3,398 research outputs found
High Energy Nuclear Collisions: Theory Overview
We review some basic concepts of Relativistic Heavy Ion Physics and discuss
our understanding of some key results from the experimental program at the
Relativistic Heavy Ion Collider (RHIC). We focus in particular on the early
time dynamics of nuclear collisions, some result from lattice QCD, hard probes
and photons.Comment: 11 pages, 3 figures; delivered at ISNP 2009, published in Praman
The QCD confinement transition: hadron formation
We review the foundations and the applications of the statistical and the
quark recombination model as hadronization models.Comment: 45 pages, 16 figures, accepted for publication in Landolt-Boernstein
Volume 1-23
Better assessment of physical function: item improvement is neglected but essential
INTRODUCTION: Physical function is a key component of patient-reported outcome (PRO) assessment in rheumatology. Modern psychometric methods, such as Item Response Theory (IRT) and Computerized Adaptive Testing, can materially improve measurement precision at the item level. We present the qualitative and quantitative item-evaluation process for developing the Patient Reported Outcomes Measurement Information System (PROMIS) Physical Function item bank.
METHODS: The process was stepwise: we searched extensively to identify extant Physical Function items and then classified and selectively reduced the item pool. We evaluated retained items for content, clarity, relevance and comprehension, reading level, and translation ease by experts and patient surveys, focus groups, and cognitive interviews. We then assessed items by using classic test theory and IRT, used confirmatory factor analyses to estimate item parameters, and graded response modeling for parameter estimation. We retained the 20 Legacy (original) Health Assessment Questionnaire Disability Index (HAQ-DI) and the 10 SF-36\u27s PF-10 items for comparison. Subjects were from rheumatoid arthritis, osteoarthritis, and healthy aging cohorts (n = 1,100) and a national Internet sample of 21,133 subjects.
RESULTS: We identified 1,860 items. After qualitative and quantitative evaluation, 124 newly developed PROMIS items composed the PROMIS item bank, which included revised Legacy items with good fit that met IRT model assumptions. Results showed that the clearest and best-understood items were simple, in the present tense, and straightforward. Basic tasks (like dressing) were more relevant and important versus complex ones (like dancing). Revised HAQ-DI and PF-10 items with five response options had higher item-information content than did comparable original Legacy items with fewer response options. IRT analyses showed that the Physical Function domain satisfied general criteria for unidimensionality with one-, two-, three-, and four-factor models having comparable model fits. Correlations between factors in the test data sets were \u3e 0.90.
CONCLUSIONS: Item improvement must underlie attempts to improve outcome assessment. The clear, personally important and relevant, ability-framed items in the PROMIS Physical Function item bank perform well in PRO assessment. They will benefit from further study and application in a wider variety of rheumatic diseases in diverse clinical groups, including those at the extremes of physical functioning, and in different administration modes
Evolution of mechanism of parton energy loss with transverse momentum at RHIC and LHC in relativistic collision of heavy nuclei
We analyze the suppression of particle production at large transverse momenta
in ( most) central collisions of gold nuclei at
200 GeV and lead nuclei at 2.76 TeV. Full
next-to-leading order radiative corrections at , and
nuclear effects like shadowing and parton energy loss are included. The parton
energy loss is implemented in a simple multiple scattering model, where the
partons lose an energy per collision, where
is their mean free path. We take for a treatment
which is suggestive of the Bethe Heitler (BH) mechanism of incoherent
scatterings, for LPM mechanism, and
constant for a mechanism which suggests that the rate of energy loss ()
of the partons is proportional to total path length () of the parton in the
plasma, as the formation time of the radiated gluon becomes much larger than
.
We find that while the BH mechanism describes the nuclear modification factor
for 5 GeV/ (especially at RHIC energy), the LPM
and more so the constant mechanism provides a good description at
larger . This confirms the earlier expectation that the energy loss
mechanism for partons changes from BH to LPM for ,
where 1 fm and 1 GeV is the average
transverse kick-squared received by the parton per collision. The energy loss
per collision at the =2.76 TeV is found to be about
twice of that at 0.2 TeV.Comment: Discussion expanded, additional references added, 14 pages, 6
figures, To appear in Journal of Physics
Hadronization in heavy ion collisions: recombination or fragmentation?
We show that hadron production in relativistic heavy ion collisions at
transverse momenta larger than 2 GeV/c can be explained by the competition of
two different hadronization mechanisms. Above 5 GeV/c hadron production can be
described by fragmentation of partons that are created perturbatively. Below 5
GeV/c recombination of partons from the dense and hot fireball dominates. This
can explain some of the surprising features of RHIC data like the constant
baryon-to-meson ratio of about one and the small nuclear suppression for
baryons between 2 to 4 GeV/c.Comment: Contribution to the 7th Conference on Strange Quark Matter (SQM
2003), submitted to J.Phys.G; 6 pages LaTeX, 4 eps figures, uses iopart.cl
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