3,122 research outputs found
Critical behaviour and scaling functions of the three-dimensional O(6) model
We numerically investigate the three-dimensional O(6) model on 12^3 to 120^3
lattices within the critical region at zero magnetic field, as well as at
finite magnetic field on the critical isotherm and for several fixed couplings
in the broken and the symmetric phase. We obtain from the Binder cumulant at
vanishing magnetic field the critical coupling J_c=1.42865(3). The universal
value of the Binder cumulant at this point is g_r(J_c)=-1.94456(10). At the
critical coupling, the critical exponents \gamma=1.604(6), \beta=0.425(2) and
\nu=0.818(5) are determined from a finite-size-scaling analysis. Furthermore,
we verify predicted effects induced by massless Goldstone modes in the broken
phase. The results are well described by the perturbative form of the model's
equation of state. Our O(6)-result is compared to the corresponding Ising, O(2)
and O(4) scaling functions. Finally, we study the finite-size-scaling behaviour
of the magnetisation on the pseudocritical line.Comment: 13 pages, 20 figures, REVTEX, fixed an error in the determination of
R_\chi and changed the corresponding line in figure 13
Spatial Correlation Function of X-ray Selected AGN
We present a detailed description of the first direct measurement of the
spatial correlation function of X-ray selected AGN. This result is based on an
X-ray flux-limited sample of 219 AGN discovered in the contiguous 80.7 deg^2
region of the ROSAT North Ecliptic Pole (NEP) Survey. Clustering is detected at
the 4 sigma level at comoving scales in the interval r = 5-60 h^-1 Mpc. Fitting
the data with a power law of slope gamma=1.8, we find a correlation length of
r_0 = 7.4 (+1.8, -1.9) h^-1 Mpc (Omega_M=0.3, Omega_Lambda=0.7). The median
redshift of the AGN contributing to the signal is z_xi=0.22. This clustering
amplitude implies that X-ray selected AGN are spatially distributed in a manner
similar to that of optically selected AGN. Furthermore, the ROSAT NEP
determination establishes the local behavior of AGN clustering, a regime which
is poorly sampled in general. Combined with high-redshift measures from optical
studies, the ROSAT NEP results argue that the AGN correlation strength
essentially does not evolve with redshift, at least out to z~2.2. In the local
Universe, X-ray selected AGN appear to be unbiased relative to galaxies and the
inferred X-ray bias parameter is near unity, b_X~1. Hence X-ray selected AGN
closely trace the underlying mass distribution. The ROSAT NEP AGN catalog,
presented here, features complete optical identifications and spectroscopic
redshifts. The median redshift, X-ray flux, and X-ray luminosity are z=0.41,
f_X=1.1*10^-13 cgs, and L_X=9.2*10^43 h_70^-2 cgs (0.5-2.0 keV), respectively.
Unobscured, type 1 AGN are the dominant constituents (90%) of this soft X-ray
selected sample of AGN.Comment: 17 pages, 8 figures, accepted for publication in ApJ, a version with
high-resolution figures is available at
http://www.eso.org/~cmullis/papers/Mullis_et_al_2004b.ps.gz, a
machine-readable version of the ROSAT NEP AGN catalog is available at
http://www.eso.org/~cmullis/research/nep-catalog.htm
HS 1857+5144 : a hot and young pre-cataclysmic variable
Aims. We report the discovery of a new white dwarf/M dwarf binary, HS 1857+5144, identified in the Hamburg Quasar Survey (HQS).
Methods. Time-resolved optical spectroscopy and photometry were carried out to determine the properties of this new cataclysmic variable progenitor (pre-CV).
Results. The light curves of HS 1857+5144 display a sinusoidal variation with a period of Porb = 383.52 min and peak-to-peak amplitudes of 0.7 mag and 1.1mag in the B-band and R-band, respectively. The large amplitude of the brightness variation results from a reflection effect on the heated inner hemisphere of the companion star, suggesting a very high temperature of the white
dwarf. Our radial velocity study confirms the photometric period as the orbital period of the system. A model atmosphere fit to the spectrum of the white dwarf obtained at minimum light provides limits to its mass and temperature of Mwd 0.6â1.0 M and Twd 70 000â100 000 K, respectively. The detection of He II λ4686 absorption classifies the primary star of HS 1857+5144 as a
DAO white dwarf. Combining the results from our spectroscopy and photometry, we estimate the mass of the companion star and the binary inclination to be Msec 0.15â0.30 M and i 45âŠâ55âŠ, respectively.
