May 1996 arXiv papers — page 14

C. Athanassopoulos, L. B. Auerbach, R. L. Burman, I. Cohen

A search for nu_bar_mu to nu_bar_e oscillations has been conducted at the Los Alamos Meson Physics Facility using nu_bar_mu from mu+ decay at rest. The nu_bar_e are detected via the reaction (nu_bar_e,p) -> (e+,n), correlated with the 2.2 MeV gamma from (n,p) -> (d,gamma). The use of tight cuts to identify e+ events with correlated gamma rays yields 22 event

I. Ya. Aref'eva, A. P. Zubarev

A parquet approximation (generalized ladder diagrams) in matrix models is considered. By means of numerical calculations we demonstrate that in the large $N$ limit the parquet approximation gives an excellent agreement with exact results.

D. V. Long, G. M. Shore

The Schrodinger picture description of vacuum states in curved spacetime is further developed. General solutions for the vacuum wave functional are given for both static and dynamic (Bianchi type I) spacetimes and for conformally static spacetimes of Robertson-Walker type. The formalism is illustrated for simple cosmological models with time-dependent metric

Kazuyuki Fujii, Kunio Funahashi

We construct the path integral formula in terms of ``multi-periodic'' coherent state as an extension of the Nielsen-Rohrlich formula for spin. We make an exact calculation of the formula and show that, when a parameter corresponding to the magnitude of spin becomes large, the leading order term of the expansion coincides with the exact result. We als

C. C. Lassig, G. C. Joshi

The violation of the Jacobi identity by the presence of magnetic charge is accomodated by using an explicitly nonassociative theory of octonionic fields. It is found that the dynamics of this theory is simplified if the Lagrangian contains only dyonic charges, but certain problems in the constrained quantisation remain. The extension of these concepts to str

Zhen Cao, Rudolph C. Hwa

The branching processes in parton showers are studied in perturbative QCD for both quark and gluon jets. The emphasis is on the nature of fluctuations of both the parton multiplicities and the spatial patterns of the final states. Effective measures of such fluctuations are calculated from the data obtained by Monte Carlo simulations. The entropy indices are

Simonetta Frittelli, Oscar A. Reula

We find a one-parameter family of variables which recast the 3+1 Einstein equations into first-order symmetric-hyperbolic form for any fixed choice of gauge. Hyperbolicity considerations lead us to a redefinition of the lapse in terms of an arbitrary factor times a power of the determinant of the 3-metric; under certain assumptions, the exponent can be chose

T. Ohta, R. B. Mann

We develop the formalism for canonical reduction of $(1+1)$--dimensional gravity coupled with a set of point particles by eliminating constraints and imposing coordinate conditions. The formalism itself is quite analogous to the $(3+1)$--dimensional case; however in $(1+1)$ dimensions an auxiliary scalar field is shown to have an important role. The reduced

Naresh Dadhich

Instead of conformal to flat spacetime, we take the metric conformal to a spacetime which can be thought of as ``minimally'' curved in the sense that free particles experience no gravitational force yet it has non-zero curvature. The base spacetime can be written in the Kerr-Schild form in spherical polar coordinates. The conformal metric then admits

Naresh Dadhich

We prove the theorem: The necessary and sufficient condition for a spherically symmetric spacetime to represent an isothermal perfect fluid (barotropic equation of state with density falling off as inverse square of the curvature radius) distribution without boundary is that it is conformal to the ``minimally'' curved (gravitation only manifesting in

L. Pryor, G. Collins

A fundamental assumption made by classical AI planners is that there is no uncertainty in the world: the planner has full knowledge of the conditions under which the plan will be executed and the outcome of every action is fully predictable. These planners cannot therefore construct contingency plans, i.e., plans in which different actions are performed in d

P. Tadepalli, B. K. Natarajan

Speedup learning seeks to improve the computational efficiency of problem solving with experience. In this paper, we develop a formal framework for learning efficient problem solving from random problems and their solutions. We apply this framework to two different representations of learned knowledge, namely control rules and macro-operators, and prove theo

