40 research outputs found
Relativistic predictions of spin observables for exclusive proton knockout reactions
Within the framework of the relativistic distorted wave impulse approximation
(DWIA), we investigate the sensitivity of complete sets of polarization
transfer observables for exclusive proton knockout from the 3s,
2d and 2d states in Pb, at an incident laboratory
kinetic energy of 202 MeV, and for coincident coplanar scattering angles
(, ), to different distorting optical potentials,
finite-range (FR) versus zero-range (ZR) approximations to the DWIA, as well as
medium-modified meson-nucleon coupling constants and meson masses. Results are
also compared to the nonrelativistic DWIA predictions based on the
Schr\"{o}dinger equation.Comment: Submitted for publication to Physicical Review C, 23 pages, 7 figure
System size and centrality dependence of charged hadron transverse momentum spectra in Au+Au and Cu+Cu collisions at sqrt(s) = 62.4 and 200 GeV
We present transverse momentum distributions of charged hadrons produced in
Cu+Cu collisions at sqrt(s) = 62.4 and 200 GeV. The spectra are measured for
transverse momenta of 0.25 < p_T < 5.0 GeV/c at sqrt(s) = 62.4 GeV and 0.25 <
p_T < 7.0 GeV/c at sqrt(s) = 200 GeV, in a pseudo-rapidity range of 0.2 < eta <
1.4. The nuclear modification factor R_AA is calculated relative to p+p data at
both collision energies as a function of collision centrality. At a given
collision energy and fractional cross-section, R_AA is observed to be
systematically larger in Cu+Cu collisions compared to Au+Au. However, for the
same number of participating nucleons, R_AA is essentially the same in both
systems over the measured range of p_T, in spite of the significantly different
geometries of the Cu+Cu and Au+Au systems.Comment: 4 pages, 5 figures, submitted to Phys. Rev. Let
System Size, Energy and Centrality Dependence of Pseudorapidity Distributions of Charged Particles in Relativistic Heavy Ion Collisions
We present the first measurements of the pseudorapidity distribution of
primary charged particles in Cu+Cu collisions as a function of collision
centrality and energy, \sqrtsnn = 22.4, 62.4 and 200 GeV, over a wide range of
pseudorapidity, using the PHOBOS detector. Making a global comparison of Cu+Cu
and Au+Au results, we find that the total number of produced charged particles
and the rough shape (height and width) of the pseudorapidity distributions are
determined by the number of nucleon participants. More detailed studies reveal
that a more precise matching of the shape of the Cu+Cu and Au+Au pseudorapidity
distributions over the full range of pseudorapidity occurs for the same
Npart/2A value rather than the same Npart value. In other words, it is the
collision geometry rather than just the number of nucleon participants that
drives the detailed shape of the pseudorapidity distribution and its centrality
dependence at RHIC energies.Comment: Submitted to Physical Review Letter
Identified charged antiparticle to particle ratios near midrapidity in Cu+Cu collisions at sqrt(s) = 62.4 and 200 GeV
Antiparticle to particle ratios for identified protons, kaons and pions at
sqrt(s) = 62.4 and 200 GeV in Cu+Cu collisions are presented as a function of
centrality for the midrapidity region of 0.2 < eta < 1.4. No strong dependence
on centrality is observed. For the / ratio at ~ 0.51 GeV/c, we
observe an average value of 0.50 +/- 0.003_(stat) +/- 0.04_(syst) and 0.77 +/-
0.008_(stat) +/- 0.05_(syst) for the 10% most central collisions of 62.4 and
200 GeV Cu+Cu, respectively. The values for all three particle species measured
at sqrt(s) = 200 GeV are in agreement within systematic uncertainties with that
seen in both heavier and lighter systems measured at the same RHIC energy. This
indicates that system size does not appear to play a strong role in determining
the midrapidity chemical freeze-out properties affecting the antiparticle to
particle ratios of the three most abundant particle species produced in these
collisions.Comment: 5 Pages, 4 figures Made changes to the figures to include the panel
numbers. Slight changes to the text. Updated data points from other
experiment
Cluster properties from two-particle angular correlations in p+p collisions at = 200 and 410 GeV
We present results on two-particle angular correlations in proton-proton
collisions at center of mass energies of 200 and 410 GeV. The PHOBOS experiment
at the Relativistic Heavy Ion Collider has a uniquely large coverage for
charged particles, giving the opportunity to explore the correlations at both
short- and long-range scales. At both energies, a complex two-dimensional
correlation structure in and is observed. In the
context of an independent cluster model of short-range correlations, the
cluster size and its decay width are extracted from the two-particle
pseudorapidity correlation function and compared with previous measurements in
proton-proton and proton-antiproton collisions, as well as PYTHIA and HIJING
predictions.Comment: 10 pages, 10 figures, submitted to Phys. Rev.
System Size, Energy, Pseudorapidity, and Centrality Dependence of Elliptic Flow
This paper presents measurements of the elliptic flow of charged particles as
a function of pseudorapidity and centrality from Cu-Cu collisions at 62.4 and
200 GeV using the PHOBOS detector at the Relativistic Heavy Ion Collider
(RHIC). The elliptic flow in Cu-Cu collisions is found to be significant even
for the most central events. For comparison with the Au-Au results, it is found
that the detailed way in which the collision geometry (eccentricity) is
estimated is of critical importance when scaling out system-size effects. A new
form of eccentricity, called the participant eccentricity, is introduced which
yields a scaled elliptic flow in the Cu-Cu system that has the same relative
magnitude and qualitative features as that in the Au-Au system
Latest Results from PHOBOS
This manuscript contains a summary of the latest physics results from PHOBOS,
as reported at Quark Matter 2006. Highlights include the first measurement from
PHOBOS of dynamical elliptic flow fluctuations as well as an explanation of
their possible origin, two-particle correlations, identified particle ratios,
identified particle spectra and the latest results in global charged particle
production.Comment: 9 pages, 7 figures, PHOBOS plenary proceedings for Quark Matter 200
System size, energy, centrality and pseudorapidity dependence of charged-particle density in Au+Au and Cu+Cu collisions at RHIC
Charged particle pseudorapidity distributions are presented from the PHOBOS
experiment at RHIC, measured in Au+Au and Cu+Cu collisions at sqrt{s_NN}=19.6,
22.4, 62.4, 130 and 200 GeV, as a function of collision centrality. The
presentation includes the recently analyzed Cu+Cu data at 22.4 GeV. The
measurements were made by the same detector setup over a broad range in
pseudorapidity, |eta|<5.4, allowing for a reliable systematic study of particle
production as a function of energy, centrality and system size. Comparing Cu+Cu
and Au+Au results, we find that the total number of produced charged particles
and the overall shape (height and width) of the pseudorapidity distributions
are determined by the number of nucleon participants, N_part. Detailed
comparisons reveal that the matching of the shape of the Cu+Cu and Au+Au
pseudorapidity distributions over the full range of eta is better for the same
N_part/2A value than for the same N_part value, where A denotes the mass
number. In other words, it is the geometry of the nuclear overlap zone, rather
than just the number of nucleon participants that drives the detailed shape of
the pseudorapidity distribution and its centrality dependence.Comment: 5 pages, 4 figures. Presented at the 20th International Conference on
Nucleus-Nucleus Collisions (Quark Matter 2008), Jaipur, Rajasthan, India,
4-10 February 200