1,022 research outputs found
Efficient Equilibria in Polymatrix Coordination Games
We consider polymatrix coordination games with individual preferences where
every player corresponds to a node in a graph who plays with each neighbor a
separate bimatrix game with non-negative symmetric payoffs. In this paper, we
study -approximate -equilibria of these games, i.e., outcomes where
no group of at most players can deviate such that each member increases his
payoff by at least a factor . We prove that for these
games have the finite coalitional improvement property (and thus
-approximate -equilibria exist), while for this
property does not hold. Further, we derive an almost tight bound of
on the price of anarchy, where is the number of
players; in particular, it scales from unbounded for pure Nash equilibria ( to for strong equilibria (). We also settle the complexity
of several problems related to the verification and existence of these
equilibria. Finally, we investigate natural means to reduce the inefficiency of
Nash equilibria. Most promisingly, we show that by fixing the strategies of
players the price of anarchy can be reduced to (and this bound is tight)
Microscopic description of Coulomb and nuclear excitation of multiphonon states in Ca + Ca collisions
We calculate the inelastic scattering cross sections to populate one- and
two-phonon states in heavy ion collisions with both Coulomb and nuclear
excitations. Starting from a microscopic approach based on RPA, we go beyond it
in order to treat anharmonicities and non-linear terms in the exciting field.
These anharmonicities and non-linearities are shown to have important effects
on the cross sections both in the low energy part of the spectrum and in the
energy region of the Double Giant Quadrupole Resonance. By properly introducing
an optical potential the inelastic cross section is calculated semiclassically
by integrating the excitation probability over all impact parameters. A
satisfactory agreement with the experimental results is obtained.Comment: 20 pages, 2 figures, revtex, to be published in Phys. Rev.
Soft Dipole Modes in Neutron-rich Ni-isotopes in QRRPA
The soft dipole modes in neutron rich even-even Ni-isotopes are investigated
in the quasiparticle relativistic random phase approximation. We study the
evolution of strengths distribution, centroid energies of dipole excitation in
low-lying and normal GDR regions with the increase of the neutron excess. It is
found in the present study that the centroid energies of the soft dipole
strengths strongly depend on the thickness of neutron skin along with the
neutron rich even-even Ni-isotopes.Comment: 14 pages, 7 figure
Polarisation Patterns and Vectorial Defects in Type II Optical Parametric Oscillators
Previous studies of lasers and nonlinear resonators have revealed that the
polarisation degree of freedom allows for the formation of polarisation
patterns and novel localized structures, such as vectorial defects. Type II
optical parametric oscillators are characterised by the fact that the
down-converted beams are emitted in orthogonal polarisations. In this paper we
show the results of the study of pattern and defect formation and dynamics in a
Type II degenerate optical parametric oscillator for which the pump field is
not resonated in the cavity. We find that traveling waves are the predominant
solutions and that the defects are vectorial dislocations which appear at the
boundaries of the regions where traveling waves of different phase or
wave-vector orientation are formed. A dislocation is defined by two topological
charges, one associated with the phase and another with the wave-vector
orientation. We also show how to stabilize a single defect in a realistic
experimental situation. The effects of phase mismatch of nonlinear interaction
are finally considered.Comment: 38 pages, including 15 figures, LATeX. Related material, including
movies, can be obtained from
http://www.imedea.uib.es/Nonlinear/research_topics/OPO
Fragmentation of exotic oxygen isotopes
Abrasion-ablation models and the empirical EPAX parametrization of projectile fragmentation are described. Their cross section predictions are compared to recent data of the fragmentation of secondary beams of neutron-rich, unstable 19,20,21O isotopes at beam energies near 600 MeV/nucleon as well as data for stable 17,18O beams
Solar System Processes Underlying Planetary Formation, Geodynamics, and the Georeactor
Only three processes, operant during the formation of the Solar System, are
responsible for the diversity of matter in the Solar System and are directly
responsible for planetary internal-structures, including planetocentric nuclear
fission reactors, and for dynamical processes, including and especially,
geodynamics. These processes are: (i) Low-pressure, low-temperature
condensation from solar matter in the remote reaches of the Solar System or in
the interstellar medium; (ii) High-pressure, high-temperature condensation from
solar matter associated with planetary-formation by raining out from the
interiors of giant-gaseous protoplanets, and; (iii) Stripping of the primordial
volatile components from the inner portion of the Solar System by super-intense
solar wind associated with T-Tauri phase mass-ejections, presumably during the
thermonuclear ignition of the Sun. As described herein, these processes lead