J. Gratch, S. Chien

Although most scheduling problems are NP-hard, domain specific techniques perform well in practice but are quite expensive to construct. In adaptive problem-solving solving, domain specific knowledge is acquired automatically for a general problem solver with a flexible control architecture. In this approach, a learning system explores a space of possible he

L. P. Kaelbling, M. L. Littman, A. W. Moore

This paper surveys the field of reinforcement learning from a computer-science perspective. It is written to be accessible to researchers familiar with machine learning. Both the historical basis of the field and a broad selection of current work are summarized. Reinforcement learning is the problem faced by an agent that learns behavior through trial-and-er

S. H. Nienhuys-Cheng, R. deWolf

The main operations in Inductive Logic Programming (ILP) are generalization and specialization, which only make sense in a generality order. In ILP, the three most important generality orders are subsumption, implication and implication relative to background knowledge. The two languages used most often are languages of clauses and languages of only Horn cla

G. I. Webb

This paper presents new experimental evidence against the utility of Occam's razor. A~systematic procedure is presented for post-processing decision trees produced by C4.5. This procedure was derived by rejecting Occam's razor and instead attending to the assumption that similar objects are likely to belong to the same class. It increases a decision

Raphael Blumenfeld

A minimal model is constructed for two-dimensional fracture propagation. The heterogeneous process zone is presumed to suppress stress relaxation rate, leading to non-quasistatic behavior. Using the Yoffe solution, I construct and solve a dynamical equation for the tip stress. I discuss a generic tip velocity response to local stress and find that noise-free

K. Levin, D. Z. Liu, Jiri Maly

In this paper we argue that d-wave symmetry is a general consequence of superconductivity driven by repulsive interactions. Van Hove (or flat band) effects, deriving from the two dimensionality of the $CuO_2$ plane are important in stabilizing this state. By extending the original Kohn-Luttinger picture to a 2 D lattice, we find that the screened Coulomb ter

Lee-Wen Chen, Leon Balents, Matthew P. A. Fisher, M. Cristina Marchetti

We have studied numerically the dynamics of sliding charge-density waves (CDWs) in the presence of impurities in d=1,2. The model considered exhibits a first order dynamical transition at a critical driving force $F_c$ between ``rough&#39;&#39; (disorder dominated) $(F<F_c)$ and ``flat&#39;&#39; $(F>F_c)$ sliding phases where disorder is washed out by the ex

Jon Rotvig, Antti-Pekka Jauho, Henrik Smith

We present an analytical study of one-dimensional semiconductor superlattices in external electric fields, which may be time-dependent. A number of general results for the (quasi)energies and eigenstates are derived. An equation of motion for the density matrix is obtained for a two-band model, and the properties of the solutions are analyzed. An expression

M. Alouani, J. M. Wills

The macroscopic dielectric function in the random-phase-approximation without local field effect has been implemented using the local density approximation with an all electron, full-potential linear muffin-tin orbital basis-set. This method is used to investigate the optical properties of the semiconductors Si, Ge, and GaAs under hydrostatic pressure. The p

B. Virgin, E. Haslund, R. Hilfer

Local porosity distributions for a three-dimensional porous medium and local porosity distributions for a two-dimensional plane-section through the medium are generally different. However, for homogeneous and isotropic media having finite correlation-lengths, a good degree of correspondence between the two sets of local porosity distributions can be obtained

Kicheon Kang, B. I. Min

We have investigated effects of an external magnetic field in the impurity Anderson model with a finite on-site Coulomb repulsion $U$. Large $N_f$ expansion is employed in the slave boson representation, by taking into account $f^0$, $f^1$, and $f^2$ subspaces. To evaluate the vertex function for the ``empty state boson&#34; self-energy, we have devised two

Akira Furusaki, Naoto Nagaosa

The transport through a barrier is studied for a Tomonaga-Luttinger liquid of finite length connected to reservoirs. An effective action for the phase variable at the barrier is derived for spatially varying electric field. In the d.c. limit only the total voltage drop between the left and right reservoirs appears in the action. We discuss crossovers of the

