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N. E. Mavromatos

Publications and source records attributed to N. E. Mavromatos.

At least 109 records · Page 6Linked to original sources
arXiv1999

Non-linear Dynamics in QED_3 and Non-trivial Infrared Structure

In this work we consider a coupled system of Schwinger-Dyson equations for self-energy and vertex functions in QED_3. Using the concept of a semi-amputated vertex function, we manage to decouple the vertex equation and transform it in the infrared into a non-linear differential equation of Emden-Fowler type. Its solution suggests the following picture: in the absence of infrared cut-offs there is only a trivial infrared fixed-point structure in the theory. However, the presence of masses, for either fermions or photons, changes the situation drastically, leading to a mass-dependent non-trivial infrared fixed point. In this picture a dynamical mass for the fermions is found to be generated consistently. The non-linearity of the equations gives rise to highly non-trivial constraints among the mass and effective (`running') gauge coupling, which impose lower and upper bounds on the latter for dynamical mass generation to occur. Possible implications of this to the theory of high-temperature superconductivity are briefly discussed.

hep-th↗
arXiv1999

Quantum-Gravitational Diffusion and Stochastic Fluctuations in the Velocity of Light

We argue that quantum-gravitational fluctuations in the space-time background give the vacuum non-trivial optical properties that include diffusion and consequent uncertainties in the arrival times of photons, causing stochastic fluctuations in the velocity of light ``in vacuo''. Our proposal is motivated within a Liouville string formulation of quantum gravity that also suggests a frequency-dependent refractive index of the particle vacuum. We construct an explicit realization by treating photon propagation through quantum excitations of $D$-brane fluctuations in the space-time foam. These are described by higher-genus string effects, that lead to stochastic fluctuations in couplings, and hence in the velocity of light. We discuss the possibilities of constraining or measuring photon diffusion ``in vacuo'' via $γ$-ray observations of distant astrophysical sources.

gr-qc↗
arXiv1999

On dynamical mass generation in three dimensional supersymmetric U(1) gauge field theory

We investigate and contrast the non-perturbative infra red structure of N=1 and N=2 supersymmetric non-compact U(1) gauge field theory in three space-time dimensions with N matter flavours. We study the Dyson-Schwinger equations in a general gauge using superfield formalism; this ensures that supersymmetry is kept manifest, though leads to spurious infra red divergences which we have to avoid carefully. In the N=1 case the superfield formalism allows us to choose a vertex which satisfies the U(1) Ward identity exactly, and we find the expected critical behaviour in the wavefunction renormalization and strong evidence for the existence of a gauge independent dynamically generated mass, but with no evidence for a critical flavour number. We study the N=2 model by dimensional reduction from four dimensional N=1 electrodynamics, and we refine the old gauge dependence argument that there is no dynamical mass generation. We recognize that the refinement only holds after dimensional reduction.

hep-th↗
arXiv1999

Comments on CP, T and CPT Violation in Neutral Kaon Decays

We comment on CP, T and CPT violation in the light of interesting new data from the CPLEAR and KTeV Collaborations on neutral kaon decay asymmetries. Other recent data from the CPLEAR experiment, constraining possible violations of CPT and the $ΔS = ΔQ$ rule, exclude the possibility that the semileptonic-decay asymmetry $A_{T}$ measured by CPLEAR could be solely due to CPT violation, confirming that their data constitute direct evidence for T violation. The CP-violating asymmetry in $K_L \to e^-e^+π^-π^+$ recently measured by the KTeV Collaboration does not by itself provide direct evidence for T violation, but we use it to place new bounds on CPT violation.

hep-ph↗
arXiv1999

Tests of the Equivalence Principle with Neutral Kaons

We test the Principle of Equivalence for particles and antiparticles, using CPLEAR data on tagged K0 and K0bar decays into pi^+ pi^-. For the first time, we search for possible annual, monthly and diurnal modulations of the observables |eta_{+-}| and phi_{+-}, that could be correlated with variations in astrophysical potentials. Within the accuracy of CPLEAR, the measured values of |eta_{+-}| and phi_{+-} are found not to be correlated with changes of the gravitational potential. We analyze data assuming effective scalar, vector and tensor interactions, and we conclude that the Principle of Equivalence between particles and antiparticles holds to a level of 6.5, 4.3 and 1.8 x 10^{-9}, respectively, for scalar, vector and tensor potentials originating from the Sun with a range much greater than the distance Earth-Sun. We also study energy-dependent effects that might arise from vector or tensor interactions. Finally, we compile upper limits on the gravitational coupling difference between K0 and K0bar as a function of the scalar, vector and tensor interaction range.

hep-ex↗
arXiv1999

On `Graceful Exit' from inflationary phase in two-dimensional Liouville String Cosmology

