15,078 research outputs found
Entanglement Detection by Local Orthogonal Observables
We propose a family of entanglement witnesses and corresponding positive maps
that are not completely positive based on local orthogonal observables. As
applications the entanglement witness of the bound entangled state
[P. Horodecki, Phys. Lett. A {\bf 232}, 333 (1997)] is explicitly constructed
and a family of -dimensional bound entangled states is designed so that the
entanglement can be detected by permuting local orthogonal observables. Further
the proposed physically not implementable positive maps can be physically
realized by measuring a Hermitian correlation matrix of local orthogonal
observables.Comment: 4 pages, 1 figur
Can one control systematic errors of QCD sum rule predictions for bound states?
We study the possibility to control systematic errors of the ground-state
parameters obtained by Shifman-Vainshtein-Zakharov (SVZ) sum rules, making use
of the harmonic-oscillator potential model as an example. In this case, one
knows the exact solution for the polarization operator, which allows one to
obtain both the OPE to any order and the parameters (masses and decay
constants) of the bound states. We determine the parameters of the ground state
making use of the standard procedures of the method of QCD sum rules, and
compare the obtained results with the known exact values. We show that in the
situation when the continuum contribution to the polarization operator is not
known and is modelled by an effective continuum, the method of sum rules does
not allow to control the systematic errors of the extracted ground-state
parameters.Comment: RevTex, 7 pages, figure 4 modified, version to be published in Phys.
Lett.
Polchinski equation, reparameterization invariance and the derivative expansion
The connection between the anomalous dimension and some invariance properties
of the fixed point actions within exact RG is explored. As an application,
Polchinski equation at next-to-leading order in the derivative expansion is
studied. For the Wilson fixed point of the one-component scalar theory in three
dimensions we obtain the critical exponents \eta=0.042, \nu=0.622 and
\omega=0.754.Comment: 28 pages, LaTeX with psfig, 12 encapsulated PostScript figures. A
number wrongly quoted in the abstract correcte
Major Merging: The Way to Make a Massive, Passive Galaxy
We analyze the projected axial ratio distribution, p(b/a), of galaxies that
were spectroscopically selected from the Sloan Digital Sky Survey (DR6) to have
low star-formation rates. For these quiescent galaxies we find a rather abrupt
change in p(b/a) at a stellar mass of ~10^{11} M_sol: at higher masses there
are hardly any galaxies with b/a<0.6, implying that essentially none of them
have disk-like intrinsic shapes and must be spheroidal. This transition mass is
~3-4 times higher than the threshold mass above which quiescent galaxies
dominate in number over star-forming galaxies, which suggests these mass scales
are unrelated. At masses lower than ~10^{11} M_sol, quiescent galaxies show a
large range in axial ratios, implying a mix of bulge- and disk-dominated
galaxies. Our result strongly suggests that major merging is the most
important, and perhaps only relevant, evolutionary channel to produce massive
(>10^{11} M_sol), quiescent galaxies, as it inevitably results in spheroids.Comment: Minor changes to match published version in ApJ Letter
Calibration update of the COMBO-17 CDFS catalogue
We present an update to the photometric calibration of the COMBO-17 catalogue
on the Extended Chandra Deep Field South, which is now consistent with the
GaBoDS and MUSYC catalogues. As a result, photometric redshifts become slightly
more accurate, with <0.01 rms and little bias in the delta_z/(1+z) of galaxies
with R<21 and of QSOs with R<24. With increasing photon noise the rms of
galaxies reaches 0.02 for R<23 and 0.035 at R~23.5. Consequences for the
rest-frame colours of galaxies at z<1 are discussed.Comment: A&A research note, resubmitted 02 Oct 2008, 4 pages in print forma
Infrared Spectral Energy Distributions of z~0.7 Star-Forming Galaxies
We analyze the infrared (IR) spectral energy distributions (SEDs) for
10micron < lambda(rest) < 100micron for ~600 galaxies at z~0.7 in the extended
Chandra Deep Field South by stacking their Spitzer 24, 70 and 160micron images.
We place interesting constraints on the average IR SED shape in two bins: the
brightest 25% of z~0.7 galaxies detected at 24micron, and the remaining 75% of
individually-detected galaxies. Galaxies without individual detections at
24micron were not well-detected at 70micron and 160micron even through
stacking. We find that the average IR SEDs of z~0.7 star-forming galaxies fall
within the diversity of z~0 templates. While dust obscuration Lir/Luv seems to
be only a function of star formation rate (SFR; ~ Lir+Luv), not of redshift,
the dust temperature of star-forming galaxies (with SFR ~ 10 solar mass per
year) at a given IR luminosity was lower at z~0.7 than today. We suggest an
interpretation of this phenomenology in terms of dust geometry: intensely
star-forming galaxies at z~0 are typically interacting, and host dense
centrally-concentrated bursts of star formation and warm dust temperatures. At
z~0.7, the bulk of intensely star-forming galaxies are relatively undisturbed
spirals and irregulars, and we postulate that they have large amounts of
widespread lower-density star formation, yielding lower dust temperatures for a
given IR luminosity. We recommend what IR SEDs are most suitable for modeling
intermediate redshift galaxies with different SFRs.Comment: 12 pages, 11 figures, 2 tables, accepted for publication in Ap
Hadron form factors from sum rules for vacuum-to-hadron correlators
We analyse the extraction of the bound-state form factor from
vacuum-to-hadron correlator, which is the basic object for the calculation of
hadron form factors in the method of light-cone sum rules in QCD. We study this
correlator in quantum mechanics, calculate it exactly, and derive the
corresponding OPE. We then apply the standard procedures of QCD sum rules to
isolate the ground-state form factor from this correlator. We demonstrate that
fixing the effective continuum threshold, one of the key ingredients of the
sum-rule calculation of bound-state parameters, poses a serious problem for sum
rules based on vacuum-to-hadron correlators.Comment: 8 page
Molecular dark matter in galaxies
Clouds containing molecular dark matter in quantities relevant for star
formation may exist in minihaloes of the type of cold dark matter included in
many cosmological simulations or in the regions of some galaxies extending far
beyond their currently known boundaries. We have systematically explored
parameter space to identify conditions under which plane-parallel clouds
contain sufficient column densities of molecular dark matter that they could be
reservoirs for future star formation. Such clouds would be undetected or at
least appear by current observational criteria to be uninteresting from the
perspective of star formation. We use a time-dependent PDR code to produce
theoretical models of the chemistry and emission arising in clouds for our
chosen region of parameter space. We then select a subset of model clouds with
levels of emission that are low enough to be undetectable or at least
overlooked by current surveys. The existence of significant column densities of
cold molecular dark matter requires that the background radiation field be
several or more orders of magnitude weaker than that in the solar
neighbourhood. Lower turbulent velocities and cosmic ray induced ionization
rates than typically associated with molecular material within a kpc of the Sun
are also required for the molecular matter to be dark. We find that there is a
large region within the parameter space that results in clouds that might
contain a significant mass of molecular gas whilst remaining effectively
undetectable or at least not particularly noticeable in surveys. We note
briefly conditions under which molecular dark matter may contain a dynamically
interesting mass.Comment: 9 pages, 2 figures, accepted for publication in A&A; additional
concluding paragraph added at proof stag
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