Relativistic Heavy Ion Collider Collisions Research Articles

Pion Transverse Momentum Spectrum, Elliptic Flow, and Interferometry in the Granular Source Model for RHIC and LHC Heavy Ion Collisions

We systematically investigate the pion transverse momentum spectrum, elliptic flow, and Hanbury-Brown-Twiss (HBT) interferometry in the granular source model for the heavy ion collisions of Au-Au atsNN=200 GeV and Pb-Pb atsNN=2.76 TeV with different centralities. The granular source model can well reproduce the experimental results of the heavy ion collisions at the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC). We examine the parameters involved in the granular source model. The experimental data of the momentum spectrum, elliptic flow, and HBT radii for the two collision energies and different centralities impose very strict constraints on the model parameters. They exhibit certain regularities for collision centrality and energy. The space-time structure and expansion velocities of the granular sources for the heavy ion collisions at the RHIC and LHC energies with different centralities are investigated.

A critical review of RHIC experimental results

The relativistic heavy-ion collider (RHIC) was constructed to achieve an asymptotic state of nuclear matter in heavy-ion collisions, a near-ideal gas of deconfined quarks and gluons denoted quark–gluon plasma or QGP. RHIC collisions are indeed very different from the hadronic processes observed at the Bevalac and AGS, but high-energy elementary-collision mechanisms are also non-hadronic. The two-component model (TCM) combines measured properties of elementary collisions with the Glauber eikonal model to provide an alternative asymptotic limit for A–A collisions. RHIC data have been interpreted to indicate formation of a strongly-coupled QGP (sQGP) or "perfect liquid". In this review, I consider the experimental evidence that seems to support such conclusions and alternative evidence that may conflict with those conclusions and suggest different interpretations.

Initial indications for the production of a strongly coupled plasma inPb+Pbcollisions atsNN=2.76TeV

Results from first measurements of charged-particle differential elliptic flow, obtained in $\mathrm{Pb}+\mathrm{Pb}$ collisions at $\sqrt{{s}_{\mathit{NN}}}=2.76$ TeV with the ALICE detector at CERN's Large Hadron Collider (LHC), are compared to those obtained for $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{{s}_{\mathit{NN}}}=0.2$ TeV with the PHENIX detector at BNL's Relativistic Heavy Ion Collider (RHIC). The comparisons, made as a function of centrality (cent) or the number of participant pairs (${N}_{\text{part}}$) and particle transverse momentum ${p}_{T}$, indicate an excellent agreement between the magnitude and trends for the flow coefficients ${v}_{2}({p}_{T},\text{cent})$. Analysis indicates that the averaged specific viscosity of the quark gluon plasma (QGP) produced in LHC collisions is similar to that for the strongly coupled QGP produced in RHIC collisions.

Examination of the relevance of hydrodynamics for data measured at the BNL relativistic heavy ion collider

Hydrodynamic (hydro) models applied to heavy ion data from the relativistic heavy ion collider (RHIC) suggest that a dense QCD medium nearly opaque to partons—a strongly coupled quark-gluon plasma—is formed in more-central Au–Au collisions and may have a small viscosity (‘perfect liquid’). Claimed evidence for radial and elliptic flows and possible coalescence of ‘constituent quarks’ seems to support the conclusion. But other measurements provide contradictory evidence. Unbiased angular correlations indicate that most back-to-back jets from initial-state scattered partons with energies as low as 3 GeV survive as ‘minijet’ hadron correlations even in central Au–Au collisions, suggesting near transparency. Two-component analysis of single-particle spectra reveals a spectrum hard component (parton fragment distribution) which can be mistaken for ‘radial flow’ in some forms of analysis. Based on recent results, reinterpretation of ‘elliptic flow’ as a QCD quadrupole scattering process including fragmentation may be possible. In this paper we review conventional analysis methods in the context of two paradigms: a hydrodynamics/hard-probes paradigm and a quadrupole/minijets paradigm. Re-examination of fiducial data suggests that hydrodynamics may not be relevant to RHIC collisions. Collision evolution may be dominated by QCD scattering and fragmentation, albeit strongly modified in more-central A–A collisions.

