230 research outputs found
Correlations and currents in studied with the (e,e'pp) reaction
Nucleon-nucleon correlations, especially those of short-range character, can be well studied with electron-induced two-nucleon knockout reactions at intermediate electron energies. However, these reactions are not only driven by one-body currents, i.e.,
coupling of the virtual photon to one of the nucleons of a correlated pair, a process that directly probes NN-correlations. Also
two-body currents, resulting from intermediate Delta-excitation and coupling to exchanged mesons, as well as final state
interactions, influence the experimental cross section. Exclusive measurements of the three-body breakup of 3He offer the
opportunity to compare data to microscopic calculations. The relative importance of competing two-proton knockout
mechanisms can be investigated by varying the energy and momentum of the virtual photon. The experiment was performed
with the electron beam extracted from the Amsterdam Pulse Stretcher (AmPS) at NIKHEF; the incident electron energy was 564
MeV. A cryogenic, high-pressure 3He gas target was used with a thickness of 270 mg/cm^2. Scattered electrons were detected
in the QDQ magnetic spectrometer and both emitted protons in the HADRON plastic scintillator arrays. Cross sections were
determined for three values of the three-momentum transfer of the virtual photon (q=305, 375, and 445 MeV/c) at an energy
transfer value omega of 220 MeV. At q=375 MeV/c, measurements were performed over a continuous range in energy transfer
from 170 to 290 MeV. The data are compared to results of continuum-Faddeev calculations performed by Golak et al., that
account for rescattering among the emitted nucleons. Various potential models were used in the calculations: Bonn-B,
CD-Bonn, Nijmegen-93 and Argonne v18 . Presentation of the data as a function of the missing or neutron momentum, pm,
shows that the cross section decreases exponentially as a function of pm. Calculations performed with only a one-body
hadronic current operator show fair agreement with data obtained at pm < 100 MeV/c at omega = 220 MeV for all q-values. It can
therefore be concluded that at omega = 220 MeV and pm < 100 MeV/c the cross section is dominated by direct knockout of two
protons via a one-body hadronic current. At higher neutron momentum values, data and theoretical predictions differ up to a
factor of five for all values of omega. Within the range of energy transfer values probed in this experiment, the high pm domain
is expected to be strongly influenced by intermediate excitation in the proton-neutron pair. Within specific regions of phase
space, where two nucleons are emitted with comparable momentum vectors, rescattering processes strongly influence the cross
section. For a such a region, measured at q=445 MeV/c, good agreement was found between data and the continuum- Faddeev
calculations as a function of the pn momentum difference in the final state. Information on the wave function of 3He may be
obtained in the domain omega = 220 MeV and pm < 100 MeV/c by representing the cross section as a function of pdiff1, which
can be related to the relative momentum of the constituents of the two-proton pair in the initial state. The observed decrease of
the cross section reflects the behaviour of the wave function and is well reproduced by calculations. At present, the data do
not permit to express preference for any one of the potential models considered
Making identity assurance and authentication strength work for federated infrastructures
In both higher Research and Education (R&E) as well as in research-/ e-infrastructures (in short: infrastructures), federated access and single sign-on by way of national federations, operated in most cases by NRENs, are used as a means to provide users with access to a variety of services. Whereas in national federations institutional accounts, e.g. provided by a university, are typically used to access services, many infrastructures also accept other sources of identity: provided by \u27\u27community identity providers\u27\u27, social identity providers, or governmental IDs. In order to assess and communicate the quality of identities being used and authentications being performed, so called Level of Assurance (LoA) frameworks are used. Because sophisticated LoA frameworks like NIST 800-63-3, Kantara IAF 1420 or eIDAS regulation are often considered too complex to be used in R&E scenarios, the REFEDS Assurance Suite, a more lightweight approach, has been developed. To select an appropriate assurance level, Service Providers need to weigh risks and potential harms in relation to the kind of service they offer. However, the management of risks is often implicitly assumed and little or no guidance to determine the appropriate assurance level is given. In this paper, first, common LoA frameworks and their relation to risk management are investigated. Following that, their components are compared against the REFEDS Assurance Suite using a graphical representation. The focus of this paper lies in providing guidance and best practices based on example scenarios for both Service Providers to request the appropriate REFEDS assurance level, as well as for Identity Provider operators on how to implement REFEDS assurance components
Managing Dynamic User Communities in a Grid of Autonomous Resources
One of the fundamental concepts in Grid computing is the creation of Virtual
Organizations (VO's): a set of resource consumers and providers that join
forces to solve a common problem. Typical examples of Virtual Organizations
include collaborations formed around the Large Hadron Collider (LHC)
experiments. To date, Grid computing has been applied on a relatively small
scale, linking dozens of users to a dozen resources, and management of these
VO's was a largely manual operation. With the advance of large collaboration,
linking more than 10000 users with a 1000 sites in 150 counties, a
comprehensive, automated management system is required. It should be simple
enough not to deter users, while at the same time ensuring local site autonomy.
