7,429 research outputs found
Electroweak 2 -> 2 amplitudes for electron-positron annihilation at TeV energies
The non-radiative scattering amplitudes for electron-positron annihilation
into quark and lepton pairs in the TeV energy range are calculated in the
double-logarithmic approximation. The expressions for the amplitudes are
obtained using infrared evolution equations with different cut-offs for virtual
photons and for W and Z bosons, and compared with previous results obtained
with an universal cut-off.Comment: Revtex4, 17 pages, 7 figures. Some minor changes made, more refs
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Hadronization in heavy ion collisions: Recombination and fragmentation of partons
We argue that the emission of hadrons with transverse momentum up to about 5
GeV/c in central relativistic heavy ion collisions is dominated by
recombination, rather than fragmentation of partons. This mechanism provides a
natural explanation for the observed constant baryon-to-meson ratio of about
one and the apparent lack of a nuclear suppression of the baryon yield in this
momentum range. Fragmentation becomes dominant at higher transverse momentum,
but the transition point is delayed by the energy loss of fast partons in dense
matter.Comment: 4 pages, 2 figures; v2: reference [8] added; v3: Eq.(2) corrected,
two references added, version to appear in PR
Genome-wide signatures of population bottlenecks and diversifying selection in European wolves
Genomic resources developed for domesticated species provide powerful tools for studying the evolutionary history of their wild relatives. Here we use 61K single-nucleotide polymorphisms (SNPs) evenly spaced throughout the canine nuclear genome to analyse evolutionary relationships among the three largest European populations of grey wolves in comparison with other populations worldwide, and investigate genome-wide effects of demographic bottlenecks and signatures of selection. European wolves have a discontinuous range, with large and connected populations in Eastern Europe and relatively smaller, isolated populations in Italy and the Iberian Peninsula. Our results suggest a continuous decline in wolf numbers in Europe since the Late Pleistocene, and long-term isolation and bottlenecks in the Italian and Iberian populations following their divergence from the Eastern European population. The Italian and Iberian populations have low genetic variability and high linkage disequilibrium, but relatively few autozygous segments across the genome. This last characteristic clearly distinguishes them from populations that underwent recent drastic demographic declines or founder events, and implies long-term bottlenecks in these two populations. Although genetic drift due to spatial isolation and bottlenecks seems to be a major evolutionary force diversifying the European populations, we detected 35 loci that are putatively under diversifying selection. Two of these loci flank the canine platelet-derived growth factor gene, which affects bone growth and may influence differences in body size between wolf populations. This study demonstrates the power of population genomics for identifying genetic signals of demographic bottlenecks and detecting signatures of directional selection in bottlenecked populations, despite their low background variability.Heredity advance online publication, 18 December 2013; doi:10.1038/hdy.2013.122
Electron-phonon interaction dressed by electronic correlations near charge ordering as the origin for superconductivity in cobaltates
We consider possible routes to superconductivity in hydrated cobaltates
Na_xCoO_2.yH_2O on the basis of the t-J-V model plus phonons on the triangular
lattice. We studied the stability conditions for the homogeneous Fermi liquid
(HFL) phase against different broken symmetry phases. Besides the
sqrt(3)xsqrt(3)-CDW phase, triggered by the nearest-neighbour Coulomb
interaction V, we have found that the HFL is unstable, at very low doping,
against a bond-ordered phase due to J. We also discuss the occurrence of phase
separation at low doping and V. The interplay between the electron-phonon
interaction and correlations near the sqrt(3)xsqrt(3)-CDW leads to
superconductivity in the unconventional next-nearest neighbour f-wave (NNN-f)
channel with a dome shape for Tc around x ~ 0.35, and with values of a few
Kelvin as seen in experiments. Near the bond-ordered phase at low doping we
found tendencies to superconductivity with d-wave symmetry for finite J and
x<0.15. Contact with experiments is given along the paper.Comment: 21 pages, 6 figure
A remark on an overdetermined problem in Riemannian Geometry
Let be a Riemannian manifold with a distinguished point and
assume that the geodesic distance from is an isoparametric function.
