17,023 research outputs found
The Open String Regge Trajectory and Its Field Theory Limit
We study the properties of the leading Regge trajectory in open string theory
including the open string planar one-loop corrections. With SU(N) Chan-Paton
factors, the sum over planar open string multi-loop diagrams describes the 't
Hooft limit N\to\infty. Our motivation is to improve the understanding of open
string theory at finite \alpha' as a model of gauge theories. SU(N) gauge
theories in D space-time dimensions are described by requiring open strings to
end on a stack of N Dp-branes of space-time dimension D=p+1. The large N
leading trajectory \alpha(t)=1+\alpha' t+\Sigma(t) can be extracted, through
order g^2, from the s\to-\infty limit, at fixed t, of the four open string tree
and planar loop diagrams. We analyze the t\to0 behavior with the result that
\Sigma(t)\sim-Cg^2(-\alpha' t)^{(D-4)/2}/(D-4). This result precisely tracks
the 1-loop Reggeized gluon of gauge theory in D>4 space-time dimensions. In
particular, for D\to4 it reproduces the known infrared divergences of gauge
theory in 4 dimensions with a Regge trajectory behaving as -\ln(-\alpha^\prime
t). We also study \Sigma(t) in the limit t\to-\infty and show that, when D<8,
it behaves as \alpha^\prime t/(\ln(-\alpha^\prime t))^{\gamma}, where \gamma>0
depends on D and the number of massless scalars. Thus, as long as 4<D<8, the
1-loop correction stays small relative to the tree trajectory for the whole
range -\infty<t<0. Finally we present the results of numerical calculations of
\Sigma(t) for all negative t.Comment: 19 pages, 5 figure
Solving multi-objective hub location problems by hybrid algorithms
In many logistic, telecommunications and computer networks, direct routing of
commodities between any origin and destination is not viable due to economic and technolog-
ical constraints. In that cases, a network with centralized units, known as hub facilities, and a
small number of links is commonly used to connect any origin-destination pair. The purpose
of these hub facilities is to consolidate, sort and transship e ciently any commodity in the
network. Hub location problems (HLPs) consider the design of these networks by locating a
set of hub facilities, establishing an interhub subnet, and routing the commodities through
the network while optimizing some objective(s) based on the cost or service.
Hub location has evolved into a rich research area, where a huge number of papers have
been published since the seminal work of O'Kelly [1]. Early works were focused on analogue
facility location problems, considering some assumptions to simplify network design. Recent
works [2] have studied more complex models that relax some of these assumptions and in-
corporate additional real-life features. In most HLPs considered in the literature, the input
parameters are assumed to be known and deterministic. However, in practice, this assumption
is unrealistic since there is a high uncertainty on relevant parameters, such as costs, demands
or even distances.
In this work, we will study the multi-objective hub location problems with uncertainty.Universidad de Málaga. Campus de Excelencia Internacional AndalucĂa Tec
Plio-Pleistocene time-averaged field in southern Patagonia recorded in lava flows
Paleomagnetic directions were obtained from stepwise alternating-field or thermal demagnetization of 53 lava flows from southern Patagonia (latitudes 49.5°-52.1 °S) that include the Pali-Aike volcanic field and the Meseta Viscachas plateau lavas. In addition to previous Miocene-late Quaternary ages of these flows, 40Ar/39Ar dates spanning from 0.1 to 15.4 Ma were obtained for 17 of the sites. All except one of the magnetic polarities coincide with the expected polarities of the magnetic polarity timescale [Cande and Kent, 1995] for the obtained 40Ar/39Ar ages. The mean direction from 33 sites (eliminating sites <4 Ma) that pass a selection criteria of α95 ≤5° is Dec = 358.7°,Inc = - 68.2°, α95 = 3.5°, a value that coincides within the statistical uncertainty with the direction of the geocentric axial dipole for that area (Inc = - 68.1°). Likewise, the mean virtual geomagnetic pole (VGP) coincides within the statistical uncertainty with the geographic North Pole. The secular variation described by the VGP angular standard deviation for these sites is 17.1°, a value expected for that latitude according to Model G of paleosecular variation [McFadden et al., 1988]. The characteristics of the data presented are optimum for time-averaged field (TAF) studies because of the good age control and good quality of the paleomagnetic data: (1) primary components of magnetization were obtained using principal component analysis [Kirschvink, 1980] from at least five points and maximum angular deviation ≤5°, (2) site means were calculated with Fisher statistics using at least three samples, and (c) 38 of the 53 flows had α95 ≤ 5°. No results (five sites) or high α95 values (≤5°) were obtained primarily from sites affected by lightning.Fil: Mejia, V.. University of Florida; Estados UnidosFil: Opdyke, N. D.. University of Florida; Estados UnidosFil: Vilas, Juan Francisco A.. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de GeologĂa; Argentina. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Oficina de CoordinaciĂłn Administrativa Ciudad Universitaria. Instituto de Geociencias Básicas, Aplicadas y Ambientales de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Geociencias Básicas, Aplicadas y Ambientales de Buenos Aires; ArgentinaFil: Singer, B. S.. University of Wisconsin; Estados UnidosFil: Stoner, J. S.. State University of Colorado at Boulder; Estados Unido
Spanning avalanches in the three-dimensional Gaussian Random Field Ising Model with metastable dynamics: field dependence and geometrical properties
Spanning avalanches in the 3D Gaussian Random Field Ising Model (3D-GRFIM)
with metastable dynamics at T=0 have been studied. Statistical analysis of the
field values for which avalanches occur has enabled a Finite-Size Scaling (FSS)
study of the avalanche density to be performed. Furthermore, direct measurement
of the geometrical properties of the avalanches has confirmed an earlier
hypothesis that several kinds of spanning avalanches with two different fractal
dimensions coexist at the critical point. We finally compare the phase diagram
of the 3D-GRFIM with metastable dynamics with the same model in equilibrium at
T=0.Comment: 16 pages, 17 figure
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