arXiv subjects

Paolo Rossi

Publications and source records attributed to Paolo Rossi.

89 records · Page 5Linked to original sources

arXiv1996

A strong-coupling analysis of two-dimensional O(N) sigma models with N<=2 on square, triangular and honeycomb lattices

The critical behavior of two-dimensional ${\rm O}(N)$ $σ$ models with $N\leq 2$ on the square, triangular, and honeycomb lattices is investigated by an analysis of the strong-coupling expansion of the two-point fundamental Green's function $G(x)$, calculated up to 21st order on the square lattice, 15th order on the triangular lattice, and 30th order on the honeycomb lattice. For $N<2$ the critical behavior is of power-law type, and the exponents $γ$ and $ν$ extracted from our strong-coupling analysis confirm exact results derived assuming universality with solvable solid-on-solid models. At $N=2$, i.e., for the 2-$d$ XY model, the results from all lattices considered are consistent with the Kosterlitz-Thouless exponential approach to criticality, characterized by an exponent $σ=1/2$, and with universality. The value $σ=1/2$ is confirmed within an uncertainty of few per cent. The prediction $η=1/4$ is also roughly verified. For various values of $N\leq 2$, we determine some ratios of amplitudes concerning the two-point function $G(x)$ in the critical limit of the symmetric phase. This analysis shows that the low-momentum behavior of $G(x)$ in the critical region is essentially Gaussian at all values of $N\leq 2$. New exact results for the long-distance behavior of $G(x)$ when $N=1$ (Ising model in the strong-coupling phase) confirm this statement.

hep-lat↗

arXiv1996

A strong-coupling analysis of two-dimensional O(N) sigma models with $N\geq 3$ on square, triangular and honeycomb lattices

Recently-generated long strong-coupling series for the two-point Green's functions of asymptotically free ${\rm O}(N)$ lattice $σ$ models are analyzed, focusing on the evaluation of dimensionless renormalization-group invariant ratios of physical quantities and applying resummation techniques to series in the inverse temperature $β$ and in the energy $E$. Square, triangular, and honeycomb lattices are considered, as a test of universality and in order to estimate systematic errors. Large-$N$ solutions are carefully studied in order to establish benchmarks for series coefficients and resummations. Scaling and universality are verified. All invariant ratios related to the large-distance properties of the two-point functions vary monotonically with $N$, departing from their large-$N$ values only by a few per mille even down to $N=3$.

hep-lat↗

arXiv1995

Running coupling constant and correlation length from Wilson loops

We consider a definition of the QCD running coupling constant $α(μ)$ related to Wilson loops of size $r{\times}t$ with arbitrary fixed $t/r$. The schemes defined by these couplings are very close to the $\overline{\rm MS}$ scheme for all values of $t/r$; in the $t/r\to\infty$ limit, the ``$q\bar q$ force'' scheme is recovered. We propose a definition of correlation length, also related to Wilson loops, which can be applied to the Monte Carlo evaluation of $α(μ)$ up to very large momentum scales by use of finite-size scaling techniques.

hep-lat↗

arXiv1995

Strong-coupling expansion of lattice O(N) sigma models

We report progress in the computation and analysis of strong-coupling series of two- and three-dimensional ${\rm O}(N)$ $σ$ models. We show that, through a combination of long strong-coupling series and judicious choice of observables, one can compute continuum quantities reliably and with a precision at least comparable with the best available Monte Carlo data.

hep-lat↗

arXiv1995

Application of the $O(N)$-Hyperspherical Harmonics to the Study of the Continuum Limits of One-Dimensional $σ$-Models and to the Generation of High-Temperature Expansions in Higher Dimensions

In this talk we present the exact solution of the most general one-dimensional $O(N)$-invariant spin model taking values in the sphere $S^{N-1}$, with nearest-neighbour interactions, and we discuss the possible continuum limits. All these results are obtained using a high-temperature expansion in terms of hyperspherical harmonics. Applications in higher dimensions of the same technique are then discussed.

