14,289 research outputs found
Current Physics Results from Staggered Chiral Perturbation Theory
We review several results that have been obtained using lattice QCD with the
staggered quark formulation. Our focus is on the quantities that have been
calculated numerically with low statistical errors and have been extrapolated
to the physical quark mass limit and continuum limit using staggered chiral
perturbation theory. We limit our discussion to a brief introduction to
staggered quarks, and applications of staggered chiral perturbation theory to
the pion mass, decay constant, and heavy-light meson decay constants.Comment: 18 pages, 4 figures, commissioned review article, to appear in Mod.
Phys. Lett.
The Stellar Content of Obscured Galactic Giant H II Regions: II. W42
We present near infrared J, H, and K images and K-band spectroscopy in the
giant HII region W42. A massive star cluster is revealed; the color-color plot
and K-band spectroscopic morphology of two of the brighter objects suggest the
presence of young stellar objects. The spectrum of the bright central star is
similar to unobscured stars with MK spectral types of O5-O6.5. If this star is
on the zero age main sequence, then the derived spectrophotometric distance is
considerably smaller than previous estimates. The Lyman continuum luminosity of
the cluster is a few times that of the Trapezium. The slope of the K-band
luminosity function is similar to that for the Trapezium cluster and
significantly steeper than that for the massive star cluster in M17 or the
Arches cluster near the Galactic center.Comment: 30 pages, 11 figures, late
Submillimetre-sized dust aggregate collision and growth properties
The collisional and sticking properties of sub-mm-sized aggregates composed
of protoplanetary dust analogue material are measured, including the
statistical threshold velocity between sticking and bouncing, their surface
energy and tensile strength within aggregate clusters. We performed an
experiment on the REXUS 12 suborbital rocket. The protoplanetary dust analogue
materials were micrometre-sized monodisperse and polydisperse SiO2 particles
prepared into aggregates with sizes around 120 m and 330 m,
respectively and volume filling factors around 0.37. During the experimental
run of 150 s under reduced gravity conditions, the sticking of aggregates and
the formation and fragmentation of clusters of up to a few millimetres in size
was observed. The sticking probability of the sub-mm-sized dust aggregates
could be derived for velocities decreasing from 22 to 3 cm/s. The transition
from bouncing to sticking collisions happened at 12.7 cm/s for the smaller
aggregates composed of monodisperse particles and at 11.5 and 11.7 cm/s for the
larger aggregates composed of mono- and polydisperse dust particles,
respectively. Using the pull-off force of sub-mm-sized dust aggregates from the
clusters, the surface energy of the aggregates composed of monodisperse dust
was derived to be 1.6x10-5 J/m2, which can be scaled down to 1.7x10-2 J/m2 for
the micrometre-sized monomer particles and is in good agreement with previous
measurements for silica particles. The tensile strengths of these aggregates
within the clusters were derived to be 1.9 Pa and 1.6 Pa for the small and
large dust aggregates, respectively. These values are in good agreement with
recent tensile strength measurements for mm-sized silica aggregates. Using our
data on the sticking-bouncing threshold, estimates of the maximum aggregate
size can be given. For a minimum mass solar nebula model, aggregates can reach
sizes of 1 cm.Comment: 21 pages (incl. 6 pages of appendix), 23 figure
Low-velocity collision behaviour of clusters composed of sub-mm sized dust aggregates
The experiments presented aim to measure the outcome of collisions between
sub-mm sized protoplanetary dust aggregate analogues. We also observed the
clusters formed from these aggregates and their collision behaviour. The
experiments were performed at the drop tower in Bremen. The protoplanetary dust
analogue materials were micrometre-sized monodisperse and polydisperse SiO
particles prepared into aggregates with sizes between 120~m and
250~m. One of the dust samples contained aggregates that were previously
compacted through repeated bouncing. During three flights of 9~s of
microgravity each, individual collisions between aggregates and the formation
of clusters of up to a few millimetres in size were observed. In addition, the
collisions of clusters with the experiment cell walls leading to compaction or
fragmentation were recorded. We observed collisions amongst dust aggregates and
