12,008 research outputs found
On a Possibility to Determine the Sign of the Polarized Gluon Distribution
We investigate the possibility to draw conclusions on the sign of the
spin-dependent gluon distribution, , from existing polarized
DIS data. The spin-dependent parton distributions , and are constructed
in the framework of a phenomenological procedure taking into account some
assumptions on signs of valence and sea parton distributions motivated by 't
Hooft's mechanism of quark-quark interaction induced by instantons. The axial
gluon anomaly and data on integral quark contributions to the proton spin,
, and , are also taken into
account. Predictions for the - and -dependencies of the polarized
proton and neutron structure functions, and , are compared to
experimental data. It is shown that the neutron structure function, , is
especially sensitive to the sign of . The results of our
analysis supports the conclusion that this sign should be positive.Comment: 14 pages, latex, 12 figure
Tuning of the spin-orbit interaction in a quantum dot by an in-plane magnetic field
Using an exact diagonalization approach we show that one- and two-electron
InAs quantum dots exhibit avoided crossing in the energy spectra that are
induced by the spin-orbit coupling in the presence of an in-plane external
magnetic field. The width of the avoided crossings depends strongly on the
orientation of the magnetic field which reveals the intrinsic anisotropy of the
spin-orbit coupling interactions. We find that for specific orientations of the
magnetic field avoided crossings vanish. Value of this orientation can be used
to extract the ratio of the strength of Rashba and Dresselhaus interactions.
The spin-orbit anisotropy effects for various geometries and orientations of
the confinement potential are discussed. Our analysis explains the physics
behind the recent measurements performed on a gated self-assembled quantum dot
[S. Takahashi et al. Phys. Rev. Lett. 104, 246801 (2010)].Comment: Corrected according to referees comment
Relativistic Iron Lines in Galactic Black Holes: Recent Results and Lines in the ASCA Archive
Recent observations with Chandra and XMM-Newton, aided by broad-band spectral
coverage from RXTE, have revealed skewed relativistic iron emission lines in
stellar-mass Galactic black hole systems. Such systems are excellent
laboratories for testing General Relativity, and relativistic iron lines
provide an important tool for making such tests. In this contribution to the
Proceedings of the 10th Annual Marcel Grossmann Meeting on General Relativity,
we briefly review recent developments and present initial results from fits to
archival ASCA observations of Galactic black holes. It stands to reason that
relativistic effects, if real, should be revealed in many systems (rather than
just one or two); the results of our archival work have borne-out this
expectation. The ASCA spectra reveal skewed, relativistic lines in XTE
J1550-564, GRO J1655-40, GRS 1915+105, and Cygnus X-1.Comment: to appear in the proc. of the 10th Annual Marcel Grossmann Meeting on
General Relativity, 5 pages, 1 figure, uses specific .cls and .sty file
Thermal vortex dynamics in thin circular ferromagnetic nanodisks
The dynamics of gyrotropic vortex motion in a thin circular nanodisk of soft
ferromagnetic material is considered. The demagnetization field is calculated
using two-dimensional Green's functions for the thin film problem and fast
Fourier transforms. At zero temperature, the dynamics of the
Landau-Lifshitz-Gilbert equation is simulated using fourth order Runge-Kutta
integration. Pure vortex initial conditions at a desired position are obtained
with a Lagrange multipliers constraint. These methods give accurate estimates
of the vortex restoring force constant and gyrotropic frequency, showing
that the vortex core motion is described by the Thiele equation to very high
precision. At finite temperature, the second order Heun algorithm is applied to
the Langevin dynamical equation with thermal noise and damping. A spontaneous
gyrotropic motion takes place without the application of an external magnetic
field, driven only by thermal fluctuations. The statistics of the vortex radial
position and rotational velocity are described with Boltzmann distributions
determined by and by a vortex gyrotropic mass ,
respectively, where is the vortex gyrovector.Comment: 18 pages, 17 figure
Correlation of Positive and Negative Reciprocity Fails to Confer an Evolutionary Advantage: Phase Transitions to Elementary Strategies
Economic experiments reveal that humans value cooperation and fairness. Punishing unfair behavior is therefore common, and according to the theory of strong reciprocity, it is also directly related to rewarding cooperative behavior. However, empirical data fail to confirm that positive and negative reciprocity are correlated. Inspired by this disagreement, we determine whether the combined application of reward and punishment is evolutionarily advantageous. We study a spatial public goods game, where in addition to the three elementary strategies of defection, rewarding, and punishment, a fourth strategy that combines the latter two competes for space. We find rich dynamical behavior that gives rise to intricate phase diagrams where continuous and discontinuous phase transitions occur in succession. Indirect territorial competition, spontaneous emergence of cyclic dominance, as well as divergent fluctuations of oscillations that terminate in an absorbing phase are observed. Yet, despite the high complexity of solutions, the combined strategy can survive only in very narrow and unrealistic parameter regions. Elementary strategies, either in pure or mixed phases, are much more common and likely to prevail. Our results highlight the importance of patterns and structure in human cooperation, which should be considered in future experiments
Conditional strategies and the evolution of cooperation in spatial public goods games
The fact that individuals will most likely behave differently in different
situations begets the introduction of conditional strategies. Inspired by this,
we study the evolution of cooperation in the spatial public goods game, where
besides unconditional cooperators and defectors, also different types of
conditional cooperators compete for space. Conditional cooperators will
contribute to the public good only if other players within the group are likely
to cooperate as well, but will withhold their contribution otherwise. Depending
on the number of other cooperators that are required to elicit cooperation of a
conditional cooperator, the latter can be classified in as many types as there
are players within each group. We find that the most cautious cooperators, such
that require all other players within a group to be conditional cooperators,
are the undisputed victors of the evolutionary process, even at very low
synergy factors. We show that the remarkable promotion of cooperation is due
primarily to the spontaneous emergence of quarantining of defectors, which
become surrounded by conditional cooperators and are forced into isolated
convex "bubbles" from where they are unable to exploit the public good. This
phenomenon can be observed only in structured populations, thus adding to the
relevance of pattern formation for the successful evolution of cooperation.Comment: 7 two-column pages, 7 figures; accepted for publication in Physical
Review
Beyond XSPEC: Towards Highly Configurable Analysis
We present a quantitative comparison between software features of the defacto
standard X-ray spectral analysis tool, XSPEC, and ISIS, the Interactive
Spectral Interpretation System. Our emphasis is on customized analysis, with
ISIS offered as a strong example of configurable software. While noting that
XSPEC has been of immense value to astronomers, and that its scientific core is
moderately extensible--most commonly via the inclusion of user contributed
"local models"--we identify a series of limitations with its use beyond
conventional spectral modeling. We argue that from the viewpoint of the
astronomical user, the XSPEC internal structure presents a Black Box Problem,
with many of its important features hidden from the top-level interface, thus
discouraging user customization. Drawing from examples in custom modeling,
numerical analysis, parallel computation, visualization, data management, and
automated code generation, we show how a numerically scriptable, modular, and
extensible analysis platform such as ISIS facilitates many forms of advanced
astrophysical inquiry.Comment: Accepted by PASP, for July 2008 (15 pages
Low-energy interactions of Nambu-Goldstone bosons with mesons in covariant chiral perturbation theory
We calculate the scattering lengths of Nambu-Goldstone bosons interacting
with mesons in a covariant formulation of chiral perturbation theory, which
satisfies heavy-quark spin symmetry and analytical properties of loop
amplitudes. We compare our results with previous studies performed using heavy
meson chiral perturbation theory and show that recoil corrections are sizable
in most cases.Comment: 3 figures and 4 table
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