hep-lat↗

arXiv1995

Strong-coupling expansion of chiral models

The strong-coupling character expansion of lattice models is reanalyzed in the perspective of its complete algorithmization. The geometric problem of identifying, counting, and grouping together all possible contributions is disentangled from the group-theoretical problem of weighting them properly. The first problem is completely solved for all spin models admitting a character-like expansion and for arbitrary lattice connectivity. The second problem is reduced to the evaluation of a class of invariant group integrals defined on simple graphs. Since these integrals only depend on the global symmetry of the model, results obtained for principal chiral models can be used without modifications in lattice gauge theories. By applying the techniques and results obtained we study the two-dimensional principal chiral models on the square and honeycomb lattice. These models are a prototype field theory sharing with QCD many properties. Strong-coupling expansions for Green's functions are derived up to 15th and 20th order respectively. Large-$N$ and $N=\infty$ results are presented explicitly. Related papers are devoted to a discussion of the results.

hep-lat↗

arXiv1994

Running coupling constant and correlation length from Wilson loops

We consider a definition of the QCD running coupling constant $α(μ)$ related to Wilson loops of size $r{\times}t$ with arbitrary fixed $t/r$. The schemes defined by these couplings are very close to the $\overline{\rm MS}$ scheme (i.e.\ the one-loop perturbative correction to the coupling is small) for all values of $t/r$; in the $t/r\to\infty$ limit, the ``$q\bar q$ force'' scheme is recovered, where the coupling constant is related to the quark-antiquark force. We discuss the possibility of applying finite-size scaling techniques to the Monte Carlo evaluation of $α(μ)$ up to very large momentum scales. We propose a definition of correlation length, also related to Wilson loops, which should make such a computation feasible.

hep-lat↗

arXiv1994

Simplicial Chiral Models

Principal chiral models on a d-1 dimensional simplex are introduced and studied analytically in the large $N$ limit. The $d = 0, 2, 4$ and $\infty$ models are explicitly solved. Relationship with standard lattice models and with few-matrix systems in the double scaling limit are discussed.

hep-th↗

arXiv1994

Comment on A. Patrascioiu and E. Seiler's paper ``Nonuniformity of the $1/N$ Expansion for Two-Dimensional $O(N)$ Models''

We remind that the non-uniformity in the temperature of the $1/N$ expansion for dimensionful quantities pointed out in Patrascioiu and Seiler's paper is not only compatible with but is predicted by asymptotic freedom, and it is present in the continuum as well as on the lattice. The convergence of the $1/N$ expansion for adimensional quantities is perfectly uniform.

hep-lat↗

arXiv1994

Asymptotic scaling from strong coupling

Strong-coupling analysis of two-dimensional chiral models, extended to 15th order, allows for the identification of a scaling region where known continuum results are reproduced with great accuracy, and asymptotic scaling predictions are fulfilled. The properties of the large-$N$ second-order phase transition are quantitatively investigated.

hep-lat↗

arXiv1994

Two dimensional SU(N)xSU(N) Chiral Models on the Lattice (II): the Green's Function

Analytical and numerical methods are applied to principal chiral models on a two-dimensional lattice and their predictions are tested and compared. New techniques for the strong coupling expansion of SU(N) models are developed and applied to the evaluation of the two-point correlation function. The momentum-space lattice propagator is constructed with precision O(β^{10}) and an evaluation of the correlation length is obtained for several different definitions. Three-loop weak coupling contributions to the internal energy and to the lattice $β$ and $γ$ functions are evaluated for all N, and the effect of adopting the ``energy'' definition of temperature is computed with the same precision. Renormalization-group improved predictions for the two-point Green's function in the weak coupling ( continuum ) regime are obtained and successfully compared with Monte Carlo data. We find that strong coupling is predictive up to a point where asymptotic scaling in the energy scheme is observed. Continuum physics is insensitive to the effects of the large N phase transition occurring in the lattice model. Universality in N is already well established for $N \ge 10$ and the large N physics is well described by a ``hadronization'' picture.

