168 research outputs found
Observation of strong final-state effects in pi+ production in pp collisions at 400 MeV
Differential cross sections of the reactions and have been measured at MeV by detecting the charged
ejectiles in the angular range . The
deduced total cross sections agree well with those published previously for
neighbouring energies. The invariant mass spectra are observed to be strongly
affected by production and final-state interaction. The data are
well described by Monte Carlo simulations including both these effects. The
ratio of and cross sections also compares
favourably to a recent theoretical prediction which suggests a dominance of
-production in the relative -state.Comment: 17 pages, 5 figure
Spin correlations in pion production near threshold
A first measurement of longitudinal as well as transverse spin correlation
coefficients for the reaction was made using a
polarized proton target and a polarized proton beam. We report kinematically
complete measurements for this reaction at 325, 350, 375 and 400 MeV beam
energy. The spin correlation coefficients and the analyzing power as well as angular
distributions for and the polarization observables
were extracted. Partial wave cross sections for dominant
transition channels were obtained from a partial wave analysis that included
the transitions with final state angular momenta of . The measurements
of the polarization observables are compared
with the predictions from the J\"ulich meson exchange model. The agreement is
very good at 325 MeV, but it deteriorates increasingly for the higher energies.
At all energies agreement with the model is better than for the reaction
.Comment: Preprint, 21 pp, submitted to Phys. Rev. C. Keywords: Mesons,
Polarization, Spin Correlations, Few body system
Preheated Advection Dominated Accretion Flow
All high temperature accretion solutions including ADAF are physically thick,
so outgoing radiation interacts with the incoming flow, sharing as much or more
resemblance with classical spherical accretion flows as with disk flows. We
examine this interaction for the popular ADAF case. We find that without
allowance for Compton preheating, a very restricted domain of ADAF solution is
permitted and with Compton preheating included a new high temperature PADAF
branch appears in the solution space. In the absence of preheating, high
temperature flows do not exist when the mass accretion rate mdot == Mdot c^2 /
L_E >~ 10^-1.5. Below this mass accretion rate, a roughly conical region around
the hole cannot sustain high temperature ions and electrons for all flows
having mdot >~ 10^-4, which may lead to a funnel possibly filled with a tenuous
hot outgoing wind. If the flow starts at large radii with the usual equilibrium
temperature ~10^4 K, the critical mass accretion rate is much lower, mdot
\~10^-3.7 above which level no self-consistent ADAF (without preheating) can
exist. However, above this critical mass accretion rate, the flow can be
self-consistently maintained at high temperature if Compton preheating is
considered. These solutions constitute a new branch of solutions as in
spherical accretion flows. High temperature PADAF flows can exist above the
critical mass accretion rate in addition to the usual cold thin disk solutions.
We also find solutions where the flow near the equatorial plane accretes
normally while the flow near the pole is overheated by Compton preheating,
possibly becoming, a polar wind, solutions which we designate WADAF.Comment: 41 pages with 10 postscript figures (aastex5). Submitted to Ap
Does the Slim-Disk Model Correctly Consider Photon-Trapping Effects?
We investigate the photon-trapping effects in the super-critical black hole
accretion flows by solving radiation transfer as well as the energy equations
of radiation and gas. It is found that the slim-disk model generally
overestimates the luminosity of the disk at around the Eddington luminosity
(L_E) and is not accurate in describing the effective temperature profile,
since it neglects time delay between energy generation at deeper inside the
disk and energy release at the surface. Especially, the photon-trapping effects
are appreciable even below L ~ L_E, while they appear above ~ 3L_E according to
the slim disk. Through the photon-trapping effects, the luminosity is reduced
and the effective temperature profile becomes flatter than r^{-3/4} as in the
standard disk. In the case that the viscous heating is effective only around
the equatorial plane, the luminosity is kept around the Eddington luminosity
even at very large mass accretion rate, Mdot>>L_E/c^2. The effective
temperature profile is almost flat, and the maximum temperature decreases in
accordance with rise in the mass accretion rate. Thus, the most luminous radius
shifts to the outer region when Mdot/(L_E/c^2) >> 10^2. In the case that the
energy is dissipated equally at any heights, the resultant luminosity is
somewhat larger than in the former case, but the energy-conversion efficiency
still decreases with increase of the mass accretion rate, as well. The most
luminous radius stays around the inner edge of the disk in the latter case.
Hence, the effective temperature profile is sensitive to the vertical
distribution of energy production rates, so is the spectral shape. Future
observations of high L/L_E objects will be able to test our model.Comment: 10 pages, 7 figures, accepted for publication in Ap
Dark Matter Accretion into Supermassive Black Holes
The relativistic accretion rate of dark matter by a black hole is revisited.
