10,432 research outputs found
On the measurement of D-meson yield in nucleus-nucleus collisions at the CERN SPS
We argue that the measurement of open charm gives a unique opportunity to test the validity of pQCD-based and statistical models of nucleus-nucleus collisions at high energies. We show that various approaches used to estimate D-meson multiplicity in central Pb+Pb collisions at 158 A GeV give predictions which differ by more than a factor of 100. Finally we demonstrate that decisive experimental results concerning the open charm yield in A+A collisions can be obtained using data of the NA49 experiment at the CERN SPS
Comparing knowledge sources for nominal anaphora resolution
We compare two ways of obtaining lexical knowledge for antecedent selection in other-anaphora
and definite noun phrase coreference. Specifically, we compare an algorithm that relies on links
encoded in the manually created lexical hierarchy WordNet and an algorithm that mines corpora
by means of shallow lexico-semantic patterns. As corpora we use the British National
Corpus (BNC), as well as the Web, which has not been previously used for this task. Our
results show that (a) the knowledge encoded in WordNet is often insufficient, especially for
anaphor-antecedent relations that exploit subjective or context-dependent knowledge; (b) for
other-anaphora, the Web-based method outperforms the WordNet-based method; (c) for definite
NP coreference, the Web-based method yields results comparable to those obtained using
WordNet over the whole dataset and outperforms the WordNet-based method on subsets of the
dataset; (d) in both case studies, the BNC-based method is worse than the other methods because
of data sparseness. Thus, in our studies, the Web-based method alleviated the lexical knowledge
gap often encountered in anaphora resolution, and handled examples with context-dependent relations
between anaphor and antecedent. Because it is inexpensive and needs no hand-modelling
of lexical knowledge, it is a promising knowledge source to integrate in anaphora resolution systems
Lambda(1520) production in d+Au collisions at RHIC
Recent results of (1520) resonance production in d+Au collisions at
200 GeV are presented and discussed in terms of the
evolution and freeze-out conditions of a hot and dense fireball medium. Yields
and spectra are compared to results from p+p and Au+Au collisions. The
(1520)/ ratio in d+Au collisions ratio is consistent with the
ratio in p+p collisions. This suggests a short time for elastic interactions
between chemical and thermal freeze-out. One can conclude that the interaction
volume in d+Au collisions is small.Comment: 4 Pages, 3 figures, conference proceedings Quark Matter 200
Formation and decay of hadronic resonances in the QGP
Hadronic resonances can play a pivotal role in providing experimental
evidence for partial chiral symmetry restoration in the deconfined quark-gluon
phase produced at RHIC. Their lifetimes, which are comparable to the lifetime
of the partonic plasma phase, make them an invaluable tool to study medium
modifications to the resonant state due to the chiral transition. In this paper
we show that the heavier, but still abundant, light and strange quark
resonances K*, phi, Delta and Lambda* have large probability to be produced
well within the plasma phase due to their short formation times. We demonstrate
that, under particular kinematic conditions, these resonances can be formed and
will decay inside the partonic state, but still carry sufficient momentum to
not interact strongly with the hadronic medium after the QCD phase transition.
Thus, K*, phi, Delta and Lambda* should exhibit the characteristic property
modifications which can be attributed to chiral symmetry restoration, such as
mass shifts, width broadening or branching ratio modifications.Comment: 8 pages, 7 figure
Strange Hadron Resonances: Freeze-Out Probes in Heavy-Ion Collisions
Hyperon resonances are becoming an extremely useful tool allowing the study
of the properties of hadronic fireballs made in heavy ion collisions. Their
yield, compared to stable particles with the same quark composition, depends on
hadronization conditions. The resonance's short lifetime makes them ideal
probes of the fireball chemical freeze-out mechanisms. An analysis of resonance
abundance in heavy ion collisions should be capable of distinguishing between
possible hadronization scenarios, in particular between sudden and gradual
hadronization. In this paper, we review the existing SPS and RHIC experimental
data on resonance production in heavy ion collisions, and discuss in terms of
both thermal and microscopic models the yields of the two observed resonances,
K* and Lambda(1520). We show how freeze-out properties, namely chemical
freeze-out temperature and the lifetime of the interacting hadron phase which
follows, can be related to resonance yields. Finally, we apply these methods to
SPS and RHIC measurements, discuss the significance and interpretations of our
findings, and suggest further measurements which may help in clarifying
existing ambiguities.Comment: To appear in the proceedings of Pan American Advanced Studies
Institute on New States of Matter in Hadronic Interactions (PASI 2002),
Campos do Jordao, Brazil, 7-18 Jan 2002; American Institute of Physics 2002.
v2: reference and language correctio
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