377 research outputs found
How important are next-to-leading order models in predicting strange particle spectra in p+p collisions at STAR ?
STAR has measured a variety of strange particle species in p + p collisions
at = 200 GeV. These high statistics data are ideal for comparing to
existing leading- and next-to-leading order perturbative QCD (pQCD) models.
Next-to-leading (NLO) models have been successful in describing inclusive
hadron production using parameterized fragmentation functions (FF) for quarks
and gluons. However, in order to describe identied strange particle spectra at
NLO, knowledge of flavor separated FF is essential. Such FF have recently been
parameterized using data by the OPAL experiment and allow for the first time to
perform NLO calculation for strange baryons. In fact, comparing the STAR Lambda
data with these calculations allow to put a constraint on the gluon
fragmentation function. We show that the Leading-order (LO) event generator
PYTHIA has to be tuned significantly to reproduce the STAR identified strange
particle data. In particular, it fails to describe the observed enhancement of
baryon-to-meson ratio at intermediate pT (2-6 GeV/c). In heavy-ion (HI)
collisions this observable has been extensively compared with models and shows
a strong dependency on collision centrality or parton density. In the HI
context the observed enhancement has been explained by recent approaches in
terms of parton coalescense and recombination models.Comment: 5 pages, HotQuarks 2006 conference proceeding
A model for net-baryon rapidity distribution
In nuclear collisions, a sizable fraction of the available energy is carried
away by baryons. As the baryon number is conserved, the net-baryon
retains information on the energy-momentum carried by the incoming nuclei. A
simple and consistent model for net-baryon production in high energy
proton-proton and nucleus-nucleus collisions is presented. The basic
ingredients of the model are valence string formation based on standard PDFs
with QCD evolution and string fragmentation via the Schwinger mechanism. The
results of the model are presented and compared with data at different
centre-of-mass energies and centralities, as well as with existing models.
These results show that a good description of the main features of net-baryon
data is possible in the framework of a simplistic model, with the advantage of
making the fundamental production mechanisms manifest.Comment: 9 pages, 12 figures; in fig. 11 a) the vertical scale was correcte
A Multi-Phase Transport model for nuclear collisions at RHIC
To study heavy ion collisions at energies available from the Relativistic
Heavy Ion Collider, we have developed a multi-phase transport model that
includes both initial partonic and final hadronic interactions. Specifically,
the parton cascade model ZPC, which uses as input the parton distribution from
the HIJING model, is extended to include the quark-gluon to hadronic matter
transition and also final-state hadronic interactions based on the ART model.
Predictions of the model for central Au on Au collisions at RHIC are reported.Comment: 7 pages, 4 figure
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Low-Dimensional Water on Ru(0001); Model System for X-ray Absorption Spectroscopy Studies of Liquid Water
We present an x-ray absorption spectroscopy results for fully broken to a complete H-bond network of water molecules on Ru(0001) by varying the morphology from isolated water molecules via two-dimensional clusters to a fully covered monolayer as probed by scanning tunneling microscopy. The sensitivity of x-ray absorption to the symmetry of H-bonding is further elucidated for the amino (-NH{sub 2}) group in glycine adsorbed on Cu(110) where the E-vector is parallel either to the NH donating an H-bond or to the non-H-bonded NH. The results give further evidence for the interpretation of the various spectral features of liquid water and for the general applicability of x-ray absorption spectroscopy to analyze H-bonded systems
Carotenoid content and reflectance of yellow and red nuptial plumages in widowbirds (Euplectes spp.)
1. Ornamental carotenoid coloration is commonly based on several different pigments with different nutritional and metabolic constraints. The identification and quantification of carotenoid pigments is therefore crucial to the understanding of signal content and signal evolution. 2. In male widowbirds (Euplectes spp.), the striking yellow and red carotenoid colours have been measured by reflectance spectrometry and studied with respect to sexual selection through male contest competition, but their biochemical mechanisms have not been analysed. 3. Here we use reflectance analysis and high performance liquid chromatography (HPLC) to describe the species-specific colours and plumage carotenoids in three widowbird species: yellow-mantled widowbird (YMW) Euplectes macrourus, red-shouldered widowbird (RSW) E. axillaris and red-collared widowbird (RCW) E. ardens. 4. YMW yellow (‘hue’ colorimetric λR50 = 522 nm) derives from the two ‘dietary yellow’ xanthophylls lutein and zeaxanthin, together with small amounts of ‘derived yellow’ pigments (3′-dehydrolutein and canary xanthophylls). 5. RCW red (λR50 = 574 nm) is achieved by the addition of low concentrations of ‘derived red ’ 4-keto-carotenoids, notably α- and β-doradexanthin and canthaxanthin. 6. RSW red (λR50 = 589 nm) is, in contrast, created by high concentrations of ‘dietary yellow ’ pigments (lutein, zeaxanthin) and ‘derived yellow ’ anhydrolutein, the latter only recently described in birds. 7. The two different mechanisms of producing red plumage are compared with other bird species and discussed with regard to costs and signal ‘honesty’
J/Psi Suppression in Heavy Ion Collisions at the CERN SPS
We reexamine the production of J/Psi and other charmonium states for a
variety of target-projectile choices at the SPS. For this study we use a newly
constructed cascade code LUCIFER II, which yields acceptable descriptions of
both hard and soft processes, specifically Drell-Yan and hidden charm
production, and soft energy loss and meson production, at the SPS. Glauber
calculations of other authors are redone, and compared directly to the cascade
results. The modeling of the charmonium states differs from that of earlier
workers in its unified treatment of the hidden charm meson spectrum, which is
introduced from the outset as a set of coupled states. The result is a
description of the NA38 and NA50 data in terms of a conventional hadronic
picture. The apparently anomalous suppression found in the most massive Pb+Pb
system arises from three sources: destruction in the initial nucleon-nucleon
cascade, use of coupled channels to exploit the larger breakup in the less
bound Chi and Psi' states, and comover interaction in the final low energy
phase.Comment: 36 pages (15 figures
Ferromagnetism in Oriented Graphite Samples
We have studied the magnetization of various, well characterized samples of
highly oriented pyrolitic graphite (HOPG), Kish graphite and natural graphite
to investigate the recently reported ferromagnetic-like signal and its possible
relation to ferromagnetic impurities. The magnetization results obtained for
HOPG samples for applied fields parallel to the graphene layers - to minimize
the diamagnetic background - show no correlation with the magnetic impurity
concentration. Our overall results suggest an intrinsic origin for the
ferromagnetism found in graphite. We discuss possible origins of the
ferromagnetic signal.Comment: 11 figure
Enhancement of charm quark production due to nonlinear corrections to the DGLAP equations
We have studied how parton distributions based on the inclusion of nonlinear
scale evolution and constraints from HERA data affect charm production in
collisions at center-of-mass energies of 5.5, 8.8 and 14 TeV. We find that,
while the resulting enhancement can be substantial, it is very sensitive to the
charm quark mass and the scale entering the parton densities and the strong
coupling constant.Comment: 14 pages, 5 eps-figure
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