Let be a bounded domain, with , and consider
the problem in with on ,
where is the -Laplacian of . We prove that if the normal
derivative of along the boundary of is a
function of satisfying suitable conditions, then must be a
geodesic ball. In particular, our result applies to open balls of
equipped with a rotationally symmetric metric of the form
, where is the standard metric of the sphere.Comment: 8 pages. This paper has been written for possible publication in a
special volume dedicated to the conference "Geometric Properties for
Parabolic and Elliptic PDE's. 4th Italian-Japanese Workshop", organized in
Palinuro in May 201
Logics for Rough Concept Analysis
Taking an algebraic perspective on the basic structures of Rough Concept
Analysis as the starting point, in this paper we introduce some varieties of
lattices expanded with normal modal operators which can be regarded as the
natural rough algebra counterparts of certain subclasses of rough formal
contexts, and introduce proper display calculi for the logics associated with
these varieties which are sound, complete, conservative and with uniform cut
elimination and subformula property. These calculi modularly extend the
multi-type calculi for rough algebras to a `nondistributive' (i.e. general
lattice-based) setting
A Tabu-search-based Algorithm for Distribution Network Restoration to Improve Reliability and Resiliency
Fault restoration techniques have always been crucial for distribution system operators (DSOs). In the last decade, it started to gain more and more importance due to the introduction of output-based regulations where DSO performances are evaluated according to frequency and duration of energy supply interruptions. The paper presents a tabu-search-based algorithm able to assist distribution network operational engineers in identifying solutions to restore the energy supply after permanent faults. According to the network property, two objective functions are considered to optimize either reliability or resiliency. The mathematical formulation includes the traditional feeders, number of switching operation limit, and radiality constraints. Thanks to the DSO of Milan, Unareti, the proposed algorithm has been tested on a real distribution network to investigate its effectiveness
Relativistic Mean-Field Theory Equation of State of Neutron Star Matter and a Maxwellian Phase Transition to Strange Quark Matter
The equation of state of neutron star matter is examined in terms of the
relativistic mean-field theory, including a scalar-isovector -meson
effective field. The constants of the theory are determined numerically so that
the empirically known characteristics of symmetric nuclear matter are
reproduced at the saturation density. The thermodynamic characteristics of both
asymmetric nucleonic matter and -equilibrium hadron-electron
-plasmas are studied. Assuming that the transition to strange quark matter
is an ordinary first-order phase transition described by Maxwell's rule, a
detailed study is made of the variations in the parameters of the phase
transition owing to the presence of a -meson field. The quark phase is
described using an improved version of the bag model, in which interactions
between quarks are accounted for in a one-gluon exchange approximation. The
characteristics of the phase transition are determined for various values of
the bag parameter within the range and it is shown
that including a -meson field leads to a reduction in the phase
transition pressure and in the concentrations and at
the phase transition point.Comment: 17 pages, 8 figure
Effect of symmetry energy on two-nucleon correlation functions in heavy-ion collisions induced by neutron-rich nuclei
Using an isospin-dependent transport model, we study the effects of nuclear
symmetry energy on two-nucleon correlation functions in heavy ion collisions
induced by neutron-rich nuclei. We find that the density dependence of the
nuclear symmetry energy affects significantly the nucleon emission times in
these collisions, leading to larger values of two-nucleon correlation functions
for a symmetry energy that has a stronger density dependence. Two-nucleon
correlation functions are thus useful tools for extracting information about
the nuclear symmetry energy from heavy ion collisions.Comment: Revised version, to appear in Phys. Rev. Let
Magnetic Reconnection and Intermittent Turbulence in the Solar Wind
A statistical relationship between magnetic reconnection, current sheets and
intermittent turbulence in the solar wind is reported for the first time using
in-situ measurements from the Wind spacecraft at 1 AU. We identify
intermittency as non-Gaussian fluctuations in increments of the magnetic field
vector, , that are spatially and temporally non-uniform. The
reconnection events and current sheets are found to be concentrated in
intervals of intermittent turbulence, identified using the partial variance of
increments method: within the most non-Gaussian 1% of fluctuations in
, we find 87%-92% of reconnection exhausts and 9% of current
sheets. Also, the likelihood that an identified current sheet will also
correspond to a reconnection exhaust increases dramatically as the least
intermittent fluctuations are removed from the dataset. Hence, the turbulent
solar wind contains a hierarchy of intermittent magnetic field structures that
are increasingly linked to current sheets, which in turn are progressively more
likely to correspond to sites of magnetic reconnection. These results could
have far reaching implications for laboratory and astrophysical plasmas where
turbulence and magnetic reconnection are ubiquitous.Comment: 5 pages, 3 figures, submitted to Physical Review Letter
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