The VO Management Service (VOMS), developed by the EU DataGrid and DataTAG
projects[1, 2], is a secured system for managing authorization for users and
resources in virtual organizations. It extends the existing Grid Security
Infrastructure[3] architecture with embedded VO affiliation assertions that can
be independently verified by all VO members and resource providers. Within the
EU DataGrid project, Grid services for job submission, file- and database
access are being equipped with fine- grained authorization systems that take VO
membership into account. These also give resource owners the ability to ensure
site security and enforce local access policies. This paper will describe the
EU DataGrid security architecture, the VO membership service and the local site
enforcement mechanisms Local Centre Authorization Service (LCAS), Local
Credential Mapping Service(LCMAPS) and the Java Trust and Authorization
Manager.Comment: Talk from the 2003 Computing in High Energy and Nuclear Physics
(CHEP03), La Jolla, Ca, USA, March 2003, 7 pages, LaTeX, 5 eps figures. PSN
TUBT00
Ground-state correlations and final state interactions in the process 3He(e,e'pp)n
The two-proton emission process is theoretically investigated
using realistic three-nucleon wave functions and taking the final state
interaction into account by an approach which can be used when the value of the
three-nucleon invariant mass is either below or above the pion emission
threshold. Various kinematical conditions which enhance or minimize the effects
of the final state interaction are thoroughly analyzed.Comment: 26 pages, 12 eps-figures. Introduction and abstract updated, few
references added and Apendix A remove
The properties of the three-nucleon system with the dressed-bag model for nn interaction. I: New scalar three-body force
A multi-component formalism is developed to describe three-body systems with
nonstatic pairwise interactions and non-nucleonic degrees of freedom. The
dressed-bag model for interaction based on the formation of an
intermediate six-quark bag dressed by a -field is applied to the
system, where it results in a new three-body force between the six-quark bag
and a third nucleon. Concise variational calculations of bound states are
carried out in the dressed-bag model including the new three-body force. It is
shown that this three-body force gives at least half the total binding
energy, while the weight of non-nucleonic components in the H and He
wavefunctions can exceed 10%. The new force model provides a very good
description of bound states with a reasonable magnitude of the
coupling constant. The model can serve as a natural bridge between dynamical
description of few-nucleon systems and the very successful Walecka approach to
heavy nuclei and nuclear matter.Comment: 26 pages, Latex, 7 figure
Stacking-Order-Dependent Excitonic Properties Reveal Interlayer Interactions in Bulk ReS<sub>2</sub>
Rhenium disulfide, a member of the transition metal dichalcogenide family of semiconducting materials, is unique among 2D van der Waals materials due to its anisotropy and, albeit weak, interlayer interactions, confining excitons within single atomic layers and leading to monolayer-like excitonic properties even in bulk crystals. While recent work has established the existence of two stacking modes in bulk, AA and AB, the influence of the different interlayer coupling on the excitonic properties has been poorly explored. Here, we use polarization-dependent optical measurements to elucidate the nature of excitons in AA and AB-stacked rhenium disulfide to obtain insight into the effect of interlayer interactions. We combine polarization-dependent Raman with low-temperature photoluminescence and reflection spectroscopy to show that, while the similar polarization dependence of both stacking orders indicates similar excitonic alignments within the crystal planes, differences in peak width, position, and degree of anisotropy reveal a different degree of interlayer coupling. DFT calculations confirm the very similar band structure of the two stacking orders while revealing a change of the spin-split states at the top of the valence band to possibly underlie their different exciton binding energies. These results suggest that the excitonic properties are largely determined by in-plane interactions, however, strongly modified by the interlayer coupling. These modifications are stronger than those in other 2D semiconductors, making ReS2 an excellent platform for investigating stacking as a tuning parameter for 2D materials. Furthermore, the optical anisotropy makes this material an interesting candidate for polarization-sensitive applications such as photodetectors and polarimetry.</p
Selected Topics in High Energy Semi-Exclusive Electro-Nuclear Reactions
We review the present status of the theory of high energy reactions with
semi-exclusive nucleon electro-production from nuclear targets. We demonstrate
how the increase of transferred energies in these reactions opens a complete
new window in studying the microscopic nuclear structure at small distances.
The simplifications in theoretical descriptions associated with the increase of
the energies are discussed. The theoretical framework for calculation of high
energy nuclear reactions based on the effective Feynman diagram rules is
described in details. The result of this approach is the generalized eikonal
approximation (GEA), which is reduced to Glauber approximation when nucleon
recoil is neglected. The method of GEA is demonstrated in the calculation of
high energy electro-disintegration of the deuteron and A=3 targets.
Subsequently we generalize the obtained formulae for A>3 nuclei. The relation
of GEA to the Glauber theory is analyzed. Then based on the GEA framework we
discuss some of the phenomena which can be studied in exclusive reactions,
these are: nuclear transparency and short-range correlations in nuclei. We
illustrate how light-cone dynamics of high-energy scattering emerge naturally
in high energy electro-nuclear reactions.Comment: LaTex file with 51 pages and 23 eps figure
Investigation of the Exclusive 3He(e,e'pp)n Reaction
Cross sections for the 3He(e,e'pp)n reaction were measured over a wide range
of energy and three- momentum transfer. At a momentum transfer q=375 MeV/c,
data were taken at transferred energies omega ranging from 170 to 290 MeV. At
omega=220 MeV, measurements were performed at three q values (305, 375, and 445
MeV/c). The results are presented as a function of the neutron momentum in the
final-state, as a function of the energy and momentum transfer, and as a
function of the relative momentum of the two-proton system. The data at neutron
momenta below 100 MeV/c, obtained for two values of the momentum transfer at
omega=220 MeV, are well described by the results of continuum-Faddeev
calculations. These calculations indicate that the cross section in this domain
is dominated by direct two-proton emission induced by a one-body hadronic
current. Cross section distributions determined as a function of the relative
momentum of the two protons are fairly well reproduced by continuum-Faddeev
calculations based on various realistic nucleon-nucleon potential models. At
higher neutron momentum and at higher energy transfer, deviations between data
and calculations are observed that may be due to contributions of isobar
currents.Comment: 14 pages, 1 table, 17 figure
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