10,882 research outputs found
The alpha-particle in nuclear matter
Among the light nuclear clusters the alpha-particle is by far the strongest
bound system and therefore expected to play a significant role in the dynamics
of nuclei and the phases of nuclear matter. To systematically study the
properties of the alpha-particle we have derived an effective four-body
equation of the Alt-Grassberger-Sandhas (AGS) type that includes the dominant
medium effects, i.e. self energy corrections and Pauli-blocking in a consistent
way. The equation is solved utilizing the energy dependent pole expansion for
the sub system amplitudes. We find that the Mott transition of an
alpha-particle at rest differs from that expected from perturbation theory and
occurs at approximately 1/10 of nuclear matter densities.Comment: 9 pages RevTex file, 1 figure, submitted to Phys. Lett.
Deuteron formation in nuclear matter
We investigate deuteron formation in nuclear matter at finite temperatures
within a systematic quantum statistical approach. We consider formation through
three-body collisions relevant already at rather moderate densities because of
the strong correlations. The three-body in-medium reaction rates driven by the
break-up cross section are calculated using exact three-body equations
(Alt-Grassberger-Sandhas type) that have been suitably modified to consistently
include the energy shift and the Pauli blocking. Important quantities are the
lifetime of deuteron fluctuations and the chemical relaxation time. We find
that the respective times differ substantially while using in-medium or
isolated cross sections. We expect implications for the description of heavy
ion collisions in particular for the formation of light charged particles at
low to intermediate energies.Comment: 19 pages, 5 figure
Coupled dark energy and dark matter from dilatation anomaly
Cosmological runaway solutions may exhibit an exact dilatation symmetry in
the asymptotic limit of infinite time. In this limit, the massless dilaton or
cosmon could be accompanied by another massless scalar field - the geon. At
finite time, small time-dependent masses for both the cosmon and geon are still
present due to imperfect dilatation symmetry. For a sufficiently large mass the
geon will start oscillating and play the role of dark matter, while the cosmon
is responsible for dark energy. The common origin of the mass of both fields
leads to an effective interaction between dark matter and dark energy.
Realistic cosmologies are possible for a simple form of the effective
cosmon-geon-potential. We find an inverse geon mass of a size where it could
reduce subgalactic structure formation.Comment: 4 pages, 2 figure
Orbital navigation, docking and obstacle avoidance as a form of three dimensional model-based image understanding
Range imagery from a laser scanner can be used to provide sufficient information for docking and obstacle avoidance procedures to be performed automatically. Three dimensional model-based computer vision algorithms in development can perform these tasks even with targets which may not be cooperative (that is, objects without special targets or markers to provide unambiguous points). Role, pitch, and yaw of a vehicle can be taken into account as image scanning takes place, so that these can be correlated when the image is converted from egocentric to world coordinated. Other attributes of the sensor, such as the registered reflectance and texture channels, provide additional data sources for algorithm robustness
Galileo early cruise, including Venus, first Earth, and Gaspra encounters
This article documents Deep Space Network (DSN) support for the Galileo cruise to Jupiter. The unique trajectory affords multiple encounters during this cruise phase. Each encounter had or will have unique requirements for data acquisition and DSN support configurations. An overview of the cruise and encounters through the asteroid Gaspra encounter is provided
Investigating the Physical Origin of Unconventional Low-Energy Excitations and Pseudogap Phenomena in Cuprate Superconductors
We investigate the physical origin of unconventional low-energy excitations
in cuprate superconductors by considering the effect of coexisting competing
orders (CO) and superconductivity (SC) and of quantum fluctuations and other
bosonic modes on the low-energy charge excitation spectra. By incorporating
both SC and CO in the bare Green's function and quantum phase fluctuations in
the self-energy, we can consistently account for various empirical findings in
both the hole- and electron-type cuprates, including the excess subgap
quasiparticle density of states, ``dichotomy'' in the fluctuation-renormalized
quasiparticle spectral density in momentum space, and the occurrence and
magnitude of a low-energy pseudogap being dependent on the relative gap
strength of CO and SC. Comparing these calculated results with experiments of
ours and others, we suggest that there are two energy scales associated with
the pseudogap phenomena, with the high-energy pseudogap probably of magnetic
origin and the low-energy pseudogap associated with competing orders.Comment: 10 pages, 5 figures. Invited paper for the 2006 Taiwan International
Conference on Superconductivity. Correspondence author: Nai-Chang Yeh
(e-mail: [email protected]
Medium corrections in the formation of light charged particles in heavy ion reactions
Within a microscopic statistical description of heavy ion collisions, we
investigate the effect of the medium on the formation of light clusters. The
dominant medium effects are self-energy corrections and Pauli blocking that
produce the Mott effect for composite particles and enhanced reaction rates in
the collision integrals. Microscopic description of composites in the medium
follows the Dyson equation approach combined with the cluster mean-field
expansion. The resulting effective few-body problem is solved within a properly
modified Alt-Grassberger-Sandhas formalism. The results are incorporated in a
Boltzmann-Uehling-Uhlenbeck simulation for heavy ion collisions. The number and
spectra of light charged particles emerging from a heavy ion collision changes
in a significant manner in effect of the medium modification of production and
absorption processes.Comment: 16 pages, 6 figure
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