3,862 research outputs found
Nucleon Polarisabilities from Compton Scattering off the One- and Few-Nucleon System
These proceedings sketch how combining recent theoretical advances with data
from the new generation of high-precision Compton scattering experiments on
both the proton and few-nucleon systems offers fresh, detailed insight into the
Physics of the nucleon polarisabilities. A multipole-analysis is presented to
simplify their interpretation. Predictions from Chiral Effective Field Theory
with special emphasis on the spin-polarisabilities can serve as guideline for
doubly-polarised experiments below 300 MeV. The strong energy-dependence of the
scalar magnetic dipole-polarisability turns out to be crucial to
understand the proton and deuteron data. Finally, a high-accuracy determination
of the proton and neutron polarisabilities shows that they are identical within
error-bars. For details and a better list of references, consult the given
references.Comment: 10 pages LaTeX2e with 7 figures in 8 .eps files, using graphicx.
Invited seminar given at the 26th Course of the International School of
Nuclear Physics: Lepton Scattering and the Structure of Hadrons and Nuclei,
Erice (Italy), 16th - 24th September 2004. To be published in Prog. Nucl.
Part. Phys. 54, No. 2 as part of the proceeding
Analyzing the Effects of Neutron Polarizabilities in Elastic Compton Scattering off He
Motivated by the fact that a polarized He nucleus behaves as an
`effective' neutron target, we examine manifestations of neutron
electromagnetic polarizabilities in elastic Compton scattering from the
Helium-3 nucleus. We calculate both unpolarized and double-polarization
observables using chiral perturbation theory to next-to-leading order
() at energies, , where is
the pion mass. Our results show that the unpolarized differential cross section
can be used to measure neutron electric and magnetic polarizabilities, while
two double-polarization observables are sensitive to different linear
combinations of the four neutron spin polarizabilities.
[Note added in 2018] The qualitative conclusions and analytic formulae
presented in this paper are correct, but several of the numerical results are
wrong: see the erratum posted as arXiv:1804.01206 for further details. A full
suite of corrected numerical results for cross sections and asymmetries can be
found in Margaryan et al., arXiv:1804.00956. They can also be obtained as an
interactive Mathematica notebook by emailing [email protected]: 40 pages, 16 figure
Crossroads in New media, Identity & Law
__Abstract__
This volume brings together a number of timely contributions at the nexus of new media, politics and law. The central intuition that ties these essays together is that information and communication technology, cultural identity, and legal and political institutions are spheres that co-evolve and interpenetrate in myriad ways. Discussing these shifting relationships, the contributions all probe the question of what shape diversity will take as a result of the changes in the way we communicate and spread information: that is, are we heading to the disintegration and fragmentation of national and cultural identity, or is society moving towards more consolidation, standardization and centralization at a transnational level? In an age of digitization and globalization, this book addresses the question of whether this calls for a new civility fit for the 21st century
A bias in cosmic shear from galaxy selection: results from ray-tracing simulations
We identify and study a previously unknown systematic effect on cosmic shear
measurements, caused by the selection of galaxies used for shape measurement,
in particular the rejection of close (blended) galaxy pairs. We use ray-tracing
simulations based on the Millennium Simulation and a semi-analytical model of
galaxy formation to create realistic galaxy catalogues. From these, we quantify
the bias in the shear correlation functions by comparing measurements made from
galaxy catalogues with and without removal of close pairs. A likelihood
analysis is used to quantify the resulting shift in estimates of cosmological
parameters. The filtering of objects with close neighbours (a) changes the
redshift distribution of the galaxies used for correlation function
measurements, and (b) correlates the number density of sources in the
background with the density field in the foreground. This leads to a
scale-dependent bias of the correlation function of several percent,
translating into biases of cosmological parameters of similar amplitude. This
makes this new systematic effect potentially harmful for upcoming and planned
cosmic shear surveys. As a remedy, we propose and test a weighting scheme that
can significantly reduce the bias.Comment: 9 pages, 9 figures, version accepted for publication in Astronomy &
Astrophysic
Gravitomagnetic Field of a Rotating Superconductor and of a Rotating Superfluid
The quantization of the extended canonical momentum in quantum materials
including the effects of gravitational drag is applied successively to the case
of a multiply connected rotating superconductor and superfluid. Experiments
carried out on rotating superconductors, based on the quantization of the
magnetic flux in rotating superconductors, lead to a disagreement with the
theoretical predictions derived from the quantization of a canonical momentum
without any gravitomagnetic term. To what extent can these discrepancies be