logically, in a causally related manner, to a coherent vision of planetary
formation with profound implications including, but not limited to, (a) Earth
formation as a giant gaseous Jupiter-like planet with vast amounts of stored
energy of protoplanetary compression in its rock-plus-alloy kernel; (b) Removal
of approximately 300 Earth-masses of primordial gases from the Earth, which
began Earth's decompression process, making available the stored energy of
protoplanetary compression for driving geodynamic processes, which I have
described by the new whole-Earth decompression dynamics and which is
responsible for emplacing heat at the mantle-crust-interface at the base of the
crust through the process I have described, called mantle decompression
thermal-tsunami; and, (c)Uranium accumulations at the planetary centers capable
of self-sustained nuclear fission chain reactions.Comment: Invited paper for the Special Issue of Earth, Moon and Planets
entitled Neutrino Geophysics Added final corrections for publicatio
Isotopic Dependence of the Nuclear Caloric Curve
The A/Z dependence of projectile fragmentation at relativistic energies has
been studied with the ALADIN forward spectrometer at SIS. A stable beam of
124Sn and radioactive beams of 124La and 107Sn at 600 MeV per nucleon have been
used in order to explore a wide range of isotopic compositions. Chemical
freeze-out temperatures are found to be nearly invariant with respect to the
A/Z of the produced spectator sources, consistent with predictions for expanded
systems. Small Coulomb effects (\Delta T \approx 0.6 MeV) appear for residue
production near the onset of multifragmentation.Comment: 11 pages, 3 figures, accepted for publ. in Phys. Rev. Let
Tracing a phase transition with fluctuations of the largest fragment size: Statistical multifragmentation models and the ALADIN S254 data
A phase transition signature associated with cumulants of the largest
fragment size distribution has been identified in statistical
multifragmentation models and examined in analysis of the ALADIN S254 data on
fragmentation of neutron-poor and neutron-rich projectiles. Characteristics of
the transition point indicated by this signature are weakly dependent on the
A/Z ratio of the fragmenting spectator source. In particular, chemical
freeze-out temperatures are estimated within the range 5.9 to 6.5 MeV. The
experimental results are well reproduced by the SMM model.Comment: 7 pages, 3 figures, Proceedings of the International Workshop on
Multifragmentation and Related Topics (IWM2009), Catania, Italy, November
2009
Particle emission following Coulomb excitation in ultrarelativistic heavy-ion collisions
We study nuclear reactions induced by virtual photons associated with
Lorentz-boosted Coulomb fields of ultrarelativistic heavy ions. Evaporation,
fission and multifragmentation mechanisms are included in a new RELDIS code,
which describes the deexcitation of residual nuclei formed after single and
double photon absorption in peripheral heavy-ion collisions. Partial cross
sections for different dissociation channels, including the multiple neutron
emission ones, are calculated and compared with data when available. Rapidity
and transverse momentum distributions of nucleons, nuclear fragments and pions,
produced electromagnetically, are also calculated. These results provide
important information for designing large-rapidity detectors and zero-degree
calorimeters at RHIC and LHC. The electromagnetic dissociation of nuclei
imposes some constrains on the investigation of exotic particle production in
gamma-gamma fusion reactions.Comment: 26 LaTeX pages including 8 figures, uses epsf.st
Neutron Halo Isomers in Stable Nuclei and their Possible Application for the Production of Low Energy, Pulsed, Polarized Neutron Beams of High Intensity and High Brilliance
We propose to search for neutron halo isomers populated via -capture
in stable nuclei with mass numbers of about A=140-180 or A=40-60, where the
or neutron shell model state reaches zero binding energy.
These halo nuclei can be produced for the first time with new -beams of
high intensity and small band width ( 0.1%) achievable via Compton
back-scattering off brilliant electron beams thus offering a promising
perspective to selectively populate these isomers with small separation
energies of 1 eV to a few keV. Similar to single-neutron halo states for very
light, extremely neutron-rich, radioactive nuclei
\cite{hansen95,tanihata96,aumann00}, the low neutron separation energy and
short-range nuclear force allows the neutron to tunnel far out into free space
much beyond the nuclear core radius. This results in prolonged half lives of
the isomers for the -decay back to the ground state in the 100
ps-s range. Similar to the treatment of photodisintegration of the
deuteron, the neutron release from the neutron halo isomer via a second,
low-energy, intense photon beam has a known much larger cross section with a
typical energy threshold behavior. In the second step, the neutrons can be
released as a low-energy, pulsed, polarized neutron beam of high intensity and
high brilliance, possibly being much superior to presently existing beams from
reactors or spallation neutron sources.Comment: accepted for publication in Applied Physics
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