Within the context of a super-critical (Liouville) string, we discuss (target-space) two-dimensional string cosmology. A numerical analysis indicates that the identification of time with the Liouville mode results in an expanding universe with matter which exhibits an inflationary phase, and `graceful exit' from it, tending asymptotically to a flat-metric fixed point.This fixed point is characterized by a dilaton configuration which, depending on the initial conditions, either decreases linearly with the cosmic time, or is a finite constant. This implies that, in contrast to the critical string case, the string coupling remains bounded during the exit from the inflationary phase, and, thus, the pertinent dynamics can be reliably described in terms of a tree-level string effective action. The rôle of matter in inducing such phenomena is emphasized. It is also interesting to note that the asymptotic value of the vacuum energy, which in the $σ$-model framework is identified with the `running' central charge deficit, depends crucially on the set of initial conditions. Thus, although preliminary, this toy model seems to share all the features expected to characterize a phenomenologically acceptable cosmological string model.

hep-th↗
arXiv1999

On the Thermodynamics of a Gas of AdS Black Holes and the Quark-Hadron Phase Transition

We discuss the thermodynamics of a gas of black holes in five-dimensional anti-de-Sitter (AdS) space, showing that they are described by a van der Waals equation of state. Motivated by the Maldacena conjecture, we relate the energy density and pressure of this non-ideal AdS black-hole gas to those of four-dimensional gauge theory in the unconfined phase. We find that the energy density rises rapidly above the deconfinement transition temperature, whilst the pressure rises more slowly towards its asymptotic high-temperature value, in qualitative agreement with lattice simulations.

hep-th↗
arXiv1999

Effective Gauge Theories, The Renormalization Group, and High-Temperature Superconductivity

These lectures serve as an introduction to the renormalization group approach to effective field theories, with emphasis on systems with a Fermi surface. For such systems, demanding appropriate scaling with respect to the renormalization group for the appropriate excitations leads directly to the important concept of quasiparticles and the connexion between large-N_f treatments and renormalization group running in theory space. In such treatments N_f denotes the number of effective fermionic degrees of freedom above the Fermi surface; this number is roughly proportional to the size of the Fermi surface. As an application of these ideas, non-trivial infra red structure in three dimensional U(1) gauge theory is discussed, along with applications to the normal phase physics of high-T_c superconductors, in an attempt to explain the experimentally observed deviations from Fermi liquid behaviour. Specifically, the direct current resistivity of the theory is computed at finite temperatures, $T$, and is found to acquire $O(1/N_f)$ corrections to the linear $T$ behaviour. Such scaling corrections are consistent with recent experimental observations in high T_c superconducting cuprates.

cond-mat↗
arXiv1998

Sensitivity of Astrophysical Observations to Gravity-Induced Wave Dispersion in Vacuo

We discuss possible signatures of quantum gravity for the propagation of light, including an energy-dependent velocity (refractive index), dispersion in velocity at a given energy, and birefringence. We also compare the sensitivities of different astrophysical observations, including BATSE data on GRB 920229, BeppoSAX data on GRB 980425, the possible HEGRA observation of GRB 920925c, and Whipple observations of the active galaxy Mrk 421. Finally, we discuss the prospective sensitivities of AMS and GLAST.

astro-ph↗
arXiv1998

Time-Dependent Vacuum Energy Induced by D-Particle Recoil

We consider cosmology in the framework of a `material reference system' of D particles, including the effects of quantum recoil induced by closed-string probe particles. We find a time-dependent contribution to the cosmological vacuum energy, which relaxes to zero as $\sim 1/ t^2$ for large times $t$. If this energy density is dominant, the Universe expands with a scale factor $R(t) \sim t^2$. We show that this possibility is compatible with recent observational constraints from high-redshift supernovae, and may also respect other phenomenological bounds on time variation in the vacuum energy imposed by early cosmology.

gr-qc↗
arXiv1998

Confinement in Gauge Theories from the Condensation of World-Sheet Defects in Liouville String

We present a Liouville-string approach to confinement in four-dimensional gauge theories, which extends previous approaches to include non-conformal theories. We consider Liouville field theory on world sheets whose boundaries are the Wilson loops of gauge theory, which exhibit vortex and spike defects. We show that world-sheet vortex condensation occurs when the Wilson loop is embedded in four target space-time dimensions, and show that this corresponds to the condensation of gauge magnetic monopoles in target space. We also show that vortex condensation generates a effective string tension corresponding to the confinement of electric degrees of freedom. The tension is independent of the string length in a gauge theory whose electric coupling varies logarithmically with the length scale. The Liouville field is naturally interpreted as an extra target dimension, with an anti-de-Sitter (AdS) structure induced by recoil effects on the gauge monopoles, interpreted as D branes of the effective string theory. Black holes in the bulk AdS space correspond to world-sheet defects, so that phases of the bulk gravitational system correspond to the different world-sheet phases, and hence to different phases of the four-dimensional gauge theory. Deconfinement is associated with a Berezinskii-Kosterlitz-Thouless transition of vortices on the Wilson-loop world sheet, corresponding in turn to a phase transition of the black holes in the bulk AdS space.

