1,920 research outputs found
Dyson-Schwinger Equations - aspects of the pion
The contemporary use of Dyson-Schwinger equations in hadronic physics is
exemplified via applications to the calculation of pseudoscalar meson masses,
and inclusive deep inelastic scattering with a determination of the pion's
valence-quark distribution function.Comment: 4 pages. Contribution to the Proceedings of ``DPF 2000,'' the Meeting
of the Division of Particles and Fields of the American Physical Society,
August 9-12, 2000, Department of Physics, the Ohio State University,
Columbus, Ohi
Post-collisional Pan-African granitoids and rare metal pegmatites in western Nigeria: age, petrogenesis, and the âpegmatite conundrumâ
The Minna area of western Nigeria lies within a Pan-African orogenic belt that extends along the margin of the West African Craton, from Algeria southwards through Nigeria, Benin and Ghana, and into the Borborema Province of Brazil. This belt is characterised by voluminous post-collisional granitoid plutons that are well exposed around the city of Minna. In this paper we present new information about their age and petrogenesis.
The Pan-African plutons around Minna can be divided into two main groups: a group of largely peraluminous biotiteâmuscovite granites that show varying levels of deformation in late Pan-African shear zones; and a younger group of relatively undeformed, predominantly metaluminous hornblende granitoids. Pegmatites, including both barren and rare-metal types, occur at the margins of some of the plutons.
New UâPb zircon dating presented here, in combination with published data, indicates an early phase of magmatism at c. 790â760 Ma in the Minna area. This magmatism could be related either to continental rifting, or to subduction around the margins of an existing continent. The peraluminous biotiteâmuscovite granites were intruded at c. 650â600 Ma during regional shearing in the orogenic belt, and are likely to have formed largely by crustal melting. Subsequent emplacement of metaluminous granitoids at c. 590 Ma indicates the onset of post-orogenic extension in this area, with a contribution from mantle-derived magmas. The rare-metal pegmatites represent the youngest intrusions in this area and thus are likely to have formed in a separate magmatic episode, post-dating granite intrusion
NLSEmagic: Nonlinear Schr\"odinger Equation Multidimensional Matlab-based GPU-accelerated Integrators using Compact High-order Schemes
We present a simple to use, yet powerful code package called NLSEmagic to
numerically integrate the nonlinear Schr\"odinger equation in one, two, and
three dimensions. NLSEmagic is a high-order finite-difference code package
which utilizes graphic processing unit (GPU) parallel architectures. The codes
running on the GPU are many times faster than their serial counterparts, and
are much cheaper to run than on standard parallel clusters. The codes are
developed with usability and portability in mind, and therefore are written to
interface with MATLAB utilizing custom GPU-enabled C codes with the
MEX-compiler interface. The packages are freely distributed, including user
manuals and set-up files.Comment: 37 pages, 13 figure
Microstructural evolution in cast Haynes 282 for applications in advanced power plants
There is a worldwide drive to increase the efficiency of power plants in order to reduce the
amount of fossil fuel consumed and associated CO2 emissions. Raising the operating temperature
and pressure can improve the thermal efficiency, however, this necessitate the use of materials
which have high temperature performance. Steels are currently used at temperature up to 600°C
with the efficiency of 38-40 %. Advanced Ultra Supercritical (A-USC) design plans power plants
to operate at steam temperatures of 700°C and pressure up to 35 MPa with a lifetime of at least
100 000 hours. Ni-base superalloys are leading materials due to their significant strength and
creep resistance.
Haynes 282 is one possible candidate to meet the A-USC conditions for turbine engines. This
alloy is a ÎłâČ precipitation strengthened material and exhibits very good creep properties and
thermal stability. The alloy examined in this research was produced by sand casting, and
therefore the aim of this research is to investigate the microstructural evolution in large scale cast
components.