collisions between dust clusters and the cell aluminium walls at speeds ranging
from about 0.1 cm/s to 20 cm/s. The velocities at which sticking occurred
ranged from 0.18 to 5.0 cm/s for aggregates composed of monodisperse dust, with
an average value of 2.1 cm/s for reduced masses ranging from 1.2x10-6 to
1.8x10-3 g with an average value of 2.2x10-4 g. From the restructuring and
fragmentation of clusters composed of dust aggregates colliding with the
aluminium cell walls, we derived a collision recipe for dust aggregates
(100 m) following the model of Dominik \& Thielens (1997) developed
for microscopic particles. We measured a critical rolling energy of 1.8x10-13 J
and a critical breaking energy of 3.5x10-13 J for 100 m-sized
non-compacted aggregates.Comment: 12 pages, 13 figure
Periostreaktionen an langen Röhrenknochen im Kindesalter: Differenzialdiagnose "battered child syndrome"
Zusammenfassung: Periostreaktionen an langen Röhrenknochen im Kindesalter gehören zu den röntgenologischen Befunden, die für das "battered child syndrome" charakteristisch sind. Es wird über einen 2Monate alten Säugling berichtet, der mit einem Hämatom am rechten Oberschenkel in ein Kinderspital gebracht worden war. Wegen einer radiologisch nachgewiesenen Periostabhebung am rechten Oberschenkel und einem angegebenen häuslichen Trauma war zunächst der Verdacht auf eine Kindesmisshandlung geäußert worden. Die anschließende umfassende radiologische Diagnostik zeigte dann jedoch gleichartige Auffälligkeiten auch am linken Ober- und Unterschenkel. Nachdem derartige Veränderungen gelegentlich im Rahmen des Skelettwachstums im Kindesalter beobachtet werden, muss eine solche physiologische Entstehungsmöglichkeit gerade bei multifokalem Auftreten grundsätzlich in die differenzialdiagnostischen Überlegungen bei der Abklärung einer fraglichen Kindesmisshandlung einbezogen werde
Resonant Processes in a Frozen Gas
We present a theory of resonant processes in a frozen gas of atoms
interacting via dipole-dipole potentials that vary as , where is
the interatomic separation. We supply an exact result for a single atom in a
given state interacting resonantly with a random gas of atoms in a different
state. The time development of the transition process is calculated both on-
and off-resonance, and the linewidth with respect to detuning is obtained as a
function of time . We introduce a random spin Hamiltonian to model a dense
system of resonators and show how it reduces to the previous model in the limit
of a sparse system. We derive approximate equations for the average effective
spin, and we use them to model the behavior seen in the experiments of Anderson
et al. and Lowell et al. The approach to equilibrium is found to be
proportional to ), where the constant is explicitly related to the system's parameters.Comment: 30 pages, 6 figure
Calculating the hadronic vacuum polarization and leading hadronic contribution to the muon anomalous magnetic moment with improved staggered quarks
We present a lattice calculation of the hadronic vacuum polarization and the
lowest-order hadronic contribution to the muon anomalous magnetic moment, a_\mu
= (g-2)/2, using 2+1 flavors of improved staggered fermions. A precise fit to
the low-q^2 region of the vacuum polarization is necessary to accurately
extract the muon g-2. To obtain this fit, we use staggered chiral perturbation
theory, including the vector particles as resonances, and compare these to
polynomial fits to the lattice data. We discuss the fit results and associated
systematic uncertainties, paying particular attention to the relative
contributions of the pions and vector mesons. Using a single lattice spacing
ensemble (a=0.086 fm), light quark masses as small as roughly one-tenth the
strange quark mass, and volumes as large as (3.4 fm)^3, we find a_\mu^{HLO} =
(713 \pm 15) \times 10^{-10} and (748 \pm 21) \times 10^{-10} where the error
is statistical only and the two values correspond to linear and quadratic
extrapolations in the light quark mass, respectively. Considering systematic
uncertainties not eliminated in this study, we view this as agreement with the
current best calculations using the experimental cross section for e^+e^-
annihilation to hadrons, 692.4 (5.9) (2.4)\times 10^{-10}, and including the
experimental decay rate of the tau lepton to hadrons, 711.0 (5.0)
(0.8)(2.8)\times 10^{-10}. We discuss several ways to improve the current
lattice calculation.Comment: 44 pages, 4 tables, 17 figures, more discussion on matching the chpt
calculation to lattice calculation, typos corrected, refs added, version to
appear in PR
- …