hep-lat↗

arXiv1993

Two dimensional SU(N) x SU(N) chiral models on the lattice

Lattice $SU(N)\times SU(N)$ chiral models are analyzed by strong and weak coupling expansions and by numerical simulations. $12^{th}$ order strong coupling series for the free and internal energy are obtained for all $N\geq 6$. Three loop contributions to the internal energy and to the lattice $β$-function are evaluated for all $N$ and non-universal corrections to the asymptotic $Λ$ parameter are computed in the ``temperature'' and the ``energy'' scheme. Numerical simulations confirm a faster approach to asymptopia of the energy scheme. A phenomenological correlation between the peak in the specific heat and the dip of the $β$-function is observed. Tests of scaling are performed for various physical quantities, finding substantial scaling at $ξ\gtrsim 2$. In particular, at $N=6$ three different mass ratios are determined numerically and found in agreement, within statistical errors of about 1\%, with the theoretical predictions from the exact S-matrix theory.

hep-lat↗

arXiv1993

The $1/N$ expansion of two-dimensional spin models

A general two-dimensional spin model with U$(N)$ invariance, interpolating between $\CPN$ and ${\rm O}(2N)$ models, is studied in detail in order to illustrate both the general features of the $1/N$ expansion on the lattice and the specific techniques devised to extract scaling (field-theoretical) behavior. The continuum version of the model is carefully analyzed deriving quantitative $O(1/N)$ physical predictions in order to establish a benchmark for lattice computations. The $1/N$ expansion on the lattice, including second-nearest-neighbor interactions, is set up by constructing explicitly effective propagators and vertices. The technique of asymptotic expansion of the lattice propagators, basic to the derivation of analytical results in the scaling domain, is presented in full detail and applied to the model. Physical quantities, like the free energy and different definitions of correlation length, are evaluated. The lattice renormalization-group trajectories are identified and universality among different lattice (and continuum) schemes in the scaling region is explicitly proven. A review of other developments based on the lattice $1/N$ expansion is presented.

hep-lat↗

arXiv1993

Finite size scaling in CP(N-1) models

Finite size effects in Euclidean ${\rm CP}^{N-1}$ models with periodic boundary conditions are investigated by means of the $1/N$ expansion and by Monte Carlo simulations. Analytic and numerical results for magnetic susceptibility and correlation length are compared and found to agree for small volumes. For large volumes a discrepancy is found and explained as an effect of the physical bound state extension. The leading order finite size effects on the Abelian string tension are computed and compared with simulations finding agreement. Finite size dependence of topological quantities is also discussed.

hep-lat↗

arXiv1992

Scaling and asymptotic scaling in two-dimensional $CP^{N-1}$ models

Two-dimensional $CP^{N-1}$ models are investigated by Monte Carlo methods on the lattice, for values of $N$ ranging from 2 to 21. Scaling and rotation invariance are studied by comparing different definitions of correlation length $ξ$. Several lattice formulations are compared and shown to enjoy scaling for $ξ$ as small as $2.5$. Asymptotic scaling is investigated using as bare coupling constant both the usual $β$ and $β_E$ (related to the internal energy); the latter is shown to improve asymptotic scaling properties. Studies of finite size effects show their $N$-dependence to be highly non-trivial, due to the increasing radius of the $\bar z z$ bound states at large $N$.

hep-lat↗

arXiv1992

$1/N$ Expansion of Two-Dimensional Models in the Scaling Region

The main technical and conceptual features of the lattice $1/N$ expansion in the scaling region are discussed in the context of a two-parameter two-dimensional spin model interpolating between $CP^{N-1}$ and $O(2N)$ $σ$ models, with standard and improved lattice actions. We show how to perform the asymptotic expansion of effective propagators for small values of the mass gap and how to employ this result in the evaluation of physical quantities in the scaling regime. The lattice renormalization group $β$ function is constructed explicitly and exactly to $O({1/N})$.

hep-lat↗

arXiv1992

Topological susceptibility and string tension in CP(N-1) models

We investigate the features of ${\rm CP}^{N-1}$ models concerning confinement and topology. In order to study the approach to the large-$N$ asymptotic regime, we determine the topological susceptibility and the string tension for a wide range of values of $N$, in particular $N=4,10,21,41$. Quantitative agreement with the large-$N$ predictions is found for the ${\rm CP}^{20}$ and the ${\rm CP}^{40}$ models. Problems related to the measure of the topological susceptibility and the string tension on the lattice are discussed.

hep-lat↗