10,539 research outputs found
U(2) Flavor Physics without U(2) Symmetry
We present a model of fermion masses based on a minimal, non-Abelian discrete
symmetry that reproduces the Yukawa matrices usually associated with U(2)
theories of flavor. Mass and mixing angle relations that follow from the simple
form of the quark and charged lepton Yukawa textures are therefore common to
both theories. We show that the differing representation structure of our
horizontal symmetry allows for new solutions to the solar and atmospheric
neutrino problems that do not involve modification of the original charged
fermion Yukawa textures, or the introduction of sterile neutrinos.Comment: 12 pages RevTeX, 1 eps figure. A few typos correcte
What brakes the Crab pulsar?
Optical observations provide convincing evidence that the optical phase of
the Crab pulsar follows the radio one closely. Since optical data do not depend
on dispersion measure variations, they provide a robust and independent
confirmation of the radio timing solution. The aim of this paper is to find a
global mathematical description of Crab pulsar's phase as a function of time
for the complete set of published Jodrell Bank radio ephemerides (JBE) in the
period 1988-2014. We apply the mathematical techniques developed for analyzing
optical observations to the analysis of JBE. We break the whole period into a
series of episodes and express the phase of the pulsar in each episode as the
sum of two analytical functions. The first function is the best-fitting local
braking index law, and the second function represents small residuals from this
law with an amplitude of only a few turns, which rapidly relaxes to the local
braking index law. From our analysis, we demonstrate that the power law index
undergoes "instantaneous" changes at the time of observed jumps in rotational
frequency (glitches). We find that the phase evolution of the Crab pulsar is
dominated by a series of constant braking law episodes, with the braking index
changing abruptly after each episode in the range of values between 2.1 and
2.6. Deviations from such a regular phase description behave as oscillations
triggered by glitches and amount to fewer than 40 turns during the above
period, in which the pulsar has made more than 2.0e10 turns. Our analysis does
not favor the explanation that glitches are connected to phenomena occurring in
the interior of the pulsar. On the contrary, timing irregularities and changes
in slow down rate seem to point to electromagnetic interaction of the pulsar
with the surrounding environment.Comment: 11 pages, 8 figures, 3 tables; accepted for publication in Astronomy
& Astrophysic
Expressive Stream Reasoning with Laser
An increasing number of use cases require a timely extraction of non-trivial
knowledge from semantically annotated data streams, especially on the Web and
for the Internet of Things (IoT). Often, this extraction requires expressive
reasoning, which is challenging to compute on large streams. We propose Laser,
a new reasoner that supports a pragmatic, non-trivial fragment of the logic
LARS which extends Answer Set Programming (ASP) for streams. At its core, Laser
implements a novel evaluation procedure which annotates formulae to avoid the
re-computation of duplicates at multiple time points. This procedure, combined
with a judicious implementation of the LARS operators, is responsible for
significantly better runtimes than the ones of other state-of-the-art systems
like C-SPARQL and CQELS, or an implementation of LARS which runs on the ASP
solver Clingo. This enables the application of expressive logic-based reasoning
to large streams and opens the door to a wider range of stream reasoning use
cases.Comment: 19 pages, 5 figures. Extended version of accepted paper at ISWC 201
Instructive composites for bone regeneration
Developing new biomaterials for tissue regeneration requires careful balance\ud
between many factors, which is challenging because, on one side, such materials\ud
must provide complex information, through their physicochemical properties to\ud
actively interact with the biological surroundings and induce tissue regeneration. On the other side, regulatory issues, costs and ease of use of the final device, require low system complexity. For this reason, an emerging strategy is not attempting to recreate the complexity of tissues in vitro, but to focus on synthetic materials that have ‘intrinsic’ features that can instruct cells in vivo finally determining their fate.\ud
Therefore, newly developed biomaterials should be carefully designed to have\ud
specific local characteristics (e.g. surface stiffness, chemistry and topography) that can induce controlled cellular behaviors ultimately leading to tissue regeneration. In bone tissue regeneration by biomaterials, such instructing phenomenon is referred as ‘osteoinduction’.\ud
In this thesis we aimed to develop simple biomaterial systems, i.e. composites of two phases (i.e. polymer and calcium phosphate) that could be able to interact with the biological system. In particular, we have striven to understand the role of some ‘intrinsic’ characteristics of the composite phases (e.g. calcium phosphate content, polymer molecular weight and monomer chemistry) in determining crucial phenomena occurring at the interface between biomaterial and biological environment. Such surface processes, e.g. surface mineralization and protein adsorption, play key roles in instructing (stem) cells leading to bone tissue regeneration. Besides this, we also studied how the mechanical and physical properties of the composites were affected by the two phases and tried to develop a material with as close properties as possible to those of bone tissue
One- and Two-Nucleon Structure form Green's Function Theory
We review some applications of self-consistent Green's function theory to
studies of one- and two-nucleon structure in finite nuclei.
Large-scale microscopic calculations that employ realistic nuclear forces are
now possible. Effects of long-range correlations are seen to play a dominant
role in determining the quenching of absolute spectroscopic factors. They also
enhance considerably (e,e'pn) cross sections in superparallel kinematics, in
agreement with observations.Comment: Proceedings of the International Symposium on "Forefronts of
Researches in Exotic Nuclear Structures" (Niigata2010)
Effects of nuclear correlations on the O reactions to discrete final states
Calculations of the O cross sections to the ground state and
first excited levels of the C and N nuclei are presented.
The effects of nuclear fragmentation have been obtained in a self-consistent
approach and are accounted for in the determination of the two-nucleon removal
amplitudes.
The Hilbert space is partitioned in order to compute the contribution of both
long- and short-range effects in a separate way.
Both the two-proton and the proton-neutron emission cross sections have been
computed within the same models for the reaction mechanism and the contribution
from nuclear structure, with the aim of better comparing the differences
between the two physical processes.
The O reaction is found to be sensitive to short-range
correlations, in agreement with previous results. The O cross
section to final states is dominated by the current and tensor
correlations. For both reactions, the interplay between collective (long-range)
effects and short-range and tensor correlations plays an important role. This
suggests that the selectivity of reactions to the final state can be
used to probe correlations also beyond short-range effects.Comment: 13 pages, 9 figure
Nori 1-motives
Let EHM be Nori's category of effective homological mixed motives. In this
paper, we consider the thick abelian subcategory EHM_1 generated by the i-th
relative homology of pairs of varieties for i = 0,1. We show that EHM_1 is
naturally equivalent to the abelian category M_1 of Deligne 1-motives with
torsion; this is our main theorem. Along the way, we obtain several interesting
results. Firstly, we realize M_1 as the universal abelian category obtained,
using Nori's formalism, from the Betti representation of an explicit diagram of
curves. Secondly, we obtain a conceptual proof of a theorem of Vologodsky on
realizations of 1-motives. Thirdly, we verify a conjecture of Deligne on
extensions of 1-motives in the category of mixed realizations for those
extensions that are effective in Nori's sense
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