arXiv subjects

Jean Alexandre

Publications and source records attributed to Jean Alexandre.

At least 91 records · Page 5Linked to original sources

arXiv2006

Non-Perturbative Formulation of Time-Dependent String Solutions

We formulate here a new world-sheet renormalization-group technique for the bosonic string, which is non-perturbative in the Regge slope alpha' and based on a functional method for controlling the quantum fluctuations, whose magnitudes are scaled by the value of alpha'. Using this technique we exhibit, in addition to the well-known linear-dilaton cosmology, a new, non-perturbative time-dependent background solution. Using the reparametrization invariance of the string S-matrix, we demonstrate that this solution is conformally invariant to alpha', and we give a heuristic inductive argument that conformal invariance can be maintained to all orders in alpha'. This new time-dependent string solution may be applicable to primordial cosmology or to the exit from linear-dilaton cosmology at large times.

hep-th↗

arXiv2006

Control of quantum fluctuations for a Yukawa interaction in the Kaluza Klein picture

We study a system of fermions interacting with a scalar field, in 4+1 dimensions where the 5th dimension is compactified, using an exact functional method, where quantum fluctuations are controlled by the amplitude of the bare fermion mass. The integration of our equationsleads to the properties of the dressed Yukawa coupling, that we study at one-loop so as to show the consistency of the approach. Beyond one loop, the non-perturbative aspect of the method gives us the possibility to derive the dynamical fermion mass. The result obtained is cut off independent and this derivation proposes an alternative to the Schwinger-Dyson approach.

hep-ph↗

arXiv2006

A novel non-perturbative approach to String Cosmology

We develop an exact functional method applied to the bosonic string on a shperical world sheet, in graviton and dilaton backgrounds, consistent with conformal invariance. In this method, quantum fluctuations are controled by the amplitude of the kinetic term of the corresponding stringy sigma-model, and we exhibit a novel non-perturbative non-critical string configuration which appears as a fixed point of our evolution equation. We argue that this string configuration is an exact solution, valid to all orders in alpha', which is consistent with string scattering amplitudes. The dilaton configuration is logarithmic in terms of the string coordinate X^0, and the amplitude of the corresponding quantum fluctuations is independent of the target space dimension D; for D=4, the corresponding Universe, in the Einstein frame, is static and flat. A linearization around this fixed point leads to a slowly expanding, decelerating Universe, reaching asymptotically (in Einstein time) the Minkowski Universe. Moreover, the well-known linear (in terms of X^0) dilaton background, which is a trivial fixed point of our evolution equation, is recovered by our non trivial fixed point for early times. This feature explains the time evolution from a linearly expanding Universe to a Minkowski Universe.

hep-th↗

arXiv2005

Concepts of Renormalization in Physics

A non technical introduction to the concept of renormalization is given, with an emphasis on the energy scale dependence in the description of a physical system. We first describe the idea of scale dependence in the study of a ferromagnetic phase transition, and then show how similar ideas appear in Particle Physics. This short review is written for non-particle physicists and/or students aiming at studying Particle Physics.

physics.ed-ph↗

arXiv2005

A note on Liouville theory

An exact differential equation is derived for the evolution of the Liouville effective action with the mass parameter. This derivation is based on properties of the exponential potential and some consequences of the equation are discussed.

hep-th↗

arXiv2005

An alternative to exact renormalization equations

An alternative point of view to exact renormalization equations is discussed, where quantum fluctuations of a theory are controlled by the bare mass of a particle. The procedure is based on an exact evolution equation for the effective action, and recovers usual renormalization results.

hep-th↗

arXiv2004

Anomalous magnetic moment in parity-conserving QED3

In this article we derive the anomalous magnetic moment of fermions in (2+1)-dimensional parity-conserving QED3, in the presence of an externally applied constant magnetic field. We use a spectral representation of the photon propagator to avoid infrared divergences. We also discuss the scaling with the magnetic field intensity in the case of strong external fields, where there is dynamical mass generation for fermions induced by the magnetic field itself (magnetic catalysis). The results of this paper may be of relevance to the physics of high-temperature superconductors.

hep-ph↗

arXiv2003

Non-renormalization for planar Wess-Zumino model

Using a non-perturbative functional method, where the quantum fluctuations are gradually set up,it is shown that the interaction of a N=1 Wess-Zumino model in 2+1 dimensions does not get renormalized. This result is valid in the framework of the gradient expansion and aims at compensating the lack of non-renormalization theorems.

