311 research outputs found
Linear approach to the orbiting spacecraft thermal problem
We develop a linear method for solving the nonlinear differential equations
of a lumped-parameter thermal model of a spacecraft moving in a closed orbit.
Our method, based on perturbation theory, is compared with heuristic
linearizations of the same equations. The essential feature of the linear
approach is that it provides a decomposition in thermal modes, like the
decomposition of mechanical vibrations in normal modes. The stationary periodic
solution of the linear equations can be alternately expressed as an explicit
integral or as a Fourier series. We apply our method to a minimal thermal model
of a satellite with ten isothermal parts (nodes) and we compare the method with
direct numerical integration of the nonlinear equations. We briefly study the
computational complexity of our method for general thermal models of orbiting
spacecraft and conclude that it is certainly useful for reduced models and
conceptual design but it can also be more efficient than the direct integration
of the equations for large models. The results of the Fourier series
computations for the ten-node satellite model show that the periodic solution
at the second perturbative order is sufficiently accurate.Comment: 20 pages, 11 figures, accepted in Journal of Thermophysics and Heat
Transfe
The equation of state for two flavor QCD at N_t=6
We calculate the two flavor equation of state for QCD on lattices with
lattice spacing a=(6T)^{-1} and find that cutoff effects are substantially
reduced compared to an earlier study using a=(4T)^{-1}. However, it is likely
that significant cutoff effects remain. We fit the lattice data to expected
forms of the free energy density for a second order phase transition at
zero-quark-mass, which allows us to extrapolate the equation of state to m_q=0
and to extract the speed of sound. We find that the equation of state depends
weakly on the quark mass for small quark mass.Comment: 24 pages, latex, 11 postscipt figure
Deconfinement transition and string tensions in SU(4) Yang-Mills Theory
We present results from numerical lattice calculations of SU(4) Yang-Mills
theory. This work has two goals: to determine the order of the finite
temperature deconfinement transition on an lattice and to study the
string tensions between static charges in the irreducible representations of
SU(4). Motivated by Pisarski and Tytgat's argument that a second-order
SU() deconfinement transition would explain some features of the SU(3)
and QCD transitions, we confirm older results on a coarser, , lattice.
We see a clear two-phase coexistence signal, characteristic of a first-order
transition, at on a lattice, on which we also
compute a latent heat of . Computing
Polyakov loop correlation functions we calculate the string tension at finite
temperature in the confined phase between fundamental charges, ,
between diquark charges, , and between adjoint charges . We
find that , and our result for the adjoint string
tension is consistent with string breaking.Comment: 10 pages with included figures. For version 2: New calculation and
discussion of latent heat added; 2 new figures and 1 new table. Typo in
abstract corrected for v3. To appear in Physical Review
S-matrix approach to quantum gases in the unitary limit II: the three-dimensional case
A new analytic treatment of three-dimensional homogeneous Bose and Fermi
gases in the unitary limit of negative infinite scattering length is presented,
based on the S-matrix approach to statistical mechanics we recently developed.
The unitary limit occurs at a fixed point of the renormalization group with
dynamical exponent z=2 where the S-matrix equals -1. For fermions we find T_c
/T_F is approximately 0.1. For bosons we present evidence that the gas does not
collapse, but rather has a critical point that is a strongly interacting form
of Bose-Einstein condensation. This bosonic critical point occurs at n lambda^3
approximately 1.3 where n is the density and lambda the thermal wavelength,
which is lower than the ideal gas value of 2.61.Comment: 26 pages, 16 figure
Phenomenological Equations of State for the Quark-Gluon Plasma
Two phenomenological models describing an SU(N) quark-gluon plasma are
presented. The first is obtained from high temperature expansions of the free
energy of a massive gluon, while the second is derived by demanding color
neutrality over a certain length scale. Each model has a single free parameter,
exhibits behavior similar to lattice simulations over the range T_d - 5T_d, and
has the correct blackbody behavior for large temperatures. The N = 2
deconfinement transition is second order in both models, while N = 3,4, and 5
are first order. Both models appear to have a smooth large-N limit. For N >= 4,
it is shown that the trace of the Polyakov loop is insufficient to characterize
the phase structure; the free energy is best described using the eigenvalues of
the Polyakov loop. In both models, the confined phase is characterized by a
mutual repulsion of Polyakov loop eigenvalues that makes the Polyakov loop
expectation value zero. In the deconfined phase, the rotation of the
eigenvalues in the complex plane towards 1 is responsible for the approach to
the blackbody limit over the range T_d - 5T_d. The addition of massless quarks
in SU(3) breaks Z(3) symmetry weakly and eliminates the deconfining phase
transition. In contrast, a first-order phase transition persists with
sufficiently heavy quarks.Comment: 22 pages, RevTeX, 9 eps file
Cover to Volume 3
