65,602 research outputs found
Modeling quasar accretion disc temperature profiles
Microlensing observations indicate that quasar accretion discs have
half-light radii larger than expected from standard theoretical predictions
based on quasar fluxes or black hole masses. Blackburne and colleagues have
also found a very weak wavelength dependence of these half-light radii. We
consider disc temperature profile models that might match these observations.
Nixon and colleagues have suggested that misaligned accretion discs around
spinning black holes will be disrupted at radii small enough for the
Lense-Thirring torque to overcome the disc's viscous torque. Gas in precessing
annuli torn off a disc will spread radially and intersect with the remaining
disc, heating the disc at potentially large radii. However, if the intersection
occurs at an angle of more than a degree or so, highly supersonic collisions
will shock-heat the gas to a Compton temperature of T~10^7 K, and the spectral
energy distributions (SEDs) of discs with such shock-heated regions are poor
fits to observations of quasar SEDs. Torn discs where heating occurs in
intermittent weak shocks that occur whenever the intersection angle reaches a
tenth of a degree pose less of a conflict with observations, but do not have
significantly larger half-light radii than standard discs. We also study two
phenomenological disc temperature profile models. We find that discs with a
temperature spike at relatively large radii and lowered temperatures at radii
inside the spike yield improved and acceptable fits to microlensing sizes in
most cases. Such temperature profiles could in principle occur in sub-Keplerian
discs partially supported by magnetic pressure. However, such discs overpredict
the fluxes from quasars studied with microlensing except in the limit of
negligible continuum emission from radii inside the temperature spike.Comment: Submitted to MNRAS. Comments welcome. 20 pages, 5 figure
Fish -- More Than Just Another Commodity
This brief highlights the contribution of wild capture fisheries to nutritional security in fish dependent developing countries. It is intended to stimulate debate around two broad themes: (1) when should the focus of fisheries policies be on local food security and human well-being as opposed to revenue generation, and (2) how does the current research agenda, with its emphasis on environmental and economic issues, assist or impair decision making processes
Relaxed Bell inequalities and Kochen-Specker theorems
The combination of various physically plausible properties, such as no
signaling, determinism, and experimental free will, is known to be incompatible
with quantum correlations. Hence, these properties must be individually or
jointly relaxed in any model of such correlations. The necessary degrees of
relaxation are quantified here, via natural distance and information-theoretic
measures. This allows quantitative comparisons between different models in
terms of the resources, such as the number of bits, of randomness,
communication, and/or correlation, that they require. For example, measurement
dependence is a relatively strong resource for modeling singlet state
correlations, with only 1/15 of one bit of correlation required between
measurement settings and the underlying variable. It is shown how various
'relaxed' Bell inequalities may be obtained, which precisely specify the
complementary degrees of relaxation required to model any given violation of a
standard Bell inequality. The robustness of a class of Kochen-Specker theorems,
to relaxation of measurement independence, is also investigated. It is shown
that a theorem of Mermin remains valid unless measurement independence is
relaxed by 1/3. The Conway-Kochen 'free will' theorem and a result of Hardy are
less robust, failing if measurement independence is relaxed by only 6.5% and
4.5%, respectively. An appendix shows the existence of an outcome independent
model is equivalent to the existence of a deterministic model.Comment: 19 pages (including 3 appendices); v3: minor clarifications, to
appear in PR
A possible supersymmetric solution to the discrepancy between B -> \phi K_S and B -> \eta' K_S CP asymmetries
We present a possible supersymmetric solution to the discrepancy between the
observed mixing CP asymmetries in B -> \phi K_S and B -> \eta' K_S. We show
that due to the different parity in the final states of these processes, their
supersymmetric contributions from the R-sector have an opposite sign, which
naturally explain the large deviation between S_{\phi K_S} and S_{\eta' K_S}.
We also consider the proposed mechanisms to solve the puzzle of the observed
large branching ratio of B -> \eta' K and study their impact on S_{eta' K_S}.Comment: 4 pages, 2 figure
Multipartite reduction criteria for separability
The reduction criterion is a well known necessary condition for separable
states, and states violating this condition are entangled and also
1-distillable. In this paper we introduce a new set of necessary conditions for
separability of multipartite states, obtained from a set of positive but not
completely positive maps. These conditions can be thought of as generalisations
of the reduction criterion to multipartite systems. We use tripartite Werner
states as an example to investigate the entanglement detecting powers of some
of these new conditions, and we also look at what these conditions mean in
terms of distillation. Finally, we show that these maps can be used to give a
partial solution to the subsystem problem, as described in Ref. [14].Comment: 6 pages, 1 figure, RevTe
Radiobiological studies with monoenergetic neutrons
The Radiological Research Accelerator Facility (RARAF) has the capability of
producing essentially monoenergetic neutron beams, ranging in energy from 16.4 MeV
down to 220 keV. In addition, two lower energy neutron beams are available which
consist of a wide spectrum of energies and are described as the 110 keV and 60 keV
spectra. Seedlings of Vicia faba have been used to measure the oxygen enhancement
ratio (OER) and the relative biological effectiveness (RBE) of each of these neutron
beams. The OER decreases as the neutron energy is reduced between 15.4 MeV and
220 keV, but does not appear to decrease further for lower energy neutrons. RBE increases
as the neutron energy is reduced from 15.4 AleV to 440 keV; the curve then
goes through a maximum at around 350 keV, and for lower energies the RBE falls again
On the statistical evaluation of dose-response functions
The linear-quadratic dependence of effect on the dose of ionizing radiation and its biophysical implications are considered. The estimation of the parameters of the response function and the derivation of the joint confidence region of the estimates are described. The method is applied to the induction of pink mutations inTradescantia which follows the linear-quadratic model. The statistical procedure is also suitable for other response functions
Recent Experiments with Bose-Condensed Gases at JILA
We consider a binary mixture of two overlapping Bose-Einstein condensates in
two different hyperfine states of \Rb87 with nearly identical magnetic moments.
Such a system has been simply realized through application of radiofrequency
and microwave radiation which drives a two-photon transition between the two
states. The nearly identical magnetic moments afford a high degree of spatial
overlap, permitting a variety of new experiments. We discuss some of the
conditions under which the magnetic moments are identical, with particular
emphasis placed on the requirements for a time-averaged orbiting potential
(TOP) magnetic trap.Comment: 9 pages, 5 figures; corrected post-publication editio
Supersymmetry without a Light Higgs Boson
Motivated by the absence, so far, of any direct signal of conventional
low-energy supersymmetry, we explore the consequences of making the lightest
Higgs boson in supersymmetry relatively heavy, up to about 300 GeV, in the most
straightforward way, i.e. via the introduction of a chiral singlet S with a
superpotential interaction with the Higgs doublets, \lambda S H_1 H_2. The
coupling \lambda dominates over all the other couplings and, to maintain the
successful perturbative analysis of the ElectroWeak Precision Tests, is only
restricted to remain perturbative up to about 10 TeV. The general features of
this "\lambda SUSY" framework, which deviates significantly from the MSSM or
the standard NMSSM, are analyzed in different areas: ElectroWeak Precision
Tests, Dark Matter, naturalness bounds on superparticle masses, and LHC
signals. There is a rich Higgs/Higgsino sector in the (200-700)GeV mass region,
which may include LSP Higgsino dark matter. All other superpartners, apart from
the top squarks, may naturally be heavier than 1-2 TeV. This picture can be
made consistent with gauge coupling unification.Comment: 27 page
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