888 research outputs found
The H-Dibaryon and the Hard Core
The H dibaryon, a single, triply magic bag containing two up, two down and
two strange quarks, has long been sought after in a variety of experiments. Its
creation has been attempted in , proton and most recently in relativistic
heavy ion induced reactions. We concentrate on the latter, but our conclusions
are more generally applicable. The two baryons coalescing to form the single
dibaryon, likely in the case of heavy ions, must penetrate
the short range repulsive barrier which is expected to exist between them. We
find that this barrier can profoundly affect the probability of producing the H
state, should it actually exist.Comment: 9 pages including 4 figure
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
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
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
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
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]
Helicity skewed quark distributions of the nucleon and chiral symmetry
We compute the helicity skewed quark distributions and
in the chiral quark-soliton model of the nucleon. This model
emphasizes correctly the role of spontaneously broken chiral symmetry in
structure of nucleon. It is based on the large-N_c picture of the nucleon as a
soliton of the effective chiral lagrangian and allows to calculate the leading
twist quark- and antiquark distributions at a low normalization point. We
discuss the role of chiral symmetry in the helicity skewed quark distributions
and . We show that generalization of soft pion
theorems, based on chiral Ward identities, leads in the region of -\xi < x <
\xi to the pion pole contribution to which dominates at small
momentum transfer.Comment: 22 pages, 5 figure
Elliptical flow -- a signature for early pressure in ultrarelativistic nucleus-nucleus collisions
Elliptical energy flow patterns in non-central Au(11.7AGeV) on Au reactions
have been studied employing the RQMD model. The strength of these azimuthal
asymmetries is calculated comparing the results in two different modes of RQMD
(mean field and cascade). It is found that the elliptical flow which is readily
observable with current experimental detectors may help to distinguish
different reasonable expansion scenarios for baryon-dense matter. The final
asymmetries are very sensitive to the pressure at maximum compression, because
they involve a partial cancelation between early squeeze-out and subsequent
flow in the reaction plane. This cancelation can be expected to occur in a
broad energy region covered by the current heavy ion fixed-target programs at
BNL and at CERN.Comment: 14 pages LaTeX including 3 postscript figure
Pentaquark as Kaon-Nucleon Resonance
Several recent experiments have reported evidence for a narrow feature in the
K(+)-neutron system, an apparent resonant state ~ 100 MeV above threshold and
with a width < 25 MeV. This state has been labelled as Theta(+) (previously as
Z(*)), and because of the implied inclusion of a anti-strange quark, is
referred to as a pentaquark, that is, five quarks within a single bag. We
present an alternative explanation for such a structure, as a higher angular
momentum resonance in the isospin zero K(+) -N system. One might call this an
exit channel or a molecular resonance. In a non-relativistic potential model we
find a possible candidate for the kaon-nucleon system with relative angular
momentum L=3, while L=1 and 2 states possess centrifugal barriers too low to
confine the kaon and nucleon in a narrow state at an energy so high above
threshold. A rather strong state-dependence in the potential is essential,
however, for eliminating an observable L=2 resonance at lower energies.Comment: 4 page
Large scale shell model calculations for odd-odd Mn isotopes
Large scale shell model calculations have been carried out for odd-odd
Mn isotopes in two different model spaces. First set of calculations
have been carried out in full shell valence space with two recently
derived shell interactions namely GXPF1A and KB3G treating Ca
as core. The second set of calculations have been performed in
valence space with the interaction treating Ca as core and
imposing a truncation by allowing up to a total of six particle excitations
from the 0f orbital to the upper orbitals for protons and
from the upper orbitals to the 0g orbital for neutron. For
low-lying states in Mn, the KB3G and GXPF1A both predicts good results
and for Mn, KB3G is much better than GXPF1A. For negative parity and
high-spin positive parity states in both isotopes interaction is
required. Experimental data on Mn is sparse and therefore it is not
possible to make any definite conclusions. More experimental data on negative
parity states is needed to ascertain the importance of 0g and higher
orbitals in neutron rich Mn isotopes.Comment: 5 pages, 4 figures, Submitted to Eur. Phys. J.
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