hep-th↗
arXiv1998

High-Energy QCD as a Topological Field Theory

We propose an identification of the conformal field theory underlying Lipatov's spin-chain model of high-energy scattering in perturbative QCD. It is a twisted N=2 supersymmetric topological field theory, which arises as the limiting case of the SL(2,R)/U(1) non-linear sigma model that also plays a role in describing the Quantum Hall effect and black holes in string theory. The doubly-infinite set of non-trivial integrals of motion of the high-energy spin-chain model displayed by Faddeev and Korchemsky are identified as the Cartan subalgebra of a W_{\infty} \otimes W_{\infty} bosonic sub-symmetry possessed by this topological theory. The renormalization group and an analysis of instanton perturbations yield some understanding why this particular topological spin-chain model emerges in the high-energy limit, and provide a new estimate of the asymptotic behaviour of multi-Reggeized-gluon exchange.

hep-ph↗
arXiv1998

Phase Structure of lattice SU(2)xU_S(1) three-dimensional Gauge Theory

We discuss a phase diagram for a relativistic SU(2) x U_{S}(1) lattice gauge theory, with emphasis on the formation of a parity-invariant chiral condensate, in the case when the $U_{S}(1)$ field is infinitely coupled, and the SU(2) field is moved away from infinite coupling by means of a strong-coupling expansion. We provide analytical arguments on the existence of (and partially derive) a critical line in coupling space, separating the phase of broken SU(2) symmetry from that where the symmetry is unbroken. We review uncoventional (Kosterlitz-Thouless type) superconducting properties of the model, upon coupling it to external electromagnetic potentials. We discuss the rôle of instantons of the unbroken subgroup U(1) of SU(2), in eventually destroying superconductivity under certain circumstances. The model may have applications to the theory of high-temperature superconductivity. In particular, we argue that in the regime of the couplings leading to the broken SU(2) phase, the model may provide an explanation on the appearance of a pseudo-gap phase, lying between the antiferromagnetic and the superconducting phases. In such a phase, a fermion mass gap appears in the theory, but there is no phase coherence, due to the Kosterlitz-Thouless mode of symmetry breaking. The absence of superconductivity in this phase is attributed to non-perturbative effects (instantons) of the subgroup U(1) of SU(2).

hep-lat↗
arXiv1998

Probing the Gauge Structure of high-temperature superconductors

We suggest that a spin-charge separating ansatz, leading to non-Abelian $SU(2) \otimes U_S(1)$ gauge symmetries in doped antiferromagnets, proposed earlier as a way of describing Kosterlitz-Thouless superconducting gaps at the nodes of the gap of d-wave (high-T_c) superconductors, may also lead to a pseudogap phase, characterised by the formation of (non-superconducting) pairing and the absence of phase coherence. The crucial assumption is again the presence of electrically charged Dirac fermionic excitations (holons) about the points of the (putative) fermi surface in the pertinent phase of the superconductor. We present arguments in support of the rôle of non-perturbative effects (instantons) on the onset of the pseudogap phase. As a means of probing such gauge interactions experimentally, we perform a study of the scaling of the thermal conductivity with an externally-applied magnetic field, in certain effective models involving gauge and/or four-fermion (contact) interactions.

cond-mat↗
arXiv1998

A Microscopic Liouville Arrow of Time

We discuss the treatment of quantum-gravitational fluctuations in the space-time background as an `environment', using the formalism for open quantum-mechanical systems, which leads to a microscopic arrow of time. After reviewing briefly the open-system formalism, and the motivations for treating quantum gravity as an `environment', we present an example from general relativity and a general framework based on non-critical strings, with a Liouville field that we identify with time. We illustrate this approach with calculations in the contexts of two-dimensional models and $D$ branes. Finally, some prospects for observational tests of these ideas are mentioned.

hep-th↗
arXiv1998

Potential Sensitivity of Gamma-Ray Burster Observations to Wave Dispersion in Vacuo

The recent confirmation that at least some gamma-ray bursters (GRBs) are indeed at cosmological distances raises the possibility that observations of these could provide interesting constraints on the fundamental laws of physics. Here we demonstrate that the fine-scale time structure and hard spectra of GRB emissions are very sensitive to the possible dispersion of electromagnetic waves in vacuo with velocity differences $δv \sim E/E_{\QG}$, as suggested in some approaches to quantum gravity. A simple estimate shows that GRB measurements might be sensitive to a dispersion scale $E_{QG}$ comparable to the Planck energy scale $E_{P} \sim 10^{19}$ GeV, sufficient to test some of these theories, and we outline aspects of an observational programme that could address this goal.

astro-ph↗
arXiv1998

World-Sheet formulation of M theory

We first review the interpretation of world-sheet defects as $D$ branes described by a critical theory in 11 dimensions, that we interpret as $M$ theory. We then show that $D$-brane recoil induces dynamically an anti-de-Sitter (AdS) space-time background, with criticality restored by a twelfth time-like dimension described by a Liouville field. Local physics in the bulk of this AdS$_{11}$ may be described by an $Osp(1|32,R) \otimes Osp(1|32,R)$ topological gauge theory (TGT), with non-local boundary states in doubleton representations. We draw analogies with structures previously exhibited in two-dimensional black-hole models. Wilson loops of `matter' in the TGT may be described by an effective string action, and defect condensation may yield string tension and cause a space-time metric to appear.

hep-th↗