2,188 research outputs found
High-Precision Thermodynamics and Hagedorn Density of States
We compute the entropy density of the confined phase of QCD without quarks on
the lattice to very high accuracy. The results are compared to the entropy
density of free glueballs, where we include all the known glueball states below
the two-particle threshold. We find that an excellent, parameter-free
description of the entropy density between 0.7Tc and Tc is obtained by
extending the spectrum with the exponential spectrum of the closed bosonic
string.Comment: 4 pages, 3 figure
Investigation of passive shock wave-boundary layer control for transonic airfoil drag reduction
The passive drag control concept, consisting of a porous surface with a cavity beneath it, was investigated with a 12-percent-thick circular arc and a 14-percent-thick supercritical airfoil mounted on the test section bottom wall. The porous surface was positioned in the shock wave/boundary layer interaction region. The flow circulating through the porous surface, from the downstream to the upstream of the terminating shock wave location, produced a lambda shock wave system and a pressure decrease in the downstream region minimizing the flow separation. The wake impact pressure data show an appreciably drag reduction with the porous surface at transonic speeds. To determine the optimum size of porosity and cavity, tunnel tests were conducted with different airfoil porosities, cavities and flow Mach numbers. A higher drag reduction was obtained by the 2.5 percent porosity and the 1/4-inch deep cavity
Alkali oxide-tantalum, niobium and antimony oxide ionic conductors
The phase equilibrium relations of four systems were investigated in detail. These consisted of sodium and potassium antimonates with antimony oxide and tantalum and niobium oxide with rubidium oxide as far as the ratio 4Rb2O:llB2O5 (B=Nb, Ta). The ternary system NaSbO3-Sb2O4-NaF was investigated extensively to determine the actual composition of the body centered cubic sodium antimonate. Various other binary and ternary oxide systems involving alkali oxides were examined in lesser detail. The phases synthesized were screened by ion exchange methods to determine mobility of the mobility of the alkali ion within the niobium, tantalum or antimony oxide (fluoride) structural framework. Five structure types warranted further investigation; these structure types are (1) hexagonal tungsten bronze (HTB), (2) pyrochlore, (3) the hybrid HTB-pyrochlore hexagonal ordered phases, (4) body centered cubic antimonates and (5) 2K2O:3Nb2O5. Although all of these phases exhibit good ion exchange properties only the pyrochlore was prepared with Na(+) ions as an equilibrium phase and as a low porosity ceramic. Sb(+3) in the channel interferes with ionic conductivity in this case, although relatively good ionic conductivity was found for the metastable Na(+) ion exchanged analogs of RbTa2O5F and KTaWO6 pyrochlore phases
Effective Lagrangian for strongly coupled domain wall fermions
We derive the effective Lagrangian for mesons in lattice gauge theory with
domain-wall fermions in the strong-coupling and large-N_c limits. We use the
formalism of supergroups to deal with the Pauli-Villars fields, needed to
regulate the contributions of the heavy fermions. We calculate the spectrum of
pseudo-Goldstone bosons and show that domain wall fermions are doubled and
massive in this regime. Since we take the extent and lattice spacing of the
fifth dimension to infinity and zero respectively, our conclusions apply also
to overlap fermions.Comment: 26 pp. RevTeX and 3 figures; corrected error in symmetry breaking
scheme and added comments to discussio
Effective Field Theories
Effective field theories encode the predictions of a quantum field theory at
low energy. The effective theory has a fairly low ultraviolet cutoff. As a
result, loop corrections are small, at least if the effective action contains a
term which is quadratic in the fields, and physical predictions can be read
straight from the effective Lagrangean.
Methods will be discussed how to compute an effective low energy action from
a given fundamental action, either analytically or numerically, or by a
combination of both methods. Basically,the idea is to integrate out the high
frequency components of fields. This requires the choice of a "blockspin",i.e.
the specification of a low frequency field as a function of the fundamental
fields. These blockspins will be the fields of the effective field theory. The
blockspin need not be a field of the same type as one of the fundamental
fields, and it may be composite. Special features of blockspins in nonabelian
gauge theories will be discussed in some detail.