Imaging of granular sources in high energy heavy ion collisions

We investigate the source imaging for a granular pion-emitting source model in high energy heavy ion collisions. The two-pion source functions of the granular sources exhibit a two-tiered structure. Using a parametrized formula of a granular two-pion source function, we examine the two-tiered structure of the source functions for the imaging data of Au+Au collisions at the Alternating Gradient Synchrotron (AGS) and the Relativistic Heavy Ion Collider (RHIC). We find that the imaging technique introduced by Brown and Danielewicz is suitable for probing the granular structure of the sources. Our data-fitting results indicate that there is no visible granularity for the sources at AGS energies. However, the data for the RHIC collisions with the selections of 40 < centrality < 90% and 0.20 < kT < 0.36 GeV/c are better described by the model with granular emission than that of one Gaussian. The model with a granular source has more parameters than the simple Gaussian, hence can describe more complicated shapes.

Future of Jets, Heavy Flavor and EM Probes at RHIC and RHIC II

Exciting results from the Relativistic Heavy Ion Collider (RHIC) have been presented at this Workshop. However, fundamental questions remain to be addressed in the future regarding whether the system is deconfined, chiral symmetry is restored, a color glass condensate exists in the initial state, and how the system evolves through eventual hadronization. Jets, heavy flavors and electromagnetic probes are sensitive to the initial high density stage of RHIC collisions, and should provide new insight. Significant additional capabilities will be added with a luminosity upgrade of RHIC (to RHIC II), upgrades of present detectors and a possible, new comprehensive detector at RHIC II.

The future of hard and electromagnetic probes at RHIC

Potential near- and long-term physics opportunities with jets, heavy flavors and electromagnetic probes at the Relativistic Heavy Ion Collider (RHIC) are presented. Much new physics remains to be unveiled using these probes, due to their sensitivity to the initial high density stage of RHIC collisions, when quark-gluon plasma (QGP) formation is expected. Additional physics will include addressing deconfinement, chiral symmetry restoration, properties of the strongly-coupled QGP and a possible weakly-interacting QGP, color glass condensate in the initial state, and hadronization. To fully realize the physics prospects of the RHIC energy regime, new detector components must be added to existing experiments, the RHIC machine luminosity upgraded, and a possible new detector with significantly extended coverage and capabilities added.

Transverse momentum phenomenology as a probe of RHIC collisions

The PHENIX experiment at Brookhaven National Laboratory's Relativistic Heavy Ion Collider (RHIC) has superb capabilities for measuring the properties of both charged and neutral particles out to very high values of transverse momentum. The systematic phenomenology of transverse momentum observables carries a wealth of information about the physical environment created by relativistic heavy-ion collisions and a comparison of results obtained from p+p, d+Au and Au+Au collisions at energies ranging up to the RHIC heavy-ion maximum of SNN=200 GeV is a useful way to investigate the physics of these collisions. These probes not only provide information about properties of nuclear matter under extreme conditions but also provide a solid connection to pQCD calculations.

Pseudorapidity and centrality dependence of the collective flow of charged particles in Au+Au collisions at sqrt[s(NN)]=130 GeV.

This paper describes the measurement of collective flow for charged particles in Au+Au collisions at sqrt[s(NN)]=130 GeV using the PHOBOS detector at the Relativistic Heavy Ion Collider (RHIC). The measured azimuthal hit anisotropy is presented over a wide range of pseudorapidity (-5.0<eta<5.3) for the first time at this energy. The result, averaged over momenta and particle species, is observed to reach 7% for peripheral collisions at midrapidity, falling off with centrality and increasing |eta|. These results call into question the common assumption of longitudinal boost invariance over a large region of rapidity in RHIC collisions.

Operation of the Relativistic Heavy Ion Collider Au− ion source

The Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory is beginning its second year of operation. A cesium sputter ion source injecting into a tandem Van de Graaff provides the gold ions for RHIC. The ion source is operated in the pulsed beam mode and produces a 500-μs-long pulse of Au− with a peak intensity of 290 μA at the entrance of the tandem. After acceleration in the tandem and post-stripping, this results in a beam of Au+32 with an intensity of 80 μA and an energy of 182 MeV. Over the last several years, a series of improvements have been made to increase the intensity of the pulsed beam from the ion source. Details of the source performance and improvements will be presented. In addition, an effort is under way to provide other beam species for RHIC collisions.