Under the assumption that the phase space density indicator,
, remains constant during the inflow, the
derived accretion rate can be higher up to five orders of magnitude than the
classical accretion formula, valid for non-relativistic and non-interacting
particles, when typical dark halo conditions are considered. For these typical
conditions, the critical point of the flow is located at distances of about
30-150 times the horizon radius. Application of our results to black hole seeds
hosted by halos issued from cosmological simulations indicate that dark matter
contributes to no more than ~10% of the total accreted mass, confirming that
the bolometric quasar luminosity is related to the baryonic accretion history
of the black hole.Comment: 7 pages, 6 figures. Accepted for publication in Phys.Rev.
Dependence of pp->pp pi0 near Threshold on the Spin of the Colliding Nucleons
A polarized internal atomic hydrogen target and a stored, polarized beam are
used to measure the spin-dependent total cross section Delta_sigma_T/sigma_tot,
as well as the polar integrals of the spin correlation coefficient combination
A_xx-A_yy, and the analyzing power A_y for pp-> pp pi0 at four bombarding
energies between 325 and 400 MeV. This experiment is made possible by the use
of a cooled beam in a storage ring. The polarization observables are used to
study the contribution from individual partial waves.Comment: 6 pages, 1 table, 4 figures, corrected equations 2 and
The structure and evolution of quasi-stars
The existence of bright quasars at high redshifts implies that supermassive
black holes were able to form in the early Universe. Though a number of
mechanisms to achieve this have been proposed, none yet stands out. A recent
suggestion is the formation of quasi-stars, initially stellar-mass black holes
accreting from hydrostatic giant-like envelopes of gas, formed from the
monolithic collapse of pre-galactic gas clouds. In this work, we modify the
Cambridge STARS stellar evolution package to construct detailed models of the
evolution of these objects. We find that, in all of our models, the black hole
inside the envelope is able to reach slightly more than one-tenth of the total
mass of the system before hydrostatic equilibrium breaks down. This breakdown
occurs after a few million years of evolution. We show that the mechanism which
causes the hydrostatic evolution to end is present in polytropic models. We
also show that the solutions are highly sensitive to the size of the inner
boundary radius and that no physical solutions exist if the inner boundary is
chosen to be less than about 0.3 of the Bondi radius.Comment: 12 pages, 11 figures. Published in MNRAS. Very belatedly updated to
(more closely) match published versio
1.5 T augmented reality navigated interventional MRI: paravertebral sympathetic plexus injections
PURPOSE:The high contrast resolution and absent ionizing radiation of interventional magnetic resonance imaging (MRI) can be advantageous for paravertebral sympathetic nerve plexus injections. We assessed the feasibility and technical performance of MRI-guided paravertebral sympathetic injections utilizing augmented reality navigation and 1.5 T MRI scanner.METHODS:A total of 23 bilateral injections of the thoracic (8/23, 35%), lumbar (8/23, 35%), and hypogastric (7/23, 30%) paravertebral sympathetic plexus were prospectively planned in twelve human cadavers using a 1.5 Tesla (T) MRI scanner and augmented reality navigation system. MRI-conditional needles were used. Gadolinium-DTPA-enhanced saline was injected. Outcome variables included the number of control magnetic resonance images, target error of the needle tip, punctures of critical nontarget structures, distribution of the injected fluid, and procedure length.RESULTS: Augmented-reality navigated MRI guidance at 1.5 T provided detailed anatomical visualization for successful targeting of the paravertebral space, needle placement, and perineural paravertebral injections in 46 of 46 targets (100%). A mean of 2 images (range, 1â5 images) were required to control needle placement. Changes of the needle trajectory occurred in 9 of 46 targets (20%) and changes of needle advancement occurred in 6 of 46 targets (13%), which were statistically not related to spinal regions (P = 0.728 and P = 0.86, respectively) and cadaver sizes (P = 0.893 and P = 0.859, respectively). The mean error of the needle tip was 3.9±1.7 mm. There were no punctures of critical nontarget structures. The mean procedure length was 33±12 min.CONCLUSION:1.5 T augmented reality-navigated interventional MRI can provide accurate imaging guidance for perineural injections of the thoracic, lumbar, and hypogastric sympathetic plexus
Elastic modulus of tree frog adhesive toe pads
Previous work using an atomic force microscope in nanoindenter mode indicated that the outer, 10- to 15-ÎŒm thick, keratinised layer of tree frog toe pads has a modulus of elasticity equivalent to silicone rubber (5â15Â MPa) (Scholz et al. 2009), but gave no information on the physical properties of deeper structures. In this study, micro-indentation is used to measure the stiffness of whole toe pads of the tree frog, Litoria caerulea. We show here that tree frog toe pads are amongst the softest of biological structures (effective elastic modulus 4â25Â kPa), and that they exhibit a gradient of stiffness, being stiffest on the outside. This stiffness gradient results from the presence of a dense network of capillaries lying beneath the pad epidermis, which probably has a shock absorbing function. Additionally, we compare the physical properties (elastic modulus, work of adhesion, pull-off force) of the toe pads of immature and adult frogs
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