The fibroblast mitogen platelet-derived growth factor -BB (PDGF-BB) induces a transient expression of the orphan nuclear receptor NR4A1 (also named Nur77, TR3 or NGFIB). The aim of the present study was to investigate the pathways through which NR4A1 is induced by PDGF-BB and its functional role. We demonstrate that in PDGF-BB stimulated NIH3T3 cells, the MEK1/2 inhibitor CI-1040 strongly represses NR4A1 expression, whereas Erk5 downregulation delays the expression, but does not block it. Moreover, we report that treatment with the NF-κB inhibitor BAY11-7082 suppresses NR4A1 mRNA and protein expression. The majority of NR4A1 in NIH3T3 was found to be localized in the cytoplasm and only a fraction was translocated to the nucleus after continued PDGF-BB treatment. Silencing NR4A1 slightly increased the proliferation rate of NIH3T3 cells; however, it did not affect the chemotactic or survival abilities conferred by PDGF-BB. Moreover, overexpression of NR4A1 promoted anchorage-independent growth of NIH3T3 cells and the glioblastoma cell lines U-105MG and U-251MG. Thus, whereas NR4A1, induced by PDGF-BB, suppresses cell growth on a solid surface, it increases anchorage-independent growth
HQET at order : II. Spectroscopy in the quenched approximation
Using Heavy Quark Effective Theory with non-perturbatively determined
parameters in a quenched lattice calculation, we evaluate the splittings
between the ground state and the first two radially excited states of the
system at static order. We also determine the splitting between first excited
and ground state, and between the and ground states to order
. The Generalized Eigenvalue Problem and the use of all-to-all
propagators are important ingredients of our approach.Comment: (1+18) pages, 3 figures (4 pdf files); pdflatex; v2: corrections to
table 1, results unaffecte
Finite-size and Particle-number Effects in an Ultracold Fermi Gas at Unitarity
We investigate an ultracold Fermi gas at unitarity confined in a periodic box
using renormalization group (RG) techniques. Within this approach we
can quantitatively assess the long range bosonic order parameter fluctuations
which dominate finite-size effects. We determine the finite-size and
particle-number dependence of universal quantities, such as the Bertsch
parameter and the fermion gap. Moreover, we analyze how these universal
observables respond to the variation of an external pairing source. Our results
indicate that the Bertsch parameter saturates rather quickly to its value in
the thermodynamic limit as a function of increasing box size. On the other
hand, we observe that the fermion gap shows a significantly stronger dependence
on the box size, in particular for small values of the pairing source. Our
results may contribute to a better understanding of finite-size and
particle-number effects present in Monte-Carlo simulations of ultracold Fermi
gases.Comment: 13 pages, 7 figure
Confining strings in SU(N) gauge theories
We calculate the string tensions of -strings in SU() gauge theories in
both 3 and 4 dimensions. In D=3+1, we find that the ratio of the string
tension to the fundamental string tension is consistent, at the level, with both the M(-theory)QCD-inspired conjecture and with
`Casimir scaling'. In D=2+1 we see a definite deviation from the MQCD formula,
as well as a much smaller but still significant deviation from Casimir scaling.
We find that in both D=2+1 and D=3+1 the high temperature spatial -string
tensions also satisfy approximate Casimir scaling. We point out that
approximate Casimir scaling arises naturally if the cross-section of the flux
tube is nearly independent of the flux carried, and that this will occur in an
effective dual superconducting description, if we are in the deep-London limit.
We estimate, numerically, the intrinsic width of -strings in D=2+1 and
indeed find little variation with . In addition to the stable -strings we
investigate some ofthe unstable strings, finding in D=2+1 that they satisfy
(approximate) Casimir scaling. We also investigate the basic assumption that
confining flux tubes are described by an effective string theory at large
distances. We estimate the coefficient of the universal L\"uscher correction
from periodic strings that are longer than 1 fermi, and find in
D=3+1 and in D=2+1. These values are within of the
simple bosonic string values and are inconsistent with other simple effective
string theories.Comment: 57 pages, 11 figures. Errors on fits reduced by altering the analysis
to a standard one. Conclusions unchanged; note addedchanged. Some typos
correcte
Quenched Lattice QCD with Domain Wall Fermions and the Chiral Limit
Quenched QCD simulations on three volumes, , and
and three couplings, , 5.85 and 6.0 using domain
wall fermions provide a consistent picture of quenched QCD. We demonstrate that
the small induced effects of chiral symmetry breaking inherent in this
formulation can be described by a residual mass (\mres) whose size decreases
as the separation between the domain walls () is increased. However, at
stronger couplings much larger values of are required to achieve a given
physical value of \mres. For and , we find
\mres/m_s=0.033(3), while for , and ,
\mres/m_s=0.074(5), where is the strange quark mass. These values are
significantly smaller than those obtained from a more naive determination in
our earlier studies. Important effects of topological near zero modes which
should afflict an accurate quenched calculation are easily visible in both the
chiral condensate and the pion propagator. These effects can be controlled by
working at an appropriately large volume. A non-linear behavior of in
the limit of small quark mass suggests the presence of additional infrared
subtlety in the quenched approximation. Good scaling is seen both in masses and
in over our entire range, with inverse lattice spacing varying between
1 and 2 GeV.Comment: 91 pages, 34 figure
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