the vanishing of the gravitational anomaly is not satisfied, suggesting that the F theory compactification does not make sense, and the elliptic fibration is well defined everywhere except at four singular points in the base. We speculate on the possible existence of N=1 tensor and hypermultiplets at these points which would cancel the gravitational anomaly in this case.

LIOUVILLE COSMOLOGY AT ZERO AND FINITE TEMPERATURES

We discuss cosmology in the context of Liouville strings, characterized by a central-charge deficit Q2, in which target time is identified with (the worldsheet zero mode of the) Liouville field: Q-Cosmology. We use a specific example of colliding braneworlds to illustrate the phase diagram of this cosmological framework. The collision provides the necessary initial cosmological instability, expressed as a departure from conformal invariance in the underlying string model. The brane motion provides a way of breaking target-space supersymmetry, and leads to various phases of the brane and bulk Universes. Specifically, we find a hot metastable phase for the bulk string Universe soon after the brane collision in which supersymmetry is broken, which we describe by means of a subcritical worldsheet σ-model dressed by a spacelike Liouville field, representing finite temperature (Euclidean time). This phase is followed by an inflationary phase for the brane Universe, in which the bulk string excitations are cold. This is described by a super-critical Liouville string with a timelike Liouville mode, whose zero mode is identified with the Minkowski target time. Finally, we speculate on possible ways of exiting the inflationary phase, either by means of subsequent collisions or by deceleration of the brane Universe due to closed-string radiation from the brane to the bulk. While phase transitions from hot to cold configurations occur in the bulk string universe, stringy excitations attached to the braneworld remain thermalized throughout, at a temperature which can be relatively high. The late-time behavior of the model results in dilaton-dominated dark energy and present-day acceleration of the expansion of the Universe, asymptoting eventually to zero.

On the space-time uncertainty relations of Liouville strings and D-branes

Within a Liouville approach to noncritical string theory, we argue for a nontrivial commutation relation between space and time observables, leading to a nonzero space–time uncertainty relation δx δt>0, which vanishes in the limit of weak string coupling.

Supersymmetry signals of supercritical string cosmology at the Large Hadron Collider

We investigate the minimal supergravity signals at the Large Hadron Collider in the context of supercritical string cosmology (SSC). In this theory, the presence of a time dependent dilaton provides us with a smoothly evolving dark energy and modifies the dark matter allowed region of the minimal supergravity model with standard cosmology. Such a dilaton dilutes the supersymmetric dark matter density (of neutralinos) by a factor O(10) and consequently the regions with too much dark matter in the standard scenario are allowed in the SSC. The final states expected at the Large Hadron Collider in this scenario, unlike the standard scenario, consist of Z bosons, Higgs bosons, and/or high energy taus. We show how to characterize these final states and determine the model parameters. Using these parameters, we determine the dark matter content and the neutralino-proton cross section. All these techniques can also be applied to determine model parameters in SSC models with different supersymmetry breaking scenarios.

COSMOLOGICAL BARYOGENESIS IN SUPERSTRING MODELS WITH STABLE PROTONS

We discuss cosmological baryogenesis in phenomenological low-energy models inspired by the superstring which have an unobservably long baryon lifetime. The Affleck-Dine mechanism of baryogenesis in a cold (≲104GeV) universe is shown to be feasible, with a large baryon density being produced by the decays of large expectation values for squark and slepton fields after inflation. We catalogue the gauge-invariant quartic scalar operators in the low-energy effective action which could appear once supersymmetry is broken, show that the D-terms in the potential can vanish, and discuss the possibility that the F-terms have flat directions allowing large values for these scalar fields.

M-theory from worldsheet defects in Liouville string

We have argued previously that black holes may be represented in a D-brane approach by monopole and vortex defects in a sine–Gordon field theory model of Liouville dynamics on the worldsheet. Supersymmetrizing this sine–Gordon system, we find critical behavior in 11 dimensions, due to defect condensation which we interpret as the worldsheet analog of D-brane condensation around an extra space–time dimension in M-theory. This supersymmetric description of Liouville dynamics has a natural embedding within a 12-dimensional framework suggestive of F-theory.

Search for supersymmetry at the p collider

Abstract Many models of broken supersymmetry predict the existence of supersymmetry fermions χ±,0 with masses less than the W± and Z0. Often there are two light neutral fermions χ0, even in models with large gaugino masses. The W± have large branching ratios for decays into χ±+χ0, with the χ± subsequently decaying into χ0 plus hadrons or leptons. We propose looking at the CERN p p collider for W± production and decay into supersymmetric fermions, a likely signature being “zen” events with one broadened hadronic jet system recoiling against invisible missing transverse energy.