31 research outputs found
Stopping and Baryon Transport in Heavy Ion Reactions
In this report I will give an experimental overview on nuclear stopping in
hadron collisions, and relate observations to understanding of baryon
transport. Baryon number transport is not only evidenced via net-proton
distributions but also by the enhancement of strange baryons near mid-rapidity.
Although the focus is on high-energy data obtained from pp and heavy ions from
RHIC, relevant data from SPS and ISR will be considered. A discussion how the
available data at higher energy relates and gives information on baryon
junction, quark-diquark breaking will be made.Comment: 8 pages, 7 figures. Invited plenary talk for the 5'th international
conference on Physics and Astrophysics of Quark Gluon Plasma, February 8-12,
2005, Salt Lake City, Kolkata, Indi
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
Au+Au Reactions at the AGS: Experiments E866 and E917
Particle production and correlation functions from Au+Au reactions have been
measured as a function of both beam energy (2-10.7AGeV) and impact parameter.
These results are used to probe the dynamics of heavy-ion reactions, confront
hadronic models over a wide range of conditions and to search for the onset of
new phenomena.Comment: 12 pages, 14 figures, Talk presented at Quark Matter '9
Comparison of Experimental Data to the Relativistic Quantum Molecular Dynamics Model For Si+Au Collisions at 14.6 A GeV/c
Predictions from the RQMD model are systematically compared to recently
published charged hadron distributions of AGS Experiment 802 for central Si+Au
collisions at 14.6 GeV/, taking into account both the experimental
trigger condition and acceptance. The main features of the data, including
K production, can be understood quantitatively to better than 20\% within
the framework of the model, although several discrepancies are found, most
importantly for the proton spectra.Comment: 16 pages (TeX) plus 6 Postscript figures (tar-gz-compressed and
uuencoded
production at low transverse momentum in p+p and d+Au collisions at = 200 GeV
We report on the measurement of production in the dielectron
channel at mid-rapidity (|y|<1) in p+p and d+Au collisions at =
200 GeV from the STAR experiment at the Relativistic Heavy Ion Collider. The
transverse momentum spectra in p+p for < 4 GeV/c and d+Au
collisions for < 3 GeV/c are presented. These measurements extend the
STAR coverage for production in p+p collisions to low .
The from the measured invariant cross section in
p+p and d+Au collisions are evaluated and compared to similar measurements at
other collision energies. The nuclear modification factor for is
extracted as a function of and collision centrality in d+Au and
compared to model calculations using the modified nuclear Parton Distribution
Function and a final-state nuclear absorption cross section
The proton\u2013\u3a9 correlation function in Au\u202f+\u202fAu collisions at s NN =200GeV
We present the first measurement of the proton\u2013\u3a9 correlation function in heavy-ion collisions for the central (0\u201340%) and peripheral (40\u201380%) Au\u202f+\u202fAu collisions at GeV by the STAR experiment at the Relativistic Heavy-Ion Collider (RHIC). Predictions for the ratio of peripheral collisions to central collisions for the proton\u2013\u3a9 correlation function are sensitive to the presence of a nucleon\u2013\u3a9 bound state. These predictions are based on the proton\u2013\u3a9 interaction extracted from -flavor lattice QCD calculations at the physical point. The measured ratio of the proton\u2013\u3a9 correlation function between the peripheral (small system) and central (large system) collisions is less than unity for relative momentum smaller than 40 MeV/c. Comparison of our measured correlation ratio with theoretical calculation slightly favors a proton\u2013\u3a9 bound system with a binding energy of 3c 27 MeV
Di-hadron correlations with identified leading hadrons in 200 GeV Au+Au and d+Au collisions at STAR
The STAR Collaboration presents for the first time two-dimensional di-hadron correlations with identified leading hadrons in 200 GeV central Au + Au and minimum-bias d + Au collisions to explore hadronization mechanisms in the quark gluon plasma. The enhancement of the jet-like yield for leading pions in Au + Au data with respect to the d + Au reference and the absence of such an enhancement for leading non-pions (protons and kaons) are discussed within the context of a quark recombination scenario. The correlated yield at large angles, specifically in the ridge region , is found to be significantly higher for leading non-pions than pions. The consistencies of the constituent quark scaling, azimuthal harmonic model and a mini-jet modification model description of the data are tested, providing further constraints on hadronization
Measurement of the longitudinal spin asymmetries for weak boson production in proton-proton collisions at s=510\u2009\u2009GeV
We report new STAR measurements of the single-spin asymmetries AL for W+ and W- bosons produced in polarized proton-proton collisions at s=510\u2009\u2009GeV as a function of the decay-positron and decay-electron pseudorapidity. The data were obtained in 2013 and correspond to an integrated luminosity of 250\u2009\u2009pb-1. The results are combined with previous results obtained with 86\u2009\u2009pb-1. A comparison with theoretical expectations based on polarized lepton-nucleon deep-inelastic scattering and prior polarized proton-proton data suggests a difference between the u\uaf and d\uaf quark helicity distributions for 0.05<x<0.25. In addition, we report new results for the double-spin asymmetries ALL for W\ub1, as well as AL for Z/\u3b3* production and subsequent decay into electron-positron pairs
Energy dependence of J/\u3c8 production in Au + Au collisions at s NN =39,62.4 and 200GeV
The inclusive J/\u3c8 transverse momentum spectra and nuclear modification factors are reported at mid-rapidity ( |y|<1.0 ) in Au + Au collisions at sNN = 39, 62.4 and 200 GeV taken by the STAR experiment. A suppression of J/\u3c8 production, with respect to the production in p+p scaled by the number of binary nucleon\u2013nucleon collisions, is observed in central Au + Au collisions at these three energies. No significant energy dependence of nuclear modification factors is found within uncertainties. The measured nuclear modification factors can be described by model calculations that take into account both suppression of direct J/\u3c8 production due to the color screening effect and J/\u3c8 regeneration from recombination of uncorrelated charm\u2013anticharm quark pairs
Collision energy dependence of moments of net-kaon multiplicity distributions at RHIC
Fluctuations of conserved quantities such as baryon number, charge, and strangeness are sensitive to the correlation length of the hot and dense matter created in relativistic heavy-ion collisions and can be used to search for the QCD critical point. We report the first measurements of the moments of net-kaon multiplicity distributions in Au+Au collisions at sNN=7.7 , 11.5, 14.5, 19.6, 27, 39, 62.4, and 200 GeV. The collision centrality and energy dependence of the mean ( M ), variance ( \u3c32 ), skewness ( S ), and kurtosis ( \u3ba ) for net-kaon multiplicity distributions as well as the ratio \u3c32/M and the products S\u3c3 and \u3ba\u3c32 are presented. Comparisons are made with Poisson and negative binomial baseline calculations as well as with UrQMD, a transport model (UrQMD) that does not include effects from the QCD critical point. Within current uncertainties, the net-kaon cumulant ratios appear to be monotonic as a function of collision energy