arXiv subjects

STAR Collaboration

Publications and source records attributed to STAR Collaboration.

At least 271 records · Page 15Linked to original sources

arXiv2005

Incident energy dependence of pt correlations at relativistic energies

We present results for two-particle transverse momentum correlations, , as a function of event centrality for Au+Au collisions at sqrt(sNN) = 20, 62, 130, and 200 GeV at the Relativistic Heavy Ion Collider. We observe correlations decreasing with centrality that are similar at all four incident energies. The correlations multiplied by the multiplicity density increase with incident energy and the centrality dependence may show evidence of processes such as thermalization, minijet production, or the saturation of transverse flow. The square root of the correlations divided by the event-wise average transverse momentum per event shows little or no beam energy dependence and generally agrees with previous measurements at the Super Proton Synchrotron.

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arXiv2005

Minijet deformation and charge-independent angular correlations on momentum subspace $(η,ϕ)$ in Au-Au collisions at $\sqrt{s_{NN}}$ = 130 GeV

First measurements of charge-independent correlations on angular difference variables $η_1 - η_2$ (pseudorapidity) and $ϕ_1 - ϕ_2$ (azimuth) are presented for primary charged hadrons with transverse momentum $0.15 \leq p_t \leq 2$ GeV/$c$ and $|η| \leq 1.3$ from Au-Au collisions at $\sqrt{s_{NN}} = 130$ GeV. Strong charge-independent angular correlations are observed associated with jet-like structures and elliptic flow. The width of the jet-like peak on $η_1 - η_2$ increases by a factor 2.3 from peripheral to central collisions, suggesting strong coupling of semi-hard scattered partons to a longitudinally-expanding medium. New methods of jet analysis introduced here provide evidence for nonperturbative QCD medium effects in heavy ion collisions.

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arXiv2005

Hard Probes with the STAR Experiment

Recent results on the use of hard probes in heavy ion collisions by the STAR experiment at RHIC are reviewed. The increased statistical reach from RHIC run 4 and utilization of the full capabilities of the STAR experiment have led to a qualitative improvement in these results. Light hadrons have been identified out to transverse momenta ($p_T$) of 12 GeV/c, allowing for clear identification of the dominant processes governing particle production in different $p_T$ windows. Clean signatures of dijets have been seen even in central Au+Au collisions. Nuclear modification factors for non-photonic electrons, predominantly from the decay of heavy-flavored hadrons, have also been measured out to $p_T$ of 8 GeV/c. For $p_T > \sim 6$ GeV/c, inclusive spectra of all charged hadrons, including heavy-flavored ones, appear to be suppressed equally strongly (by a factor of four to five) in central Au+Au collisions relative to p+p collisions; interestingly enough, the probability of finding a hadron from a dijet partner is suppressed to this same level.

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arXiv2005

Distributions of Charged Hadrons Associated with High Transverse Momentum Particles in pp and Au+Au Collisions at sqrt(s_NN)=200 GeV

Charged hadrons in 0.15 < pt < 4 GeV/c associated with particles of pt^trig > 4 GeV/c are reconstructed in pp and Au+Au collisions at sqrt(s_NN)=200 GeV. The associated multiplicity and pt magnitude sum are found to increase from pp to central Au+Au collisions. The associated pt distributions, while similar in shape on the near side, are significantly softened on the away side in central Au+Au relative to pp and not much harder than that of inclusive hadrons. The results, consistent with jet quenching, suggest that the away-side fragments approach equilibration with the medium traversed.

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arXiv2005

Pion interferometry in Au+Au collisions at $\sqrt{\mathrm{s}_{_{\mathrm{NN}}}}$ = 200 GeV

We present a systematic analysis of two-pion interferometry in Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV using the STAR detector at RHIC. We extract the HBT radii and study their multiplicity, transverse momentum, and azimuthal angle dependence. The Gaussianess of the correlation function is studied. Estimates of the geometrical and dynamical structure of the freeze-out source are extracted by fits with blast wave parameterizations. The expansion of the source and its relation with the initial energy density distribution is studied.

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arXiv2005

Pion, kaon, proton and anti-proton transverse momentum distributions from p+p and d+Au collisions at $\sqrt{s_{NN}} = 200$ GeV

Identified mid-rapidity particle spectra of $π^{\pm}$, $K^{\pm}$, and $p(\bar{p})$ from 200 GeV p+p and d+Au collisions are reported. A time-of-flight detector based on multi-gap resistive plate chamber technology is used for particle identification. The particle-species dependence of the Cronin effect is observed to be significantly smaller than that at lower energies. The ratio of the nuclear modification factor ($R_{dAu}$) between protons $(p+\bar{p})$ and charged hadrons ($h$) in the transverse momentum range $1.2<{p_{T}}<3.0$ GeV/c is measured to be $1.19\pm0.05$(stat)$\pm0.03$(syst) in minimum-bias collisions and shows little centrality dependence. The yield ratio of $(p+\bar{p})/h$ in minimum-bias d+Au collisions is found to be a factor of 2 lower than that in Au+Au collisions, indicating that the Cronin effect alone is not enough to account for the relative baryon enhancement observed in heavy ion collisions at RHIC.

