9 research outputs found
New results from the PHOBOS experiment
Over the past five years, PHOBOS has collected data on p+p, d+Au, Au+Au and Cu+Cu collisions, covering a wide range of collision energy, collision centrality and system size. Using these data, we have identified scaling features that are common for all types of high-energy collisions, as well as collective effects that are unique to the conditions created in collisions of relativistic nuclei. In this paper, we will focus on recent results obtained for collisions of Cu nuclei. Both in terms of universal features of particle production, and in the development of truly collective effects, the results for Cu nuclei confirm and extend our present understanding of nuclear collisions at the highest energies. In addition, we will describe recent unique results on multiplicity fluctuations and particle production at very low transverse momenta
Relativistic Nucleus-Nucleus Collisions: from the BEVALAC to RHIC
I briefly describe the initial goals of relativistic nuclear collisions
research, focusing on the LBL Bevatron/Bevalac facility in the 1970's. An early
concept of high hadronic density fireball formation, and subsequent isentropic
decay (preserving information as to the high density stage) led to an outline
of physics observables that could determine the nuclear matter equation of
state at several times nuclear ground state matter density. With the advent of
QCD the goal of locating, and characterizing the hadron-parton deconfinement
phase transformation suggested the need for higher , the research
thus moving to the BNL AGS and CERN SPS, finally to RHIC at BNL. A set of
physics observables is discussed where present data span the entire
domain, from Bevalac and SIS at GSI, to top RHIC energy. Referring,
selectively, to data concerning bulk hadron production, the overall
evolution of directed and radial flow observables, and of pion pair
Bose-Einstein correlation are discussed. The hadronization process is studied
in the grand canonical statistical model. The resulting hadronization points in
the plane T vs. converge onto the parton-hadron phase boundary
predicted by finite lattice QCD, from top SPS to RHIC energy. At lower
SPS and top AGS energy a steep strangeness maximum occurs at which the
Wroblewski parameter 0.6; a possible connection to the QCD
critical point is discussed. Finally the unique new RHIC physics is addressed:
high hadron suppression and jet "tomography".Comment: 19 pages, 11 figure
Bulk properties and flow
In this report, I summarize the experimental results on {\bf bulk properties
and flow} presented at Quark Matter 2004. It is organized in four sections: 1)
Initial condition and stopping; 2) Particle spectra and freeze-outs; 3)
Anisotropic flow; 4) Outlook for future measurements.Comment: 10 pages, 4 figures, "Rapporteur-Conference Highlights", Quark Matter
2004, Oakland, January 11-1
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
Pseudorapidity Distributions of Charged Particles in d + Au and p + p Collisions at = 200GeV
The measured pseudorapidity distributions of primary charged particles are
presented for d + Au and p + p collisions at 200 GeV
over a wide pseudorapidity range of 5.4. The results
for d + Au collisions are presented for minimum-bias events and as a function
of collision centrality. The measurements for p + p collisions are shown for
minimum-bias events. The ratio of the charged particle multiplicity in d + Au
and p + A collisions relative to that for inelastic p + p collisions is found
to depend only on , and it is remarkably independent of
collision energy and system mass. The deuteron and gold fragmentation regions
in d + Au collisions are in good agreement with proton nucleus data at lower
energies.Comment: 4 pages, 3 figures. To appear in the proceedings of Seventeenth
International Conference on Ultra-Relativistic Nucleus-Nucleus Collisions
(Quark Matter 2004), Oakland, California from January 11-17, 2004. Submitted
to Journal of Physics G: Nuclear and Particle Physic
Rapidity and k_T dependence of HBT correlations in Au+Au collisions at 200 GeV with PHOBOS
Two-particle correlations of identical charged pion pairs from Au+Au
collisions at sqrt(s_NN) = 200 GeV were measured by the PHOBOS experiment at
RHIC. Data for the most central (0--15%) events were analyzed with
Bertsch-Pratt (BP) and Yano-Koonin-Podgoretskii (YKP) parameterizations using
pairs with rapidities of 0.4 < y < 1.3 and transverse momenta 0.1 < k_T < 1.4
GeV/c. The Bertsch-Pratt radii decrease as a function of pair transverse
momentum. The pair rapidity Y_pipi roughly scales with the source rapidity
Y_YKP, indicating strong dynamical correlations.Comment: 5 pages, 2 figures. To appear in the proceedings of Seventeenth
International Conference on Ultra-Relativistic Nucleus-Nucleus Collisions
(Quark Matter 2004), Oakland, California from January 11-17, 2004. Submitted
to Journal of Physics G: Nuclear and Particle Physic
The Landscape of Particle Production: Results from PHOBOS
Recent results from the PHOBOS experiment at RHIC are presented, both from
Au+Au collisions from the 2001 run and p+p and d+Au collisions from 2003. The
centrality dependence of the total charged particle multiplicity in p+p and
d+Au show features, such as Npart-scaling and limiting fragmentation, similar
to p+A collisions at lower energies. Multiparticle physics in Au+Au is found to
be local in (pseudo)rapidity, both when observed by HBT correlations and by
forward-backward pseudorapidity correlations. The shape of elliptic flow in
Au+Au, measured over the full range of pseudorapidity, appears to have a very
weak centrality dependence. Identified particle ratios in d+Au reactions show
little difference between the shape of proton and anti-proton spectra, while
the absolute yields show an approximate m_T scaling.Comment: 8 Pages, 11 Figures, Plenary talk at Quark Matter 2004, Oakland, CA,
January 11-18, 200
Thermal Dileptons at LHC
We predict dilepton invariant-mass spectra for central 5.5 ATeV Pb-Pb
collisions at LHC. Hadronic emission in the low-mass region is calculated using
in-medium spectral functions of light vector mesons within hadronic many-body
theory. In the intermediate-mass region thermal radiation from the Quark-Gluon
Plasma, evaluated perturbatively with hard-thermal loop corrections, takes
over. An important source over the entire mass range are decays of correlated
open-charm hadrons, rendering the nuclear modification of charm and bottom
spectra a critical ingredient.Comment: 2 pages, 2 figures, contributed to Workshop on Heavy Ion Collisions
at the LHC: Last Call for Predictions, Geneva, Switzerland, 14 May - 8 Jun
2007 v2: acknowledgment include
Scaling properties in bulk and p-dependent particle production near midrapidity in relativistic heavy ion collisions
The centrality dependence of the midrapidity charged-particle multiplicity
density (1) is presented for Au+Au and Cu+Cu collisions at RHIC over
a broad range of collision energies. The multiplicity measured in the Cu+Cu
system is found to be similar to that measured in the Au+Au system, for an
equivalent N, with the observed factorization in energy and
centrality still persistent in the smaller Cu+Cu system. The extent of the
similarities observed for bulk particle production is tested by a comparative
analysis of the inclusive transverse momentum distributions for Au+Au and Cu+Cu
collisions near midrapidity. It is found that, within the uncertainties of the
data, the ratio of yields between the various energies for both Au+Au and Cu+Cu
systems are similar and constant with centrality, both in the bulk yields as
well as a function of p, up to at least 4 GeV/. The effects of
multiple nucleon collisions that strongly increase with centrality and energy
appear to only play a minor role in bulk and intermediate transverse momentum
particle production.Comment: Submitted for publication in Phys. Rev. C, 5 pages, 3 figure