Universal ratios along a line of critical points. The Ashkin--Teller model

The two-dimensional Ashkin-Teller model provides the simplest example of a statistical system exhibiting a line of critical points along which the critical exponents vary continously. The scaling limit of both the paramagnetic and ferromagnetic phases separated by the critical line are described by the sine-Gordon quantum field theory in a given range of its dimensionless coupling. After computing the relevant matrix elements of the order and disorder operators in this integrable field theory, we determine the universal amplitude ratios along the critical line within the two-particle approximation in the form factor approach.Comment: 31 pages, late

Single-photon signals at LEP in supersymmetric models with a light gravitino

We study the single-photon signals expected at LEP in models with a very light gravitino. The dominant process is neutralino-gravitino production (e+e- -> chi+ G) with subsequent neutralino decay via chi->gamma+G, giving a gamma+E_miss signal. We first calculate the cross section at arbitrary center-of-mass energies and provide new analytic expressions for the differential cross section valid for general neutralino compositions. We then consider the constraints on the gravitino mass from LEP 1 and LEP161 single-photon searches, and possible such searches at the Tevatron. We show that it is possible to evade the stringent LEP 1 limits and still obtain an observable rate at LEP 2, in particular in the region of parameter space that may explain the CDF e+e+gamma+gamma+E_T,miss event. As diphoton events from neutralino pair-production would not be kinematically accessible in this scenario, the observation of whichever photonic signal will discriminate among the various light-gravitino scenarios in the literature. We also perform a Monte Carlo simulation of the expected energy and angular distributions of the emitted photon, and of the missing invariant mass expected in the events. Finally we specialize the results to the case of a recently proposed one-parameter no-scale supergravity model.Comment: 31 pages, LaTeX, 14 figures (included

Constructing a uniform plane-filling path in the ternary heptagrid of the hyperbolic plane

In this paper, we distinguish two levels for the plane-filling property. We consider a simple and a strong one. In this paper, we give the construction which proves that the simple plane-filling property also holds for the hyperbolic plane. The plane-filling property was established for the Euclidean plane by J. Kari, in 1994, in the strong version

Scaling and Crossover in the Large-N Model for Growth Kinetics

The dependence of the scaling properties of the structure factor on space dimensionality, range of interaction, initial and final conditions, presence or absence of a conservation law is analysed in the framework of the large-N model for growth kinetics. The variety of asymptotic behaviours is quite rich, including standard scaling, multiscaling and a mixture of the two. The different scaling properties obtained as the parameters are varied are controlled by a structure of fixed points with their domains of attraction. Crossovers arising from the competition between distinct fixed points are explicitely obtained. Temperature fluctuations below the critical temperature are not found to be irrelevant when the order parameter is conserved. The model is solved by integration of the equation of motion for the structure factor and by a renormalization group approach.Comment: 48 pages with 6 figures available upon request, plain LaTe

Fate of the classical false vacuum

Thermalisation of configurations with initial white noise power spectrum is studied in numerical simulations of a classical one-component $\Phi^4$ theory in 2+1 dimensions, coupled to a small amplitude homogenous external field. The study is performed for energy densities corresponding to the broken symmetry phase of the system in equilibrium. The effective equation of the order parameter motion is reconstructed from its trajectory which starts from an initial value near the metastable point and ends in the stable ground state. This phenomenological theory quantitatively accounts for the decay of the false vacuum. The large amplitude transition of the order parameter between the two minima displays characteristics reflecting dynamical aspects of the Maxwell construction.Comment: RevTeX 16 pages, 10 Postscript figures, version accepted in Phys. Rev.

Temporally disordered Ising models

We present a study of the influence of different types of disorder on systems in the Ising universality class by employing both a dynamical field theory approach and extensive Monte Carlo simulations. We reproduce some well known results for the case of quenched disorder (random temperature and random field), and analyze the effect of four different types of time-dependent disorder scarcely studied so far in the literature. Some of them are of obvious experimental and theoretical relevance (as for example, globally fluctuating temperatures or random fields). All the predictions coming from our field theoretical analysis are fully confirmed by extensive simulations in two and three dimensions, and novel qualitatively different, non-Ising transitions are reported. Possible experimental setups designed to explore the described phenomenologies are also briefly discussed.Comment: Submitted to Phys. Rev. E. Rapid Comm. 4 page