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arXiv2005

Multiplicity and Pseudorapidity Distributions of Photons in Au + Au Collisions at $\sqrt{s_{NN}}$ = 62.4 GeV

We present the first measurement of multiplicity and pseudorapidity distributions of photons in the pseudorapidity region 2.3 $\le$ $η$ $\le$ 3.7 for different centralities in Au + Au collisions at $\sqrt{s_{NN}}$ = 62.4 GeV. We find that the photon yield in this pseudorapidity range scales with the number of participating nucleons at all collision centralities studied. The pseudorapidity distribution of photons, dominated by neutral pion decays, has been compared to those of identified charged pions, photons, and inclusive charged particles from heavy ion and nucleon-nucleon collisions at various energies. The photon production in the measured pseudorapidity region has been shown to be consistent with the energy and centrality independent limiting fragmentation scenario.

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arXiv2005

Azimuthal anisotropy in Au+Au collisions at sqrtsNN = 200 GeV

The results from the STAR Collaboration on directed flow (v_1), elliptic flow (v_2), and the fourth harmonic (v_4) in the anisotropic azimuthal distribution of particles from Au+Au collisions at sqrtsNN = 200 GeV are summarized and compared with results from other experiments and theoretical models. Results for identified particles are presented and fit with a Blast Wave model. Different anisotropic flow analysis methods are compared and nonflow effects are extracted from the data. For v_2, scaling with the number of constituent quarks and parton coalescence is discussed. For v_4, scaling with v_2^2 and quark coalescence is discussed.

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arXiv2005

Experimental and Theoretical Challenges in the Search for the Quark Gluon Plasma: The STAR Collaboration's Critical Assessment of the Evidence from RHIC Collisions

We review the most important experimental results from the first three years of nucleus-nucleus collision studies at RHIC, with emphasis on results from the STAR experiment, and we assess their interpretation and comparison to theory. The theory-experiment comparison suggests that central Au+Au collisions at RHIC produce dense, rapidly thermalizing matter characterized by: (1) initial energy densities above the critical values predicted by lattice QCD for establishment of a Quark-Gluon Plasma (QGP); (2) nearly ideal fluid flow, marked by constituent interactions of very short mean free path, established most probably at a stage preceding hadron formation; and (3) opacity to jets. Many of the observations are consistent with models incorporating QGP formation in the early collision stages, and have not found ready explanation in a hadronic framework. However, the measurements themselves do not yet establish unequivocal evidence for a transition to this new form of matter. The theoretical treatment of the collision evolution, despite impressive successes, invokes a suite of distinct models, degrees of freedom and assumptions of as yet unknown quantitative consequence. We pose a set of important open questions, and suggest additional measurements, at least some of which should be addressed in order to establish a compelling basis to conclude definitively that thermalized, deconfined quark-gluon matter has been produced at RHIC.

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arXiv2005

Event-wise mean p_t fluctuations in Au-Au collisions at sqrt{s_{NN}} = 130 GeV

We present the first large-acceptance measurement of event-wise mean transverse momentum mean p_t fluctuations for Au-Au collisions at nucleon-nucleon center-of-momentum collision energy sqrt{s_{NN}} = 130 GeV. The observed non-statistical mean p_t fluctuations substantially exceed in magnitude fluctuations expected from the finite number of particles produced in a typical collision. The r.m.s. fractional width excess of the event-wise mean p_t distribution is 13.7 +/- 0.1(stat) +/- 1.3(syst)% relative to a statistical reference, for the 15% most-central collisions and for charged hadrons within pseudorapidity range |eta|<1, 2pi azimuth and 0.15 < p_t < 2 GeV/c. The width excess varies smoothly but non-monotonically with collision centrality, and does not display rapid changes with centrality which might indicate the presence of critical fluctuations. The reported mean p_t fluctuation excess is qualitatively larger than those observed at lower energies and differs markedly from theoretical expectations. Contributions to mean p_t mean fluctuations from semi-hard parton scattering in the initial state and dissipation in the bulk colored medium are discussed.

