76,827 research outputs found
Four-quark flux distribution and binding in lattice SU(2)
The full spatial distribution of the color fields of two and four static
quarks is measured in lattice SU(2) field theory at separations up to 1 fm at
beta=2.4. The four-quark case is equivalent to a qbar q qbar q system in SU(2)
and is relevant to meson-meson interactions. By subtracting two-body flux tubes
from the four-quark distribution we isolate the flux contribution connected
with the four-body binding energy. This contribution is further studied using a
model for the binding energies. Lattice sum rules for two and four quarks are
used to verify the results.Comment: 46 pages including 71 eps figures. 3D color figures are available at
www.physics.helsinki.fi/~ppennane/pics
Non-equilibrium conductivity at quantum critical points
Quantum criticality provides an important route to revealing universal
non-equilibrium behaviour. A canonical example of a quantum critical point is
the Bose-Hubbard model, which we study under the application of an electric
field. A Boltzmann transport formalism and -expansion are used to
obtain the non-equilibrium conductivity and current noise. This approach allows
us to explicitly identify how a universal non-equilibrium steady state is
maintained, by identifying the rate-limiting step in balancing Joule heating
and dissipation to a heat bath. It also reveals that the non-equilibrium
distribution function is very far from a thermal distribution.Comment: 5 pages, 2 figure
Design concepts for the ASTROMAG cryogenic system
Described is a proposed cryogenic system used to cool the superconducting magnet for the Space Station based ASTROMAG Particle Astrophysics Facility. This 2-meter diameter superconducting magnet will be cooled using stored helium II. The paper presents a liquid helium storage concept which would permit cryogenic lifetimes of up to 3 years between refills. It is proposed that the superconducting coil be cooled using superfluid helium pumped by the thermomechanical effect. It is also proposed that the storage tank be resupplied with helium in orbit. A method for charging and discharging the magnet with minimum helium loss using split gas-cooled leads is discussed. A proposal to use a Stirling cycle cryocooler to extend the storage life of the cryostat will also be presented
A model for multi-quark systems
As a step towards understanding multi-quark systems abundant in nature we
construct a model that reproduces the binding energies of static four-quark
systems. These energies have been calculated using SU(2) lattice gauge theory
for a set of six different geometries representative of the general case. The
model is based on ground and excited state two-body potentials and multi-quark
interaction terms.Comment: 10 pages, one LaTeX figur
An interquark potential model for multi-quark systems
A potential model for four interacting quarks is constructed in SU(2) from
six basis states -- the three partitions into quark pairs, where the gluon
field is either in its ground state or first excited state. With four
independent parameters to describe the interactions connecting these basis
states, it is possible to fit 100 pieces of data -- the ground and first
excited states of configurations from six different four-quark geometries
calculated on a 16^3*32 lattice.Comment: 14 page
Radio Images of 3C 58: Expansion and Motion of its Wisp
New 1.4 GHz VLA observations of the pulsar-powered supernova remnant 3C 58
have resulted in the highest-quality radio images of this object to date. The
images show filamentary structure over the body of the nebula. The present
observations were combined with earlier ones from 1984 and 1991 to investigate
the variability of the radio emission on a variety of time-scales. No
significant changes are seen over a 110 day interval. In particular, the upper
limit on the apparent projected velocity of the wisp is 0.05c. The expansion
rate of the radio nebula was determined between 1984 and 2004, and is
0.014+/-0.003%/year, corresponding to a velocity of 630+/-70 km/s along the
major axis. If 3C 58 is the remnant of SN 1181, it must have been strongly
decelerated, which is unlikely given the absence of emission from the supernova
shell. Alternatively, the low expansion speed and a number of other arguments
suggest that 3C 58 may be several thousand years old and not be the remnant of
SN 1181.Comment: 12 pages; accepted for publication in the Astrophysical Journa
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