950 research outputs found
Inclusive Particle Spectra at RHIC
A simulation is performed of the recently reported data from PHOBOS at
energies of 56 and 130 A GeV using the relativistic heavy ion cascade LUCIFER
which had previously given a good description of the NA49 inclusive spectra at
E=17.2 A GeV. The results compare well with these early measurements at RHIC.Comment: 4 pages, 2 figure
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
J/Psi Production by Charm Quark Coalescence
Production of pairs in elementary hadron-hadron collisions is
introduced in a simulation of relativistic heavy ion collisions. Coalescence of
charmed quarks and antiquarks into various charmonium states is performed and
the results are compared to PHENIX J Au+Au data. The and '
bound states must be included as well as the ground state J, given the
appreciable feeding from the excited states down to the J via gamma
decays. Charmonium coalescence is found to take place at relatively late times:
generally after ()-medium interactions have ceased. Direct
production of charmonia through hadron-hadron interactions, {\it ie.} without
explicit presence of charm quarks, occurring only at early times, is suppressed
by collisions with comoving particles and accounts for some of the
total J production. Coalescence is especially sensitive to the level of
open charm production, scaling naively as . The J
transverse momentum distribution is dependent on the charm quark transverse
momentum distribution and early charm quark-medium interaction, thus providing
a glimpse of the initial collision history.Comment: 19 pages, 10 figure
Neuronal Activity in the Human Subthalamic Nucleus Encodes Decision Conflict during Action Selection
The subthalamic nucleus (STN), which receives excitatory inputs from the cortex and has direct connections with the inhibitory pathways\ud
of the basal ganglia, is well positioned to efficiently mediate action selection. Here, we use microelectrode recordings captured during\ud
deep brain stimulation surgery as participants engage in a decision task to examine the role of the human STN in action selection. We\ud
demonstrate that spiking activity in the STN increases when participants engage in a decision and that the level of spiking activity\ud
increases with the degree of decision conflict. These data implicate the STN as an important mediator of action selection during decision\ud
processes.\u
Suppression of High Transverse Momentum Spectra in Au+Au Collisions at RHIC
Au+Au, A GeV measurements at RHIC, obtained with the PHENIX,
STAR, PHOBOS and BRAHMS detectors, have all indicated a suppression of neutral
pion production, relative to an appropriately normalized NN level. For central
collisions and vanishing pseudo-rapidity these experiments exhibit suppression
in charged meson production, especially at medium to large transverse momenta.
In the PHENIX experiment similar behavior has been reported for
spectra.
In a recent work on the simpler D+Au interaction, to be considered perhaps as
a tune-up for Au+Au, we reported on a pre-hadronic cascade mechanism which
explains the mixed observation of moderately reduced suppression at
higher pseudo-rapidity as well as the Cronin enhancement at mid-rapidity. Here
we present the extension of this work to the more massive ion-ion collisions.
Our major thesis is that much of the suppression is generated in a late stage
cascade of colourless pre-hadrons produced after an initial short-lived
coloured phase. We present a pQCD argument to justify this approach and to
estimate the time duration of this initial phase. Of essential
importance is the brevity in time of the coloured phase existence relative to
that of the strongly interacting pre-hadron phase. The split into two phases is
of course not sharp in time, but adequate for treating the suppression of
moderate and high mesons.Comment: 19 pages, 10 figure
Modeling Cluster Production at the AGS
Deuteron coalescence, during relativistic nucleus-nucleus collisions, is
carried out in a model incorporating a minimal quantal treatment of the
formation of the cluster from its individual nucleons by evaluating the overlap
of intial cascading nucleon wave packets with the final deuteron wave function.
In one approach the nucleon and deuteron center of mass wave packet sizes are
estimated dynamically for each coalescing pair using its past light-cone
history in the underlying cascade, a procedure which yields a parameter free
determination of the cluster yield. A modified version employing a global
estimate of the deuteron formation probability, is identical to a general
implementation of the Wigner function formalism but can differ from the most
frequent realisation of the latter. Comparison is made both with the extensive
existing E802 data for Si+Au at 14.6 GeV/c and with the Wigner formalism. A
globally consistent picture of the Si+Au measurements is achieved. In light of
the deuteron's evident fragility, information obtained from this analysis may
be useful in establishing freeze-out volumes and help in heralding the presence
of high-density phenomena in a baryon-rich environment.Comment: 31 pages REVTeX, 19 figures (4 oversized included as JPEG). For full
postscript figures (LARGE): contact [email protected]
Lattice Gauge Theory -- Present Status
Lattice gauge theory is our primary tool for the study of non-perturbative
phenomena in hadronic physics. In addition to giving quantitative information
on confinement, the approach is yielding first principles calculations of
hadronic spectra and matrix elements. After years of confusion, there has been
significant recent progress in understanding issues of chiral symmetry on the
lattice. (Talk presented at HADRON 93, Como, Italy, June 1993.)Comment: 11 pages, BNL-4946
Magnetic Moments of the SU(3) Octet Baryons in the semibosonized SU(3) Nambu-Jona-Lasinio Model
We investigate the magnetic moments of the SU(3) octet baryons in the
framework of the semibosonized Nambu--Jona--Lasinio model. The
rotational corrections and strange quark mass in linear order are taken
into account. We derive general relations between magnetic moments of the SU(3)
octet baryons, based on the symmetry of our model. These relations indicate
that higher order corrections such as and are
relatively small. The magnetic moments of the octet baryons predicted by our
model are quantitatively in a good agreement with experimental results within
about 15.Comment: 17 pages, RevTex, 1 postscript figur
Self-Consistent Pushing and Cranking Corrections to the Meson Fields of the Chiral Quark-Loop Soliton
We study translational and spin-isospin symmetry restoration for the
two-flavor chiral quark-loop soliton. Instead of a static soliton at rest we
consider a boosted and rotating hedgehog soliton. Corrected classical meson
fields are obtained by minimizing a corrected energy functional which has been
derived by semi-classical methods ('variation after projection'). We evaluate
corrected meson fields in the region 300 MeV \le M \le 600 MeV of constituent
quark masses M and compare them with the uncorrected fields. We study the
effect of the corrections on various expectation values of nuclear observables
such as the root-mean square radius, the axial-vector coupling constant,
magnetic moments and the delta-nucleon mass splitting.Comment: 19 pages, LaTeX, 7 postscript figures included using 'psfig.sty', to
appear in Int.J.Mod.Phys.
New positivity bounds on polarized parton distributions in multicolored QCD
We derive new positivity bounds on spin-dependent parton distributions in
multicolored QCD. They are stronger than Soffer inequality. We check that the
new inequalities are stable under one-loop DGLAP evolution to higher
normalization points.Comment: 4 pages, typos corrected, more details, references adde
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