The alloy has been examined in both the as-cast condition and as a function of a range of
different pre-service heat treatments. The microstructural changes during different heat treatments
have been fully identified and quantified. The results have also been compared with predictions
from thermodynamic equilibrium calculations using a Ni alloy database. It has been found that
variations in the heat treatment conditions can have a significant effect on microstructural
development and hence, potentially, the mechanical properties of Haynes 282 alloy
Bayesian Nonparametric Inverse Reinforcement Learning
Inverse reinforcement learning (IRL) is the task of learning the reward function of a Markov Decision Process (MDP) given the transition function and a set of observed demonstrations in the form of state-action pairs. Current IRL algorithms attempt to find a single reward function which explains the entire observation set. In practice, this leads to a computationally-costly search over a large (typically infinite) space of complex reward functions. This paper proposes the notion that if the observations can be partitioned into smaller groups, a class of much simpler reward functions can be used to explain each group. The proposed method uses a Bayesian nonparametric mixture model to automatically partition the data and find a set of simple reward functions corresponding to each partition. The simple rewards are interpreted intuitively as subgoals, which can be used to predict actions or analyze which states are important to the demonstrator. Experimental results are given for simple examples showing comparable performance to other IRL algorithms in nominal situations. Moreover, the proposed method handles cyclic tasks (where the agent begins and ends in the same state) that would break existing algorithms without modification. Finally, the new algorithm has a fundamentally different structure than previous methods, making it more computationally efficient in a real-world learning scenario where the state space is large but the demonstration set is small
Valence-quark distributions in the pion
We calculate the pion's valence-quark momentum-fraction probability
distribution using a Dyson-Schwinger equation model. Valence-quarks with an
active mass of 0.30 GeV carry 71% of the pion's momentum at a resolving scale
q_0=0.54 GeV = 1/(0.37 fm). The shape of the calculated distribution is
characteristic of a strongly bound system and, evolved from q_0 to q=2 GeV, it
yields first, second and third moments in agreement with lattice and
phenomenological estimates, and valence-quarks carrying 49% of the pion's
momentum. However, pointwise there is a discrepancy between our calculated
distribution and that hitherto inferred from parametrisations of extant
pion-nucleon Drell-Yan data.Comment: 8 pages, 3 figures, REVTEX, aps.sty, epsfig.sty, minor corrections,
version to appear in PR
Self-organized criticality in deterministic systems with disorder
Using the Bak-Sneppen model of biological evolution as our paradigm, we
investigate in which cases noise can be substituted with a deterministic signal
without destroying Self-Organized Criticality (SOC). If the deterministic
signal is chaotic the universality class is preserved; some non-universal
features, such as the threshold, depend on the time correlation of the signal.
We also show that, if the signal introduced is periodic, SOC is preserved but
in a different universality class, as long as the spectrum of frequencies is
broad enough.Comment: RevTex, 8 pages, 8 figure
Changes in serum neurofilament light chain levels following narrowband ultraviolet B phototherapy in clinically isolated syndrome
Objective
To determine whether serum neurofilament light chain (sNfL) levels are suppressed in patients with the clinically isolated syndrome (CIS) following narrowband ultraviolet B phototherapy (UVB-PT).
Methods
sNfL levels were measured using a sensitive single-molecule array assay at baseline and up to 12 months in 17 patients with CIS, 10 of whom received UVB-PT, and were compared with healthy control (HC) and early relapsing remitting multiple sclerosis (RRMS) group. sNfL levels were correlated with magnetic resonance imaging total lesion volume (LV) determined using icobrain version 4.4.1 and with clinical outcomes.
Results
Baseline median sNfL levels were significantly higher in the CIS (20.6 pg/mL, interquartile range [IQR] 13.7â161.4) and RRMS groups (36.6 pg/ml [IQR] 16.2â212.2) than in HC (10.7 pg/ml [IQR] 4.9â21.5) (p = .012 and p = .0002, respectively), and were strongly correlated with T2 and T1 LV at 12 months (r = .800; p = .014 and r = .833; p = .008, respectively) in the CIS group. Analysis of changes in sNfL levels over time in the CIS group showed a significant cumulative suppressive effect of UVB-PT in the first 3 months (UVB-PT â10.6% vs non-UVB-PT +58.3%; p = .04) following which the levels in the two groups converged and continued to fall.
Conclusions
Our findings provide the basis for further studies to determine the utility of sNfL levels as a marker of neuro-axonal damage in CIS and early MS and for assessing the efficacy of new therapeutic interventions such as UVB-PT
Evidence for the fourth P11 resonance predicted by the constituent quark model
It is pointed out that the third of five low-lying P11 states predicted by a
constituent quark model can be identified with the third of four states in a
solution from a three-channel analysis by the Zagreb group. This is one of the
so-called ``missing'' resonances, predicted at 1880 MeV. The fit of the Zagreb
group to the pi N -> eta N data is the crucial element in finding this fourth
resonance in the P11 partial wave.Comment: 8 pages, revtex; expanded acknowledgement
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