1,259 research outputs found
Propagation of high-energy cosmic rays in extragalactic turbulent magnetic fields: resulting energy spectrum and composition
We extend previous studies of mixed-composition extragalactic cosmic-ray
source models, by investigating the influence of a non-negligible extragalactic
magnetic field on the propagated cosmic-ray spectrum and composition. We study
the transport of charged particles in turbulent fields and the transition from
a ballistic to a diffusive propagation regime. We introduce a method allowing a
fast integration of the particle trajectories, which allows us to calculate
extragalactic cosmic-ray spectra in the general case, without using either the
diffusive or the rectilinear approximation. We find that the main features of
the mixed-composition models -- regarding the interpretation of the ankle and
the non-monotonous evolution of the average cosmic-ray mass -- remain
essentially unchanged as long as the magnetic field intensity does not exceed a
few nG.Comment: 15 pages, 20 figure
Numerical propagation of high energy cosmic rays in the Galaxy I: technical issues
We present the results of a numerical simulation of propagation of cosmic
rays with energy above eV in a complex magnetic field, made in
general of a large scale component and a turbulent component. Several
configurations are investigated that may represent specific aspects of a
realistic magnetic field of the Galaxy, though the main purpose of this
investigation is not to achieve a realistic description of the propagation in
the Galaxy, but rather to assess the role of several effects that define the
complex problem of propagation. Our simulations of Cosmic Rays in the Galaxy
will be presented in Paper II. We identified several effects that are difficult
to interpret in a purely diffusive approach and that play a crucial role in the
propagation of cosmic rays in the complex magnetic field of the Galaxy. We
discuss at length the problem of the extrapolation of our results to much lower
energies where data are available on the confinement time of cosmic rays in the
Galaxy. The confinement time and its dependence on particles' rigidity are
crucial ingredients for 1) relating the source spectrum to the observed cosmic
ray spectrum; 2) quantifying the production of light elements by spallation; 3)
predicting the anisotropy as a function of energy.Comment: 29 pages, 12 figures, submitted to JCA
Association between structural connectivity and generalized cognitive spectrum in alzheimer’s disease
Modeling disease progression through the cognitive scores has become an attractive challenge in the field of computational neuroscience due to its importance for early diagnosis of Alzheimer’s disease (AD). Several scores such as Alzheimer’s Disease Assessment Scale cognitive total score, Mini Mental State Exam score and Rey Auditory Verbal Learning Test provide a quantitative assessment of the cognitive conditions of the patients and are commonly used as objective criteria for clinical diagnosis of dementia and mild cognitive impairment (MCI). On the other hand, connectivity patterns extracted from diffusion tensor imaging (DTI) have been successfully used to classify AD and MCI subjects with machine learning algorithms proving their potential application in the clinical setting. In this work, we carried out a pilot study to investigate the strength of association between DTI structural connectivity of a mixed ADNI cohort and cognitive spectrum in AD. We developed a machine learning framework to find a generalized cognitive score that summarizes the different functional domains reflected by each cognitive clinical index and to identify the connectivity biomarkers more significantly associated with the score. The results indicate that the efficiency and the centrality of some regions can effectively track cognitive impairment in AD showing a significant correlation with the generalized cognitive score (R = 0.7)
Excitation of low-lying states in 144Nd by means of (e,e') scattering
Abstract The low-lying states of 144Nd have been investigated up to an excitation energy of 3.1 MeV by means of high-resolution inelastic electron scattering. Transition charge densities have been extracted for natural-parity states. The experimental data have been compared with the predictions of the quasiparticle-phonon model. The calculations show that both collective and single-particle degrees of freedom are important for describing the low-lying states of 144Nd. A comparison of the present data with data for 142Nd and 142Ce emphasizes the role played by the two valence neutrons outside the N = 82 closed shell
Topological Vector Symmetry of BRSTQFT and Construction of Maximal Supersymmetry
The scalar and vector topological Yang-Mills symmetries determine a closed
and consistent sector of Yang-Mills supersymmetry. We provide a geometrical
construction of these symmetries, based on a horizontality condition on
reducible manifolds. This yields globally well-defined scalar and vector
topological BRST operators. These operators generate a subalgebra of maximally
supersymmetric Yang-Mills theory, which is small enough to be closed off-shell
with a finite set of auxiliary fields and large enough to determine the
Yang-Mills supersymmetric theory. Poincar\'e supersymmetry is reached in the
limit of flat manifolds. The arbitrariness of the gauge functions in BRSTQFTs
is thus removed by the requirement of scalar and vector topological symmetry,
which also determines the complete supersymmetry transformations in a twisted
way. Provided additional Killing vectors exist on the manifold, an equivariant
extension of our geometrical framework is provided, and the resulting
"equivariant topological field theory" corresponds to the twist of super
Yang-Mills theory on Omega backgrounds.Comment: 50 page
Experimental determination of the J(pi) components of the spin-dipole resonance in B-12
AbstractThe inclusive C12(d→,He2) and exclusive C12(d→,He2+n) reactions have been studied with a beam energy of 171 MeV and scattering angles for the (d,He2) reaction θ=0° and 3°. The studies focused on the separation of the isovector spin-dipole resonance (IVSGDR) into its components by measuring tensor-analysing powers and observing the direct neutron decay to the low-lying proton-hole states in 11B. Merging the information obtained from both measurements resulted in the first-time verification of model-independent predictions of tensor-analysing powers at extreme forward angles and the experimental decomposition of the IVSGDR into its Jπ components. The experimental results are in reasonable agreement with theoretical estimates based on shell-model calculations
The origin of ultra high energy cosmic rays
We briefly discuss some open problems and recent developments in the
investigation of the origin and propagation of ultra high energy cosmic rays
(UHECRs).Comment: Invited Review Talk at TAUP 2005 (Zaragoza - September 10-14, 2005).
7 page
Stochastic conversions of TeV photons into axion-like particles in extragalactic magnetic fields
Very-high energy photons emitted by distant cosmic sources are absorbed on
the extragalactic background light (EBL) during their propagation. This effect
can be characterized in terms of a photon transfer function at Earth. The
presence of extragalactic magnetic fields could also induce conversions between
very high-energy photons and hypothetical axion-like particles (ALPs). The
turbulent structure of the extragalactic magnetic fields would produce a
stochastic behaviour in these conversions, leading to a statistical
distribution of the photon transfer functions for the different realizations of
the random magnetic fields. To characterize this effect, we derive new
equations to calculate the mean and the variance of this distribution. We find
that, in presence of ALP conversions, the photon transfer functions on
different lines of sight could have relevant deviations with respect to the
mean value, producing both an enhancement or a suppression in the observable
photon flux with respect to the expectations with only absorption. As a
consequence, the most striking signature of the mixing with ALPs would be a
reconstructed EBL density from TeV photon observations which appears to vary
over different directions of the sky: consistent with standard expectations in
some regions, but inconsistent in others.Comment: v2: 22 pages, 5 eps figures. Minor changes. A reference added.
Matches the version published on JCA
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