In analytical work and in multigrid updating schemes one needs interpolation
kernels \A from coarse to fine grid in addition to the averaging kernels
which determines the blockspin. A neural net strategy for finding optimal
kernels is presented.
Numerical methods are applicable to obtain actions of effective theories on
lattices of finite volume. The constraint effective potential) is of particular
interest. In a Higgs model it yields the free energy, considered as a function
of a gauge covariant magnetization. Its shape determines the phase structure of
the theory. Its loop expansion with and without gauge fields can be used to
determine finite size corrections to numerical data.Comment: 45 pages, 9 figs., preprint DESY 92-070 (figs. 3-9 added in ps
format
Phase transitions and volunteering in spatial public goods games
Cooperative behavior among unrelated individuals in human and animal
societies represents a most intriguing puzzle to scientists in various
disciplines. Here we present a simple yet effective mechanism promoting
cooperation under full anonymity by allowing for voluntary participation in
public goods games. This natural extension leads to rock--scissors--paper type
cyclic dominance of the three strategies cooperate, defect and loner i.e. those
unwilling to participate in the public enterprise. In spatial settings with
players arranged on a regular lattice this results in interesting dynamical
properties and intriguing spatio-temporal patterns. In particular, variations
of the value of the public good leads to transitions between one-, two- and
three-strategy states which are either in the class of directed percolation or
show interesting analogies to Ising-type models. Although volunteering is
incapable of stabilizing cooperation, it efficiently prevents successful
spreading of selfish behavior and enables cooperators to persist at substantial
levels.Comment: 4 pages, 5 figure
On The Pomeron at Large 't Hooft Coupling
We begin the process of unitarizing the Pomeron at large 't Hooft coupling.
We do so first in the conformal regime, which applies to good accuracy to a
number of real and toy problems in QCD. We rewrite the conformal Pomeron in the
-plane and transverse position space, and then work out the eikonal
approximation to multiple Pomeron exchange. This is done in the context of a
more general treatment of the complex -plane and the geometric consequences
of conformal invariance. The methods required are direct generalizations of our
previous work on single Pomeron exchange and on multiple graviton exchange in
AdS space, and should form a starting point for other investigations. We
consider unitarity and saturation in the conformal regime, noting elastic and
absorptive effects, and exploring where different processes dominate. Our
methods extend to confining theories and we briefly consider the Pomeron kernel
in this context. Though there is important model dependence that requires
detailed consideration, the eikonal approximation indicates that the Froissart
bound is generically both satisfied and saturated.Comment: 63 pages, 7 figures; published version: references updated and
several typos correcte
Polarized DIS in N=4 SYM: Where is spin at strong coupling?
Using the AdS/CFT correspondence, we calculate the polarized structure
functions in strongly coupled N=4 supersymmetric Yang-Mills theory deformed in
the infrared. We find that the flavor singlet contribution to the g_1 structure
function is vanishingly small, while the flavor non-singlet contribution shows
the Regge behavior at small-x with an intercept slightly less than 1. We
explicitly check that the latter satisfies the moment sum rule. We discuss the
`spin crisis' problem and suggest that at strong coupling the spin of a hadron
entirely comes from the orbital angular momentum.Comment: 24 page
Dual Monte Carlo and Cluster Algorithms
We discuss the development of cluster algorithms from the viewpoint of
probability theory and not from the usual viewpoint of a particular model. By
using the perspective of probability theory, we detail the nature of a cluster
algorithm, make explicit the assumptions embodied in all clusters of which we
are aware, and define the construction of free cluster algorithms. We also
illustrate these procedures by rederiving the Swendsen-Wang algorithm,
presenting the details of the loop algorithm for a worldline simulation of a
quantum 1/2 model, and proposing a free cluster version of the
Swendsen-Wang replica method for the random Ising model. How the principle of
maximum entropy might be used to aid the construction of cluster algorithms is
also discussed.Comment: 25 pages, 4 figures, to appear in Phys.Rev.
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