attributed to the additional gravitomagnetic term of the extended canonical
momentum? This is an open and important question. For the case of multiply
connected rotating neutral superfluids, gravitational drag effects derived from
rotating superconductor data appear to be hidden in the noise of present
experiments according to a first rough analysis
Explicit Delta(1232) Degrees of Freedom in Compton Scattering off the Deuteron
We examine elastic Compton scattering off the deuteron for photon energies
between 50 MeV and 100 MeV in the framework of chiral effective field theories
to next-to-leading order. We compare one theoretical scheme with only pions and
nucleons as explicit degrees of freedom to another in which the Delta(1232)
resonance is treated as an explicit degree of freedom. Whereas pion degrees of
freedom suffice to describe the experimental data measured at about 70 MeV, the
explicit Delta(1232) gives important contributions that help to reproduce the
angular dependence at higher energies. The static isoscalar dipole
polarizabilities alpha_E^s and beta_M^s are fitted to the available data,
giving results for the neutron polarizabilities
alpha_E^n=(14.2+-2.0(stat)+-1.9(syst))*10^(-4)fm^3,
beta_M^n=(1.8+-2.2(stat)+-0.3(syst))*10^(-4)fm^3. These values are in good
agreement with previous experimental analyses. Comparing them to the well-known
proton values we conclude that there is currently no evidence for significant
differences between the proton and neutron electromagnetic dipole
polarizabilities.Comment: 24 pages, 11 figure
Higher order forward spin polarizability
As a guideline for future experiments to extract the four (leading) spin
polarizabilities of the nucleon, we have constructed the forward amplitude for
polarized Compton scattering by dispersion integrals. These integrals have been
saturated by recently measured helicity-dependent photoabsorption cross
sections as well as predictions for pion photoproduction multipoles from
several phenomenological descriptions and chiral perturbation theory. The
comparison of these results corroborates the strategy to extract the spin
polarizabilities by fitting them to polarized Compton data and fixing all
higher order spin effects by dispersion relations based on pion photoproduction
multipoles.Comment: 21 pages, 6 figures, 3 Tables; version to appear in Phys. Lett.
Structure of the nucleon and spin-polarizabilities
Spin-polarizabilities are predicted by calculating the cross-section
difference from available data for the resonance
couplings and and CGLN amplitudes. The forward
spin-polarizabilities are predicted to be and
in units of fm where the different
signs are found to be due to the isospin dependencies of the and the
amplitudes. The backward spin-polarizabilities are
predicted to be and , to be
compared with the experimental values and
. Electric and magnetic
spin-polarizabilities are introduced and discussed in terms of the and
components of the photo-absorption cross section of the nucleon.Comment: 20 pages, 5 figures, 6 table
Group-finding with photometric redshifts: The Photo-z Probability Peaks algorithm
We present a galaxy group-finding algorithm, the Photo-z Probability Peaks
(P3) algorithm, optimized for locating small galaxy groups using photometric
redshift data by searching for peaks in the signal-to-noise of the local
overdensity of galaxies in a three-dimensional grid. This method is an
improvement over similar two-dimensional matched-filter methods in reducing
background contamination through the use of redshift information, allowing it
to accurately detect groups at lower richness. We present the results of tests
of our algorithm on galaxy catalogues from the Millennium Simulation. Using a
minimum S/N of 3 for detected groups, a group aperture size of 0.25 Mpc/h, and
assuming photometric redshift accuracy of sigma_z = 0.05 it attains a purity of
84% and detects ~295 groups/deg.^2 with an average group richness of 8.6
members. Assuming photometric redshift accuracy of sigma_z = 0.02, it attains a
purity of 97% and detects ~143 groups/deg.^2 with an average group richness of
12.5 members. We also test our algorithm on data available for the COSMOS field
and the presently-available fields from the CFHTLS-Wide survey, presenting
preliminary results of this analysis.Comment: Accepted for publication by MNRAS, 16 pages, 11 color figure
Wide-bandwidth mode-hop-free tuning of extended-cavity GaN diode lasers
We present a new approach for extended-cavity diode-laser tuning to achieve wide mode-hop-free tuning ranges. By using a multiple piezoactuated grating mount, the cavity length and grating angle in the laser can be adjusted independently, allowing mode-hop-free tuning without the need for a mechanically optimized pivot-point mount. Furthermore, synchronized diode injection-current tuning allows diode lasers without antireflection coatings to be employed. In combination these two techniques make the construction of a cheap, efficient, and easily optimized extended-cavity diode laser possible. A theoretical analysis is presented for optimal control of piezoactuator displacements and injection current to achieve the widest possible mode-hop-free tuning ranges, and a comparison is made with measurements. The scheme is demonstrated for blue and violet GaN lasers operating at similar to 450 nm and similar to 410 nm, for which continuous tuning ranges exceeding 90 GHz have been achieved. Examples of applications of these lasers are also given
- …