Flipped Cryptons and the UHECRs

Cryptons are metastable bound states of fractionally-charged particles that arise generically in the hidden sectors of models derived from heterotic string. We study their properties and decay modes in a specific flipped SU(5) model with long-lived four-particle spin-zero bound states called {\it tetrons}. We show that the neutral tetrons are metastable, and exhibit the tenth-order non-renormalizable superpotential operators responsible for their dominant decays. By analogy with QCD, we expect charged tetrons to be somewhat heavier, and to decay relatively rapidly via lower-order interactions that we also exhibit. The expected masses and lifetimes of the neutral tetrons make them good candidates for cold dark matter (CDM), and a potential source of the ultra-high energy cosmic rays (UHECRs) which have been observed, whereas the charged tetrons would have decayed in the early Universe.Comment: 8 Pages RevTex. New version with expanded introduction to flipped SU(5). Accepted for publication in PR

Quantum Decoherence in a D-Foam Background

Within the general framework of Liouville string theory, we construct a model for quantum D-brane fluctuations in the space-time background through which light closed-string states propagate. The model is based on monopole and vortex defects on the world sheet, which have been discussed previously in a treatment of 1+1-dimensional black-hole fluctuations in the space-time background, and makes use of a T-duality transformation to relate formulations with Neumann and Dirichlet boundary conditions. In accordance with previous general arguments, we derive an open quantum-mechanical description of this D-brane foam which embodies momentum and energy conservation and small mean energy fluctuations. Quantum decoherence effects appear at a rate consistent with previous estimates.Comment: 16 pages, Latex, two eps figures include

Search for Quantum Gravity

A satisfactory theory of quantum gravity may necessitate a drastic modification of our perception of space-time, by giving it a foamy structure at distances comparable to the Planck length. It is argued in this essay that the experimental detection of such structures may be a realistic possibility in the foreseeable future. After a brief review of different theoretical approaches to quantum gravity and the relationships between them, we discuss various possible experimental tests of the quantum nature of space-time. Observations of photons from distant astrophysical sources such as Gamma-Ray Bursters and laboratory experiments on neutral kaon decays may be sensitive to quantum-gravitational effects if they are only minimally suppressed. Experimental limits from the Whipple Observatory and the CPLEAR Collaboration are already probing close to the Planck scale, and significant increases in sensitivity are feasible.Comment: 7 pages LATEX, no figures, Awarded First Prize in the Gravity Research Foundation Essay Competition for 199

Liouville Cosmology

Liouville string theory is a natural framework for discussing the non-equilibrium evolution of the Universe. It enables non-critical strings to be treated in mathematically consistent manner, in which target time is identified with a world-sheet renormalization-group scale parameter, preserving target-space general coordinate invariance and the existence of an S-matrix. We review our proposals for a unified treatment of inflation and the current acceleration of the Universe. We link the current acceleration of the Universe with the value of the string coupling. In such a scenario, the dilaton plays an essential background role, driving the acceleration of the Universe during the present era after decoupling as a constant during inflation.Comment: 23 pages latex, 2 eps figures, contribution to the proceedings of the Dark 2004 conference, College Station, October 200

M-Theory Model-Building and Proton Stability

We study the problem of baryon stability in M theory, starting from realistic four-dimensional string models constructed using the free-fermion formulation of the weakly-coupled heterotic string. Suitable variants of these models manifest an enhanced custodial gauge symmetry that forbids to all orders the appearance of dangerous dimension-five baryon-decay operators. We exhibit the underlying geometric (bosonic) interpretation of these models, which have a $Z_2 \times Z_2$ orbifold structure similar, but not identical, to the class of Calabi-Yau threefold compactifications of M and F theory investigated by Voisin and Borcea. A related generalization of their work may provide a solution to the problem of proton stability in M theory.Comment: 14 pages. Standard Late

Testing Quantum Mechanics in the Neutral Kaon System

The neutral kaon system is a sensitive probe of quantum mechanics. We revive a parametrization of non-quantum-mechanical effects that is motivated by considerations of the nature of space-time foam, and show how it can be constrained by new measurements of $K_L \rightarrow 2\pi$ and $K_{L,S}$ semileptonic decays at LEAR or a $\phi$ factory.Comment: 10 page

Exclusive electroproduction revisited: treating kinematical effects

Generalized parton distributions of the nucleon are accessed via exclusive leptoproduction of the real photon. While earlier analytical considerations of phenomenological observables were restricted to twist-three accuracy, i.e., taking into account only terms suppressed by a single power of the hard scale, in the present study we revisit this differential cross section within the helicity formalism and restore power-suppressed effects stemming from the process kinematics exactly. We restrict ourselves to the phenomenologically important case of lepton scattering off a longitudinally polarized nucleon, where the photon flips its helicity at most by one unit.Comment: 22 pages, 1 figur

The String Universe: High TcT_c Superconductor or Quantum Hall Conductor?

Our answer is the latter. Space-time singularities, including the initial one, are described by world-sheet topological Abelian gauge theories with a Chern-Simons term. Their effective $N=2$ supersymmetry provides an initial fixed point where the Bogomolny bound is saturated on the world-sheet, corresponding to an extreme Reissner-Nordstrom solution in space-time. Away from the singularity the gauge theory has world-sheet matter fields, bosons and fermions, associated with the generation of target space-time. Because the fermions are complex (cf the Quantum Hall Effect) rather than real (cf high-$T_c$ superconductors) the energetically-preferred vacuum is not parity or time-reversal invariant, and the associated renormalization group flow explains the cosmological arrow of time, as well as the decay of real or virtual black holes, with a monotonic increase in entropy.Comment: 19 page

Cosmic Acceleration and the String Coupling

In the context of a cosmological string model describing the propagation of strings in a time-dependent Robertson-Walker background space-time, we show that the asymptotic acceleration of the Universe can be identified with the square of the string coupling. This allows for a direct measurement of the ten-dimensional string coupling using cosmological data. We conjecture that this is a generic feature of a class of non-critical string models that approach asymptotically a conformal (critical) sigma model whose target space is a four-dimensional space-time with a dilaton background that is linear in sigma-model time. The relation between the cosmic acceleration and the string coupling does not apply in critical strings with constant dilaton fields in four dimensions.Comment: 16 page

A Non-Critical String Approach to Black Holes, Time and Quantum Dynamics

We review our approach to time and quantum dynamics based on non-critical string theory, developing its relationship to previous work on non-equilibrium quantum statistical mechanics and the microscopic arrow of time. We exhibit specific non-factorizing contributions to the {\nd S} matrix associated with topological defects on the world sheet, explaining the r\^ole that the leakage of ${W_{\infty}}$ charges plays in the loss of quantum coherence. We stress the analogy with the quantum Hall effect, discuss the violation of $CPT$, and also apply our approach to cosmology.Comment: CERN-TH.7195/94, 54 pages (references on Lie-admissibility added