A renormalization group analysis of turbulent transport

The field-theoretic renormalization group is used to derive scaling relations for the transport of passive scalars by an incompressible velocity field with a specified energy spectrum. Results are obtained with the analog of the expansion of critical phenomena and compared to exact results which are available for shear flows in two dimensions.A 1/N expansion is proposed for the regions in which the expansion fails.     

A one-loop renormalization group analysis of the Gelmini-Roncadelli model

A one-loop renormalization group analysis is presented of the Gelmini-Roncadelli model of neutrino mass generation via an extended Higgs sector. We are unable to find values for the quartic scalar couplings at the W mass scale which cause the combined Higgs-gauge couplings to evolve to a stable fixed point of the renormalization group. As a consequuence this model may well be “trivial” in the same sense as λφ⁴ theory is believed to be in four dimensions.     

Analytic solution of the renormalization equations of the Kosterlitz-Thouless transition

The Kosterlitz-Thouless renormalization equations are shown to have an analytic solution, for the whole range of their validity. The solution enables us to express one of the previously unknown parameters used for flux-flow resistance in thin film superconductors, in term of the parameters of the Kosterlitz-Thouless transition.     

The non-perturbative renormalization group in the ordered phase

We study some analytical properties of the solutions of the non-perturbative renormalization group flow equations for a scalar field theory with Z₂ symmetry in the ordered phase, i.e. at temperatures below the critical temperature. The study is made in the framework of the local potential approximation. We show that the required physical discontinuity of the magnetic susceptibility χ(M) at M=±M₀ (M₀ spontaneous magnetization) is reproduced only if the cut-off function which separates high and low energy modes satisfies to some restrictive explicit mathematical conditions; we stress that these conditions are not satisfied by a sharp cut-off in dimensions of space d<4.By generalizing a method proposed earlier by Bonanno and Lacagnina [Nucl. Phys. B 693 (2004) 36] to any kind of cut-off we propose to solve numerically the renormalization group flow equations for the threshold functions rather than for the local potential. It yields an algorithm sufficiently robust and precise to extract universal as well as non-universal quantities from numerical experiments at any temperature, in particular at sub-critical temperatures in the ordered phase. Numerical results obtained for the φ⁴ potential with three different cut-off functions are reported and compared. The data confirm our theoretical predictions concerning the analytical behavior of χ(M) at M=±M₀.Fixed point solutions of the adimensioned renormalization group flow equations are also obtained in the same vein, that is by solving the fixed points equations and the associated eigenvalue problem for the threshold functions rather than for the potential. We report high precision data for the odd and even spectra of critical exponents for different cut-offs obtained in this way.     

A renormalization group analysis of leading logarithms in ChPT

We give strong evidence that the linear sigma model at small external momenta is an effective theory for the leading logarithms of chiral perturbation theory. Based on this evidence an attempt is made to sum the leading logarithms of chiral perturbation theory to all orders. We illustrate why this summation nonetheless fails when one uses standard renormalization group techniques of renormalizable quantum field theories.     

A renormalization group analysis of correlation function series expansions

A new method, inspired by renormalization group ideas, is proposed for extracting information on critical behaviour from series expansions of correlation functions. For the two-dimensional spin $\text{1}\text{2}$XY model, the results suggest the existence of a line of critical points.     

Migdal renormalization group approach to deconfinement phase transition

The Migdal renormalization group approach is applied to a finite temperature lattice gauge theory. Imposing the periodic boundary condition in the timelike orientation, the phase structure of the finite temperature lattice gauge system with a gauge groupG in (d+1)-dimensional space is determined by two kinds of recursion equations, describing spacelike and timelike correlations, respectively. One is the recursion equation for ad-dimensional gauge system with the gauge groupG, and the other corresponds to ad-dimensional spin system for which the effective theory is described by the nearest neighbor interaction of the Wilson lines. Detailed phase structure is investigated for theSU(2) gauge theory in (3+1)-dimensional space. Deconfinement phase transition is obtained. Using the recursion equation for the three dimensional spin system of the Wilson lines, it is shown that the flow of the renormalization group trajectories leads to a phase transition of the three dimensional Ising model.     

