852 research outputs found
Electromagnetic sources distributed on shells in a Schwarzschild background
In the Introduction we briefly recall our previous results on stationary
electromagnetic fields on black-hole backgrounds and the use of spin-weighted
spherical harmonics. We then discuss static electric and magnetic test fields
in a Schwarzschild background using some of these results. As sources we do not
consider point charges or current loops like in previous works, rather, we
analyze spherical shells with smooth electric or magnetic charge distributions
as well as electric or magnetic dipole distributions depending on both angular
coordinates. Particular attention is paid to the discontinuities of the field,
of the 4-potential, and their relation to the source.Comment: dedicated to Professor Goldberg's 86th birthday, accepted for
publication in Gen. Relat. Gravit., 12 page
#Globalhealth Twitter Conversations on #Malaria, #HIV, #TB, #NCDS, and #NTDS: a Cross-Sectional Analysis
Charge Fluctuations on Membrane Surfaces in Water
We generalize the predictions for attractions between over-all neutral
surfaces induced by charge fluctuations/correlations to non-uniform systems
that include dielectric discontinuities, as is the case for mixed charged lipid
membranes in an aqueous solution. We show that the induced interactions depend
in a non-trivial way on the dielectric constants of membrane and water and show
different scaling with distance depending on these properties. The generality
of the calculations also allows us to predict under which dielectric conditions
the interaction will change sign and become repulsive
Spin polarised nuclear matter and its application to neutron stars
An equation of state(EOS) of nuclear matter with explicit inclusion of a
spin-isospin dependent force is constructed from a finite range, momentum and
density dependent effective interaction. This EOS is found to be in good
agreement with those obtained from more sophisticated models for unpolarised
nuclear matter. Introducing spin degrees of freedom, it is found that at
density about 2.5 times the density of normal nuclear matter the neutron matter
undergoes a ferromagnetic transition. The maximum mass and the radius of the
neutron star agree favourably with the observations. Since finding quark matter
rather than spin polarised nuclear matter at the core of neutron stars is more
probable, the proposed EOS is also applied to the study of hybrid stars. It is
found using the bag model picture that one can in principle describe both the
mass and size as well as the surface magnetic field of hybrid stars
satisfactorily.Comment: 26 pages, 11 figures available on reques
Transforming data by calculation
Thispaperaddressesthefoundationsofdata-modeltransformation.A catalog of data mappings is presented which includes abstraction and representa- tion relations and associated constraints. These are justified in an algebraic style via the pointfree-transform, a technique whereby predicates are lifted to binary relation terms (of the algebra of programming) in a two-level style encompassing both data and operations. This approach to data calculation, which also includes transformation of recursive data models into “flat” database schemes, is offered as alternative to standard database design from abstract models. The calculus is also used to establish a link between the proposed transformational style and bidi- rectional lenses developed in the context of the classical view-update problem.Fundação para a Ciência e a Tecnologia (FCT
Statin therapy/ HMG CO-A reductase inhibitor (statins) treatment in the prevention of coronary heart disease
Associated Higgs production with top quarks at the Large Hadron Collider: NLO QCD corrections
We present in detail the calculation of the O(alpha_s^3) inclusive total
cross section for the process pp -> t-tbar-h, in the Standard Model, at the
CERN Large Hadron Collider with center-of-mass energy sqrt(s_H)=14 TeV. The
calculation is based on the complete set of virtual and real O(alpha_s)
corrections to the parton level processes q-qbar -> t-tbar-h and gg ->
t-tbar-h, as well as the tree level processes (q,qbar)g -> t-tbar-h-(q,qbar).
The virtual corrections involve the computation of pentagon diagrams with
several internal and external massive particles, first encountered in this
process. The real corrections are computed using both the single and the two
cutoff phase space slicing method. The next-to-leading order QCD corrections
significantly reduce the renormalization and factorization scale dependence of
the Born cross section and moderately increase the Born cross section for
values of the renormalization and factorization scales above m_t.Comment: 70 pages, 12 figures, RevTeX4: one word changed in the abstract, one
sentence reworded in the introduction. To appear in Phys. Rev.
Evolution of cosmic superstring networks: a numerical simulation
We study the formation and evolution of an interconnected string network in
large-scale field-theory numerical simulations, both in flat spacetime and in
expanding universe. The network consists of gauge U(1) strings of two different
kinds and their bound states, arising due to an attractive interaction
potential. We find that the network shows no tendency to ``freeze'' and appears
to approach a scaling regime, with all characteristic lengths growing linearly
with time. Bound strings constitute only a small fraction of the total string
length in the network.Comment: 16 pages, 13 figures; Minor changes; Matches published versio
Star Formation and Dynamics in the Galactic Centre
The centre of our Galaxy is one of the most studied and yet enigmatic places
in the Universe. At a distance of about 8 kpc from our Sun, the Galactic centre
(GC) is the ideal environment to study the extreme processes that take place in
the vicinity of a supermassive black hole (SMBH). Despite the hostile
environment, several tens of early-type stars populate the central parsec of
our Galaxy. A fraction of them lie in a thin ring with mild eccentricity and
inner radius ~0.04 pc, while the S-stars, i.e. the ~30 stars closest to the
SMBH (<0.04 pc), have randomly oriented and highly eccentric orbits. The
formation of such early-type stars has been a puzzle for a long time: molecular
clouds should be tidally disrupted by the SMBH before they can fragment into
stars. We review the main scenarios proposed to explain the formation and the
dynamical evolution of the early-type stars in the GC. In particular, we
discuss the most popular in situ scenarios (accretion disc fragmentation and
molecular cloud disruption) and migration scenarios (star cluster inspiral and
Hills mechanism). We focus on the most pressing challenges that must be faced
to shed light on the process of star formation in the vicinity of a SMBH.Comment: 68 pages, 35 figures; invited review chapter, to be published in
expanded form in Haardt, F., Gorini, V., Moschella, U. and Treves, A.,
'Astrophysical Black Holes'. Lecture Notes in Physics. Springer 201
Evidence for an Excess of Soft Photons in Hadronic Decays of Z^0
Soft photons inside hadronic jets converted in front of the DELPHI main
tracker (TPC) in events of qqbar disintegrations of the Z^0 were studied in the
kinematic range 0.2 < E_gamma < 1 GeV and transverse momentum with respect to
the closest jet direction p_T < 80 MeV/c. A clear excess of photons in the
experimental data as compared to the Monte Carlo predictions is observed. This
excess (uncorrected for the photon detection efficiency) is (1.17 +/- 0.06 +/-
0.27) x 10^{-3} gamma/jet in the specified kinematic region, while the expected
level of the inner hadronic bremsstrahlung (which is not included in the Monte
Carlo) is (0.340 +/- 0.001 +/- 0.038) x 10^{-3} gamma/jet. The ratio of the
excess to the predicted bremsstrahlung rate is then (3.4 +/- 0.2 +/- 0.8),
which is similar in strength to the anomalous soft photon signal observed in
fixed target experiments with hadronic beams.Comment: 37 pages, 9 figures, Accepted by Eur. Phys. J.
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