14 research outputs found
Evolution of the nuclear modification factors with rapidity and centrality in d+Au collisions at $\sqrt{s_{NN}} = 200 GeV
We report on a study of the transverse momentum dependence of nuclear
modification factors for charged hadrons produced in deuteron + gold
collisions at GeV, as a function of collision centrality
and of the pseudorapidity () of the produced hadrons. We
find significant and systematic decrease of with increasing rapidity.
The midrapidity enhancement and the forward rapidity suppression are more
pronounced in central collisions relative to peripheral collisions. These
results are relevant to the study of the possible onset of gluon saturation at
RHIC energies.Comment: Four pages, four figures. Published in PRL. Figures 1 and 2 have been
updated, and several changes made to the tex
Recent Results from the BRAHMS Experiment
We present recent results obtained by the BRAHMS experiment at the
Relativistic Heavy Ion Collider (RHIC) for the systems of Au + Au and Cu + Cu
at \rootsnn{200} and at 62.4 GeV, and p + p at \rootsnn{200}. Nuclear
modification factors for Au + Au and Cu + Cu collisions are presented. Analysis
of anti-particle to particle ratios as a function of rapidity and collision
energy reveal that particle populations at the chemical freeze-out stage for
heavy-ion reactions at and above SPS energies are controlled by the baryon
chemical potential. From the particle spectra we deduce significant radial
expansion ( 0.75), as expected for systems created with a large
initial energy density. We also measure the elliptic flow parameter
versus rapidity and \ptn. We present rapidity dependent ratios within
for Au + Au and Cu + Cu at \rootsnn{200}. \Raa is found to increase
with decreasing collision energy, decreasing system size, and when going
towards more peripheral collisions. However, \Raa shows only a very weak
dependence on rapidity (for ), both for pions and protons.Comment: 16 pages and 14 figures, proceedings for plenary talk at Quark Matter
2005, Budapest, Hungar
Nuclear Stopping in Au+Au Collisions at sqrt(sNN) = 200 GeV
Transverse momentum spectra and rapidity densities, dN/dy, of protons,
anti-protons, and net--protons (p-pbar) from central (0-5%) Au+Au collisions at
sqrt(sNN) = 200 GeV were measured with the BRAHMS experiment within the
rapidity range 0 < y < 3. The proton and anti-proton dN/dy decrease from
mid-rapidity to y=3. The net-proton yield is roughly constant for y<1 at
dN/dy~7, and increases to dN/dy~12 at y~3. The data show that collisions at
this energy exhibit a high degree of transparency and that the linear scaling
of rapidity loss with rapidity observed at lower energies is broken. The energy
loss per participant nucleon is estimated to be 73 +- 6 GeV.Comment: 5 pages, 4 figure
Quark Gluon Plasma an Color Glass Condensate at RHIC? The perspective from the BRAHMS experiment
We review the main results obtained by the BRAHMS collaboration on the
properties of hot and dense hadronic and partonic matter produced in
ultrarelativistic heavy ion collisions at RHIC. A particular focus of this
paper is to discuss to what extent the results collected so far by BRAHMS, and
by the other three experiments at RHIC, can be taken as evidence for the
formation of a state of deconfined partonic matter, the so called
quark-gluon-plasma (QGP). We also discuss evidence for a possible precursor
state to the QGP, i.e. the proposed Color Glass Condensate.Comment: 32 pages, 18 figure
Pseudorapidity distributions of charged particles from Au+Au collisions at the maximum RHIC energy, Sqrt(s_NN) = 200 GeV
We present charged particle densities as a function of pseudorapidity and
collision centrality for the 197Au+197Au reaction at Sqrt{s_NN}=200 GeV. For
the 5% most central events we obtain dN_ch/deta(eta=0) = 625 +/- 55 and
N_ch(-4.7<= eta <= 4.7) = 4630+-370, i.e. 14% and 21% increases, respectively,
relative to Sqrt{s_NN}=130 GeV collisions. Charged-particle production per pair
of participant nucleons is found to increase from peripheral to central
collisions around mid-rapidity. These results constrain current models of
particle production at the highest RHIC energy.Comment: 4 pages, 5 figures; fixed fig. 5 caption; revised text and figures to
show corrected calculation of and ; final version accepted for
publicatio
The New Physics at RHIC. From Transparency to High p Suppression
Heavy ion collisions at RHIC energies (Au+Au collisions at
GeV) exhibit significant new features as compared to
earlier experiments at lower energies. The reaction is characterized by a high
degree of transparency of the collisions partners leading to the formation of a
baryon-poor central region. In this zone, particle production occurs mainly
from the stretching of the color field. The initial energy density is well
above the one considered necessary for the formation of the Quark Gluon Plasma,
QGP. The production of charged particles of various masses is consistent with
chemical and thermal equilibrium. Recently, a suppression of the high
transverse momentum component of hadron spectra has been observed in central
Au+Au collisions. This can be explained by the energy loss experienced by
leading partons in a medium with a high density of unscreened color charges. In
contrast, such high jets are not suppressed in d+Au collisions suggesting
that the high suppression is not due to initial state effects in the
ultrarelativistic colliding nuclei.Comment: 15 pages, 11 figures. to appear in Nucl. Physics A. Invited talk at
'Nucleus-Nucleus Collisions 2003' conference, Mosco
Nuclear Modification Factor for Charged Pions and Protons at Forward Rapidity in Central Au+Au Collisions at 200 GeV
We present spectra of charged pions and protons in 0-10% central Au+Au
collisions at GeV at mid-rapidity () and forward
pseudorapidity () measured with the BRAHMS experiment at RHIC. The
spectra are compared to spectra from p+p collisions at the same energy scaled
by the number of binary collisions. The resulting nuclear modification factors
for central Au+Au collisions at both and exhibit suppression
for charged pions but not for (anti-)protons at intermediate . The
ratios have been measured up to GeV/ at the two
rapidities and the results indicate that a significant fraction of the charged
hadrons produced at intermediate range are (anti-)protons at both
mid-rapidity and
Rapidity dependence of high p_T suppression at sqrt{s_{NN}}=62.4{GeV}
We present measurements of charged hadron p_T spectra from Au+Au collisions
at sqrt(snn)=62.4 GeV for pseudorapidities eta=0, eta=1 and eta=3.2. Around
midrapidity (eta=0, eta=1) we find nuclear modification factors at levels
suggesting a smaller degree of high p_T suppression than in the same reaction
at higher energy sqrt(snn)=200 GeV. At the high pseudorapidity, eta=3.2, where
nuclear modification factors cannot be constructed due to the lack of p+p
reference data, we find a significant reduction of R_{CP} (central to
peripheral ratio) as compared to midrapidity
Rapidity dependence of antiproton to proton ratios in Au+Au collisions at sqrt{s_{NN}}=130 GeV
Measurements, with the BRAHMS detector, of the antiproton to proton ratio at
central and forward rapidities are presented for Au+Au reactions at
sqrt{s_{NN}}=130 GeV, and for three different collision centralities. For
collisions in the 0-40% centrality range we find $N(\bar{{\rm p}})/N({\rm p}) =
0.64 +- 0.04 (stat.) +- 0.06 (syst.) at y ~0, 0.66 +- 0.03 +- 0.06 at y ~ 0.7,
and 0.41 +- 0.04 +- 0.06 at y ~ 2. The ratios are found to be nearly
independent of collision centrality and transverse momentum. The measurements
demonstrate that the antiproton and proton rapidity densities vary differently
with rapidity, and indicate that a net-baryon free midrapidity plateau (Bjorken
limit) is not reached at this RHIC energy.Comment: 8 pages, 3 figure