A renormalization group analysis of correlation functions for the dipole gas

We develop a renormalization group method for analyzing the generating functional for charge correlations of a dilute classical dipole gas. It is based on and extends the renormalization group analysis introduced by Brydges and Yau for the dipole gas partition function. Our method leads to systematic formulas for the large-distance behavior of correlation functions of all orders. We prove that in any dimensiond2, at any value>0 of the inverse temperature, and at sufficiently small activityz, the correlation functions exhibit at large distances the same behavior as for a vacuum (z=0), but with a new dielectric constant 1+ over which we have good control. The results proved here extend existing results on the two-point correlations to all higher correlations, and constitute a general confirmation of the fact that dipoles do not screen.     

Self-dual renormalization group analysis of the Potts models

We study the Potts models P(N) in 1 + 1 dimensions using a self-dual renormalization-group (RG) approach. The method is very successful in calculating the critical index ν. For N > 4 a pseudocritical behaviour is obtained.     

A study of the character of the phase transition to the superconducting phase by the renormalization group method

This study deals with phase transitions in magnetic systems when spin fluctuations and electronphonon interaction exchange enhancement occur. It is shown that in this context fluctuations play a substantial role and determine the character of the first-order phase transition that is close to a second-order transition.     

A renormalization group analysis of lattice models of two-dimensional membranes

We study lattice models of two-dimensional membranes of interest in statistical physics. The energy functional of a membrane is expressed as a sum of terms proportional to the surface area of the membrane, an extrinsic curvature and an intrinsic curvature quantity, respectively, but we neglect excluded volume effects. We introduce a renormalization transformation for these models which preserves the form of the energy functional, up to nonlocal terms. Our renormalization group construction is used to analyze the phase diagram and the different critical regimes of our models. We find evidence for a crumpling transition, separating a regime where surfaces are crystalline from one where the surfaces collapse to branched polymers, and we find a third genuine random-surface regime.     

Singular renormalization group transformations and first order phase transitions (II). Monte Carlo renormalization group results

The first order phase transitions in the two-dimensional 10-state Potts model and in the two-dimensional Ising model with magnetic field are studied with Monte Carlo renormalization group methods. The deconfining phase transition of the four-dimensional U(1) lattice gauge theory is treated similarly. The results are not consistent with the standard discontinuity fixed point picture of first order phase transitions. In the U(1) case, where this possibility exists, they are not consistent with a second order phase transition either. The results show a discontinuous flow on the first order transition surface, which is a Monte Carlo renormalization group signal of singular RG transformations.     

Normalizing the renormalization group analysis of deep inelastic leptoproduction

The nucleon matrix elements of the leading twist non-singlet operators which arise in the standard QCD analysis of leptoproduction are computed in an improved version of the bag model. QCD radiative corrections are used to evolve the bag predictions which are applicable at a low value of Q² to a high Q² regime where they can be compared with moments of non-singlet structure functions. The agreement with data is good and suggests that higher twist effects are not large.     

Monte Carlo renormalization group study of first order phase transitions

The two-lattice matching MCRG method is used to study first order phase transitions. While the method gives the critical exponent in agreement with the predicted value 1/d, where d is the dimension of the system, it indicates discontinuous flows on the phase transition surface.     

Competing interactions, the renormalization group, and the isotropic-nematic phase transition

We discuss 2D systems with Ising symmetry and competing interactions at different scales. In the framework of the renormalization group, we study the effect of relevant quartic interactions. In addition to the usual constant interaction term, we analyze the effect of quadrupole interactions in the self-consistent Hartree approximation. We show that in the case of a repulsive quadrupole interaction, there is a first-order phase transition to a stripe phase in agreement with the well-known Brazovskii result. However, in the case of attractive quadrupole interactions there is an isotropic-nematic second-order transition with higher critical temperature.     

A new proposal for the determination of the renormalization group trajectories by the Monte Carlo renormalization group method

We present a new method for calculating block renormalized couplings by Monte Carlo renormalization group. This method has several advantages with respect to the existing ones and can be applied for any value of the coupling constants. A preliminary numerical study of the 2-dimensional O(3) non linear σ-model is also presented.     

A renormalization group with periodic behaviour

An explicit example of a renormalization group with periodic behaviour is constructed and analyzed using both truncated recurrence relations and direct numerical computations. This renormalization procedure arises in the context of transition to turbulence.