Biplab Chattopadhyay, D. M. Gaitonde, A. Taraphder

Effect of phase fluctuations on superconducting states with anisotropic order parameters is studied in a BCS like lattice model of cuprate superconductors. The degradation of the mean field transition temperature due to phase fluctuations is estimated within a Kosterlitz-Thouless scenario. Values of the interaction parameters for optimal doping, correspondin

Y. C. Chen, T. K. Lee

We propose a correlated spin-singlet-pairs wave function to describe the spin-gap phase of the one-dimensional $t-J$ model at low density. Adding a Jastrow factor with a variational parameter, $ν$, first introduced by Hellberg and Mele, is shown to correctly describe the long-range behavior expected for the Luther-Emery phase. Using the variational Monte Car

Fumitaka Matsubara, Junya Sasaki

We make for the first time a large-scale Monte-Carlo simulation of a ferromagnetic Heisenberg model with dipolar interactions on a two dimensional square lattice with open boundaries using an efficient new technique. We find that a phase transition occurs in the model and the ordered phase is characterized by a circular arrangement of the spins.

N. Berglund, Alex Hansen, E. H. Hauge, J. Piasecki

We study the classical dynamics of a charged particle in two dimensions, under the influence of a perpendicular magnetic and an in-plane electric field. We prove the surprising fact that there is a finite region in phase space that corresponds to the otherwise drifting particle being trapped by a local repulsive potential. Our result is a direct consequence

Stephen W. Morris, Eberhard Bodenschatz, David S. Cannell, Guenter Ahlers

We present a study of the recently discovered spatially-extended chaotic state known as spiral-defect chaos, which occurs in low-Prandtl-number, large-aspect-ratio Rayleigh-Benard convection. We employ the modulus squared of the space-time Fourier transform of time series of two-dimensional shadowgraph images to construct the structure factor ${S}({\vec k},ω

Re&#39;em Sari, Ramesh Narayan, Tsvi Piran

A leading mechanism for producing cosmological gamma-ray bursts (GRBs) is via ultra-relativistic particles in an expanding fireball. The kinetic energy of the particles is converted into thermal energy in a forward shock and a reverse shock, when the outward flowing particles encounter the interstellar medium. The thermal energy is radiated via synchrotron e

T. Manmoto, M. Takeuchi, S. Mineshige, R. Matsumoto

The response of advection-dominated accretion disks to local disturbances is examined by one-dimensional numerical simulations. It is generally believed that advection-dominated disks are thermally stable. We, however, find that any disurbance added onto accretion flow at large radii does not decay so rapidly that it can move inward with roughly the free-fal

Rainer Spurzem, Sverre J. Aarseth

A collisional $N$-body simulation using NBODY5 on a single CRAY YMP processor is followed well into the post-collapse regime. This is presently one of the largest particle numbers of all such models published, but some data for an even larger $N$ produced by using special-purpose computers have recently been presented. In contrast to previous ensemble-averag

Michael G. Akritas, Matthew A. Bershady

Two new methods are proposed for linear regression analysis for data with measurement errors. Both methods are designed to accommodate intrinsic scatter in addition to measurement errors. The first (BCES) is a direct extension of the ordinary least squares (OLS) estimator to allow for measurement errors. It is quite general, allowing a) for measurement error

Heino Falcke

We have proposed previously that Sgr A* is simply a scaled down AGN with a black hole, an accretion disk and a radio jet operating at a very low power. It appears as if M81* - the nuclear source in the nearby galaxy M81 - is an ideal laboratory to study a Sgr A*-like source at a higher power level. The jet/disk model can explain M81* in great detail with no

Joel R. Primack

The cosmological parameters that I discuss are the Hubble parameter $H_0 \equiv 100 h$ km s$^{-1}$ Mpc$^{-1}$, the age of the universe $t_0$, the average density $Ω_0$, and the cosmological constant $Λ$. To focus the discussion, I concentrate on the the value of $Ω_0$ in currently popular models in which most of the dark matter is cold, especially Cold + Hot