471 research outputs found
Violation of the zeroth law of thermodynamics for a non-ergodic interaction
The phenomenon described by our title should surprise no one. What may be
surprising though is how easy it is to produce a quantum system with this
feature; moreover, that system is one that is often used for the purpose of
showing how systems equilibrate. The violation can be variously manifested. In
our detailed example, bringing a detuned 2-level system into contact with a
monochromatic reservoir does not cause it to relax to the reservoir
temperature; rather, the system acquires the reservoir's
level-occupation-ratio
Characterization of an INVS Model IV Neutron Counter for High Precision () Cross-Section Measurements
A neutron counter designed for assay of radioactive materials has been
adapted for beam experiments at TUNL. The cylindrical geometry and 60% maximum
efficiency make it well suited for () cross-section measurements near
the neutron emission threshold. A high precision characterization of the
counter has been made using neutrons from several sources. Using a combination
of measurements and simulations, the absolute detection efficiency of the
neutron counter was determined to an accuracy of 3% in the neutron energy
range between 0.1 and 1 MeV. It is shown that this efficiency characterization
is generally valid for a wide range of targets.Comment: 22 pages, 13 figure
Gaussian coordinate systems for the Kerr metric
We present the whole class of Gaussian coordinate systems for the Kerr
metric. This is achieved through the uses of the relationship between Gaussian
observers and the relativistic Hamilton-Jacobi equation. We analyze the
completeness of this coordinate system. In the appendix we present the
equivalent JEK formulation of General Relativity -- the so-called
quasi-Maxwellian equations -- which acquires a simpler form in the Gaussian
coordinate system. We show how this set of equations can be used to obtain the
internal metric of the Schwazschild solution, as a simple example. We suggest
that this path can be followed to the search of the internal Kerr metric
Characterization of an INVS model IV neutron counter for high precision cross-section measurements
A neutron counter designed for assay of radioactive materials has been adapted for beam experiments at TUNL. The cylindrical geometry and 60% maximum efficiency make it well suited for (γ, n) cross-section measurements near the neutron emission threshold. A high precision characterization of the counter has been made using neutrons from several sources. Using a combination of measurements and simulations, the absolute detection efficiency of the neutron counter was determined to an accuracy of ± 3% in the neutron energy range between 0.1 and 1 MeV. It is shown that this efficiency characterization is generally valid for a wide range of targets
The pressure of QCD at finite temperatures and chemical potentials
The perturbative expansion of the pressure of hot QCD is computed here to
order g^6ln(g) in the presence of finite quark chemical potentials. In this
process all two- and three-loop one-particle irreducible vacuum diagrams of the
theory are evaluated at arbitrary T and mu, and these results are then used to
analytically verify the outcome of an old order g^4 calculation of Freedman and
McLerran for the zero-temperature pressure. The results for the pressure and
the different quark number susceptibilities at high T are compared with recent
lattice simulations showing excellent agreement especially for the chemical
potential dependent part of the pressure.Comment: 35 pages, 6 figures; text revised, one figure replace
Cross-section measurement of 9Be(γ,n)8Be and implications for α+α+n→9Be in the r process
Models of the r process are sensitive to the production rate of 9Be because, in explosive environments rich in neutrons, α(αn,γ) 9Be is the primary mechanism for bridging the stability gaps at A=5 and A=8. The α(αn,γ)9Be reaction represents a two-step process, consisting of α+α→8Be followed by 8Be(n,γ)9Be. We report here on a new absolute cross-section measurement for the 9Be(γ,n)8Be reaction conducted using a highly efficient, 3He-based neutron detector and nearly monoenergetic photon beams, covering energies from Eγ=1.5 MeV to Eγ=5.2 MeV, produced by the High Intensity γ-ray Source of Triangle Universities Nuclear Laboratory. In the astrophysically important threshold energy region, the present cross sections are 40% larger than those found in most previous measurements and are accurate to ±10% (95% confidence). The revised thermonuclear α(αn,γ)9Be reaction rate could have implications for the r process in explosive environments such as type II supernovae
Topological entropy for some isotropic cosmological models
The chaotical dynamics is studied in different Friedmann-Robertson- Walker
cosmological models with scalar (inflaton) field and hydrodynamical matter. The
topological entropy is calculated for some particular cases. Suggested scheme
can be easily generalized for wide class of models. Different methods of
calculation of topological entropy are compared.Comment: Final version to appear in Phys. Rev D. Minor changes, typos
corrected; 29 pages with 2 eps figure
Information-Geometric Indicators of Chaos in Gaussian Models on Statistical Manifolds of Negative Ricci Curvature
A new information-geometric approach to chaotic dynamics on curved
statistical manifolds based on Entropic Dynamics (ED) is proposed. It is shown
that the hyperbolicity of a non-maximally symmetric 6N-dimensional statistical
manifold M_{s} underlying an ED Gaussian model describing an arbitrary system
of 3N degrees of freedom leads to linear information-geometric entropy growth
and to exponential divergence of the Jacobi vector field intensity, quantum and
classical features of chaos respectively.Comment: 8 pages, final version accepted for publicatio
Hamiltonian dynamics and Noether symmetries in Extended Gravity Cosmology
We discuss the Hamiltonian dynamics for cosmologies coming from Extended
Theories of Gravity. In particular, minisuperspace models are taken into
account searching for Noether symmetries. The existence of conserved quantities
gives selection rule to recover classical behaviors in cosmic evolution
according to the so called Hartle criterion, that allows to select correlated
regions in the configuration space of dynamical variables. We show that such a
statement works for general classes of Extended Theories of Gravity and is
conformally preserved. Furthermore, the presence of Noether symmetries allows a
straightforward classification of singularities that represent the points where
the symmetry is broken. Examples of nonminimally coupled and higher-order
models are discussed.Comment: 20 pages, Review paper to appear in EPJ
Perturbative QCD and factorization of coherent pion photoproduction on the deuteron
We analyze the predictions of perturbative QCD for pion photoproduction on
the deuteron, gamma D -> pi^0 D, at large momentum transfer using the reduced
amplitude formalism. The cluster decomposition of the deuteron wave function at
small binding only allows the nuclear coherent process to proceed if each
nucleon absorbs an equal fraction of the overall momentum transfer.
Furthermore, each nucleon must scatter while remaining close to its mass shell.
Thus the nuclear photoproduction amplitude, M_{gamma D -> pi^0 D}(u,t),
factorizes as a product of three factors: (1) the nucleon photoproduction
amplitude, M_{gamma N_1 -> pi^0 N_1}(u/4,t/4), at half of the overall momentum
transfer, (2) a nucleon form factor, F_{N_2}(t/4), at half the overall momentum
transfer, and (3) the reduced deuteron form factor, f_d(t), which according to
perturbative QCD, has the same monopole falloff as a meson form factor. A
comparison with the recent JLAB data for gamma D -> pi^0 D of Meekins et al.
[Phys. Rev. C 60, 052201 (1999)] and the available gamma p -> pi^0 p data shows
good agreement between the perturbative QCD prediction and experiment over a
large range of momentum transfers and center of mass angles. The reduced
amplitude prediction is consistent with the constituent counting rule, p^11_T
M_{gamma D -> pi^0 D} -> F(theta_cm), at large momentum transfer. This is found
to be consistent with measurements for photon lab energies E_gamma > 3 GeV at
theta_cm=90 degrees and \elab > 10 GeV at 136 degrees.Comment: RevTeX 3.1, 17 pages, 6 figures; v2: incorporates minor changes as
version accepted by Phys Rev
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