hep-th↗

arXiv2003

An alternative approach to dynamical mass generation in QED3

Some quantum properties of QED3 are studied with the help of an exact evolution equation of the effective action with the bare fermion mass. The resulting effective theory and the occurrence of a dynamical mass are discussed in the framework of the gradient expansion.

hep-th↗

arXiv2003

A control on quantum fluctuations in 2+1 dimensions

A functional method is discussed, where the quantum fluctuations of a theory are controlled by a mass parameter and the evolution of the theory with this parameter is connected to its renormalization. It is found, in the framework of the gradient expansion, that the coupling constant of a N=1 Wess-Zumino theory in 2+1 dimensions does not get quantum corrections.

hep-th↗

arXiv2003

From N=1 to N=2 supersymmetries in 2+1 dimensions

Starting from N=1 scalar and vector supermultiplets in 2+1 dimensions, we construct superfields which constitute Lagrangians invariant under N=2 supersymmetries. We first recover the N=2 supersymmetric Abelian-Higgs model and then the N=2 pure super Yang-Mills model. The conditions for this elevation are consistent with previous results found by other authors.

hep-th↗

arXiv2002

Composite Supersymmetries in low-dimensional systems

Starting from a N=1 scalar supermultiplet in 2+1 dimensions, we demonstrate explicitly the appearance of induced N=1 vector and scalar supermultiplets of composite operators made out of the fundamental supersymmetric constituents. We discuss an extension to a N=2 superalgebra with central extension, due to the existence of topological currents in 2+1 dimensions. As a specific model we consider a supersymmetric $CP^1$ $σ$-model as the constituent theory, and discuss the relevance of these results for an effective description of the infrared dynamics of planar high-temperature superconducting condensed matter models with quasiparticle excitations near nodal points of their Fermi surface.

hep-th↗

arXiv2001

Radiatively-induced Magnetic moment in four-dimensional anisotropic QED in an external magnetic field

We discuss one-loop radiatively-induced magnetic moment in four-dimensional quantum electrodynamics (QED) with anisotropic coupling, and examine various cases which may be of interest in effective gauge theories of antiferromagnets, whose planar limit coresponds to highly anisotropic QED couplings. We find a different scaling with the magnetic field intensity in case there are extra statistical gauge interactions in the model with spontaneous symmetry breaking. Such a case is encountered in the CP-1 sigma-model sector of effective spin-charge separated gauge theories of antiferromagnetic systems. Our work provides therefore additional ways of possible experimental probing of the gauge nature of such systems.

hep-ph↗

arXiv2001

QED in external fields, a functional point of view

A functional partial differential equation is set for the proper graphs generating functional of QED in external electromagnetic fields. This equation leads to the evolution of the proper graphs with the external field amplitude and the external field gauge dependence of the complete fermion propagator and vertex is derived non-perturbativally.

hep-th↗

arXiv2001

Vacuum polarization in thermal QED with an external magnetic field

The one-loop vacuum polarization tensor is computed in QED with an external constant, homogeneous magnetic field at finite temperature. The Schwinger proper-time formalism is used and the computations are done in Euclidian space. The well-known results are recovered when the temperature and/or the magnetic field are switched off and the effect of the magnetic field on the Debye screening is discussed.

hep-th↗

arXiv2000

Functional Callan-Symanzik equation

We describe a functional method to obtain the exact evolution equation of the effective action with a parameter of the bare theory. When this parameter happens to be the bare mass of the scalar field, we find a functional generalization of the Callan-Symanzik equations. Another possibility is when this parameter is the Planck constant and controls the amplitude of the fluctuations. We show the similarity of these equations with the Wilsonian renormalization group flows and also recover the usual one loop effective action.

hep-th↗

arXiv2000

Renormalization group for the internal space

The renormalization group method is a successive integration over the fluctuations which are ordered according to their length scale, a parameter in the external space. A different procedure is described, where the fluctuations are treated in a successive manner, as well, but their order is given by an internal space parameter, their amplitude. The differential version of the renormalization group equation is given which is the functional generalization of the Callan-Symanzik equation in one special case and resums the loop expansion in another one.

hep-th↗

arXiv1999

Tree-level renormalization

It is shown in the framework of the N-component scalar model that the saddle point structure may generate non-trivial renormalization group flow. The spinodal phase separation can be described in this manner and a flat action is found as an exact result which is valid up to any order of the loop expansion. The correlation function is computed in a mean-field approximation.

hep-th↗