Conclusions. We classify HS 1857+5144 as one of the youngest pre-CV known to date. The cooling age of the white dwarf suggests that the present system has just emerged from a common envelope phase âŒ105 yr ago. HS 1857+5144 will start mass transfer within or below the 2â3 h period gap
Adjoint Wilson Line in SU(2) Lattice Gauge Theory
The behavior of the adjoint Wilson line in finite-temperature, ,
lattice gauge theory is discussed. The expectation value of the line and the
associated excess free energy reveal the response of the finite-temperature
gauge field to the presence of an adjoint source. The value of the adjoint line
at the critical point of the deconfining phase transition is highlighted. This
is not calculable in weak or strong coupling. It receives contributions from
all scales and is nonanalytic at the critical point. We determine the general
form of the free energy. It includes a linearly divergent term that is
perturbative in the bare coupling and a finite, nonperturbative piece. We use a
simple flux tube model to estimate the value of the nonperturbative piece. This
provides the normalization needed to estimate the behavior of the line as one
moves along the critical curve into the weak coupling region.Comment: 21 pages, no figures, Latex/Revtex 3, UCD-93-1
An Institutional Framework for Heterogeneous Formal Development in UML
We present a framework for formal software development with UML. In contrast
to previous approaches that equip UML with a formal semantics, we follow an
institution based heterogeneous approach. This can express suitable formal
semantics of the different UML diagram types directly, without the need to map
everything to one specific formalism (let it be first-order logic or graph
grammars). We show how different aspects of the formal development process can
be coherently formalised, ranging from requirements over design and Hoare-style
conditions on code to the implementation itself. The framework can be used to
verify consistency of different UML diagrams both horizontally (e.g.,
consistency among various requirements) as well as vertically (e.g.,
correctness of design or implementation w.r.t. the requirements)
Polarized Proton Beams from Laser-induced Plasmas
We report on the concept of an innovative source to produce polarized
proton/deuteron beams of a kinetic energy up to several GeV from a laser-driven
plasma accelerator. Spin effects have been implemented into the PIC simulation
code VLPL to make theoretical predictions about the behavior of proton spins in
laser-induced plasmas. Simulations of spin-polarized targets show that the
polarization is conserved during the acceleration process. For the experimental
realization, a polarized HCl gas-jet target is under construction using the
fundamental wavelength of a Nd:YAG laser system to align the HCl bonds and
simultaneously circular polarized light of the fifth harmonic to
photo-dissociate, yielding nuclear polarized H atoms. Subsequently, their
degree of polarization is measured with a Lamb-shift polarimeter. The final
experiments, aiming at the first observation of a polarized particle beam from
laser-generated plasmas, will be carried out at the 10 PW laser system SULF at
SIOM/Shanghai.Comment: 7 pages, 7 figure
Asymptotic Behavior of the Correlator for Polyakov Loops
The asymptotic behavior of the correlator for Polyakov loop operators
separated by a large distance is determined for high temperature QCD. It is
dominated by nonperturbative effects related to the exchange of magnetostatic
gluons. To analyze the asymptotic behavior, the problem is formulated in terms
of the effective field theory of QCD in 3 space dimensions. The Polyakov loop
operator is expanded in terms of local gauge-invariant operators constructed
out of the magnetostatic gauge field, with coefficients that can be calculated
using resummed perturbation theory. The asymptotic behavior of the correlator
is , where is the mass of the lowest-lying glueball in
-dimensional QCD. This result implies that existing lattice calculations
of the Polyakov loop correlator at the highest temperatures available do not
probe the true asymptotic region in .Comment: 10 pages, NUHEP-TH-94-2
QCD Phase Transition at Finite Temperature in the Dual Ginzburg-Landau Theory
We study the pure-gauge QCD phase transition at finite temperatures in the
dual Ginzburg-Landau theory, an effective theory of QCD based on the dual Higgs
mechanism. We formulate the effective potential at various temperatures by
introducing the quadratic source term, which is a new useful method to obtain
the effective potential in the negative-curvature region. Thermal effects
reduce the QCD-monopole condensate and bring a first-order deconfinement phase
transition. We find a large reduction of the self-interaction among
QCD-monopoles and the glueball masses near the critical temperature by
considering the temperature dependence of the self-interaction. We also
calculate the string tension at finite temperatures.Comment: 13 pages, uses PHYZZX ( 5 figures - available on request from
[email protected]
Collapse and revival of oscillations in a parametrically excited Bose-Einstein condensate in combined harmonic and optical lattice trap
In this work, we study parametric resonances in an elongated cigar-shaped BEC
in a combined harmonic trap and a time dependent optical lattice by using
numerical and analytical techniques. We show that there exists a relative
competition between the harmonic trap which tries to spatially localize the BEC
and the time varying optical lattice which tries to delocalize the BEC. This
competition gives rise to parametric resonances (collapse and revival of the
oscillations of the BEC width). Parametric resonances disappear when one of the
competing factors i.e strength of harmonic trap or the strength of optical
lattice dominates. Parametric instabilities (exponential growth of Bogoliubov
modes) arise for large variations in the strength of the optical lattice.Comment: 9 pages, 20 figure
Heavy Quark Free Energies and Screening in SU(2) Gauge Theory
We investigate the singlet, triplet and colour average heavy quark free
energies in SU(2) pure gauge theory at various temperatures T. We focus on the
long distance behaviour of the free energies, studying in particular the
temperature dependence of the string tension and the screening masses. The
results are qualitatively similar to the SU(3) scenario, except near the
critical temperature Tc of the deconfining transition. Finally we test a
recently proposed method to renormalize the Polyakov loop.Comment: 5 pages, 4 figures, contribution to the Proceedings of SEWM 2002
(Heidelberg
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