8,800 research outputs found
A Variable-Flavour Number Scheme for NNLO
At NNLO it is particularly important to have a Variable-Flavour Number Scheme
(VFNS) to deal with heavy quarks because there are major problems with both the
zero mass variable-flavour number scheme and the fixed-flavour number scheme. I
illustrate these problems and present a general formulation of a
Variable-Flavour Number Scheme (VFNS)for heavy quarks that is explicitly
implemented up to NNLO in the strong coupling constant alpha_S, and may be used
in NNLO global fits for parton distributions. The procedure combines elements
of the ACOT(chi) scheme and the Thorne-Roberts scheme. Despite the fact that at
NNLO the parton distributions are discontinuous as one changes the number of
active quark flavours, all physical quantities are continuous at flavour
transitions and the comparison with data is successful.Comment: 17 pages, 5 figures included as .ps files, uses axodraw. One
additional explanatory sentence after eq. (25). Correction of typos and
updated references. To be published in Phys. Rev.
Centrifugally driven electrostatic instability in extragalactic jets
The stability problem of the rotation induced electrostatic wave in
extragalactic jets is presented. Solving a set of equations describing dynamics
of a relativistic plasma flow of AGN jets, an expression of the instability
rate has been derived and analyzed for typical values of AGNs. The growth rate
was studied versus the wave length and the inclination angle and it has been
found that the instability process is much efficient with respect to the
accretion disk evolution, indicating high efficiency of the instability.Comment: 7 pages, 4 figure
Mitochondrial Dna Replacement Versus Nuclear Dna Persistence
In this paper we consider two populations whose generations are not
overlapping and whose size is large. The number of males and females in both
populations is constant. Any generation is replaced by a new one and any
individual has two parents for what concerns nuclear DNA and a single one (the
mother) for what concerns mtDNA. Moreover, at any generation some individuals
migrate from the first population to the second.
In a finite random time , the mtDNA of the second population is completely
replaced by the mtDNA of the first. In the same time, the nuclear DNA is not
completely replaced and a fraction of the ancient nuclear DNA persists. We
compute both and . Since this study shows that complete replacement of
mtDNA in a population is compatible with the persistence of a large fraction of
nuclear DNA, it may have some relevance for the Out of Africa/Multiregional
debate in Paleoanthropology
Absorption Effects due to Spin in the Worldline Approach to Black Hole Dynamics
We generalize the effective point particle approach to black hole dynamics to
include spin. In this approach dissipative effects are captured by degrees of
freedom localized on the wordline. The absorptive properties of the black hole
are determined by correlation functions which can be matched with the graviton
absorption cross section in the long wavelength approximation. For rotating
black holes, superradiance is responsible for the leading contribution. The
effective theory is then used to predict the power loss due to spin in the
dynamics of non-relativistic binary systems. An enhancement of three powers of
the relative velocity is found with respect to the non-rotating case. Then we
generalize the results to other type of constituents in the binary system, such
as rotating neutron stars. Finally we compute the power loss absorbed by a test
spinning black hole in a given spacetime background.Comment: 17 pages, 2 figures. v2 Typos and misprints fixe
Area evolution, bulk viscosity and entropy principles for dynamical horizons
We derive from Einstein equation an evolution law for the area of a trapping
or dynamical horizon. The solutions to this differential equation show a causal
behavior. Moreover, in a viscous fluid analogy, the equation can be interpreted
as an energy balance law, yielding to a positive bulk viscosity. These two
features contrast with the event horizon case, where the non-causal evolution
of the area and the negative bulk viscosity require teleological boundary
conditions. This reflects the local character of trapping horizons as opposed
to event horizons. Interpreting the area as the entropy, we propose to use an
area/entropy evolution principle to select a unique dynamical horizon and time
slicing in the Cauchy evolution of an initial marginally trapped surface.Comment: Some references added, 5 pages, Phys. Rev. D, in pres
General-Relativistic MHD for the Numerical Construction of Dynamical Spacetimes
We assemble the equations of general relativistic magnetohydrodynamics (MHD)
in 3+1 form. These consist of the complete coupled set of Maxwell equations for
the electromagnetic field, Einstein's equations for the gravitational field,
and the equations of relativistic MHD for a perfectly conducting ideal gas. The
adopted form of the equations is suitable for evolving numerically a
relativistic MHD fluid in a dynamical spacetime characterized by a strong
gravitational field.Comment: 8 pages; scheduled for March 10 issue of Ap
Dynamical Collapse of Charged Scalar Field in Phantom Gravity
We investigated the problem of the dynamical collapse of a self-gravitating
complex charged scalar field in Einstein-Maxwell-dilaton theory with a phantom
copuling for the adequate fields in the system under consideration. We also
considered two simplifications of it, i.e., the separate collapses of phantom
Maxwell and phantom scalar fields under the influence of Einstein gravity. One
starts with the regular spacetime and leads the evolution through the formation
of the horizons and the final singularity. We discuss the structures of
spacetimes emerging in the process of the dynamical collapse and comment on the
role of the considered fields in its course.Comment: 15 pages, RevTex, 18 figures, to be published in Phys.Rev.D1
Model-independent test of gravity with a network of ground-based gravitational-wave detectors
The observation of gravitational waves with a global network of
interferometric detectors such as advanced LIGO, advanced Virgo, and KAGRA will
make it possible to probe into the nature of space-time structure. Besides
Einstein's general theory of relativity, there are several theories of
gravitation that passed experimental tests so far. The gravitational-wave
observation provides a new experimental test of alternative theories of gravity
because a gravitational wave may have at most six independent modes of
polarization, of which properties and number of modes are dependent on theories
of gravity. This paper proposes a method to reconstruct the independent modes
of polarization in time-series data of an advanced detector network. Since the
method does not rely on any specific model, it gives model-independent test of
alternative theories of gravity
Domain Wall Depinning in Random Media by AC Fields
The viscous motion of an interface driven by an ac external field of
frequency omega_0 in a random medium is considered here for the first time. The
velocity exhibits a smeared depinning transition showing a double hysteresis
which is absent in the adiabatic case omega_0 --> 0. Using scaling arguments
and an approximate renormalization group calculation we explain the main
characteristics of the hysteresis loop. In the low frequency limit these can be
expressed in terms of the depinning threshold and the critical exponents of the
adiabatic case.Comment: 4 pages, 3 figure
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