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arXiv2005

K(892)* Resonance Production in Au+Au and p+p Collisions at sqrt(sNN) = 200 GeV at RHIC

The short-lived $K(892)^{*}$ resonance provides an efficient tool to probe properties of the hot and dense medium produced in relativistic heavy-ion collisions. We report measurements of $K^{*}$ in $\sqrt{s_{_{NN}}}$ = 200 GeV Au+Au and $p+p$ collisions reconstructed via its hadronic decay channels $K(892)^{*0} \to Kπ$ and $K(892)^{*\pm} \to K_S^0π^{\pm}$ using the STAR detector at RHIC. The $K^{*0}$ mass has been studied as a function of $p_T$ in minimum bias $p+p$ and central Au+Au collisions. The $K^{*}$ $p_T$ spectra for minimum bias $p+p$ interactions and for Au+Au collisions in different centralities are presented. The $K^{*}/K$ yield ratios for all centralities in Au+Au collisions are found to be significantly lower than the ratio in minimum bias $p+p$ collisions, indicating the importance of hadronic interactions between chemical and kinetic freeze-outs. A significant non-zero $K^{*0}$ elliptic flow ($v_2$) is observed in Au+Au collisions and compared to the $K_S^0$ and $Λ$ $v_2$. The nuclear modification factor of $K^{*}$ at intermediate $p_{T}$ is similar to that of $K_{S}^{0}$, but different from $Λ$. This establishes a baryon-meson effect over a mass effect in the particle production at intermediate $p_T$ ($2 < p_T \leq 4$ GeV/$c$).

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arXiv2005

Multi-strange baryon elliptic flow in Au+Au collisions at sqrt(s_NN)= 200 GeV

We report on the first measurement of elliptic flow $v_2(p_T)$ of multi-strange baryons $Ξ+\bar{Xi}$ and $Ω+\bar{Omega} in heavy-ion collisions. In minimum bias Au+Au collisions at sqrt(s_NN) = 200 GeV, a significant amount of elliptic flow, comparable to other non-strange baryons, is observed for multi-strange baryons which are expected to be particularly sensitive to the dynamics of the partonic stage of heavy-ion collisions. The $p_T$ dependence of $v_2$ of the multi-strange baryons confirms the number of constituent quark scaling previously observed for lighter hadrons. These results support the idea that a substantial fraction of the observed collective motion is developed at the early partonic stage in ultra-relativistic nuclear collisions at RHIC.

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arXiv2005

Pseudorapidity Asymmetry and Centrality Dependence of Charged Hadron Spectra in d+Au Collisions at $\sqrt{s_{NN}}=200$ GeV

The pseudorapidity asymmetry and centrality dependence of charged hadron spectra in d+Au collisions at $\sqrt{s_{NN}}=200$ GeV are presented. The charged particle density at mid-rapidity, its pseudorapidity asymmetry and centrality dependence are reasonably reproduced by a Multi-Phase Transport model, by HIJING, and by the latest calculations in a saturation model. Ratios of transverse momentum spectra between backward and forward pseudorapidity are above unity for \pT below 5 GeV/$c$. The ratio of central to peripheral spectra in d+Au collisions shows enhancement at 2 $<$ \pT $<$ 6 GeV/$c$, with a larger effect at backward rapidity than forward rapidity. Our measurements are in qualitative agreement with gluon saturation and in contrast to calculations based on incoherent multiple partonic scatterings.

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arXiv2005

Open charm yields in d+Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV

Mid-rapidity open charm spectra from direct reconstruction of $D^{0}$($\bar{D^0}$)$\to K^{\mp}π^{\pm}$ in d+Au collisions and indirect electron/positron measurements via charm semileptonic decays in p+p and d+Au collisions at \srt = 200 GeV are reported. The $D^{0}$($\bar{D^0}$) spectrum covers a transverse momentum ($p_T$) range of 0.1 $<p_T<$ 3 \GeVc whereas the electron spectra cover a range of 1 $<p_T<$ 4 GeV/$c$. The electron spectra show approximate binary collision scaling between p+p and d+Au collisions. From these two independent analyses, the differential cross section per nucleon-nucleon binary interaction at mid-rapidity for open charm production from d+Au collisions at RHIC is $dσ^{NN}_{c\bar{c}}/dy$=0.30$\pm$0.04 (stat.)$\pm$0.09(syst.) mb. The results are compared to theoretical calculations. Implications for charmoniumm results in A+A collisions are discussed.

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arXiv2004

Azimuthal anisotropy and correlations at large transverse momenta in $p+p$ and Au+Au collisions at $\sqrt{s_{_{NN}}}$= 200 GeV

Results on high transverse momentum charged particle emission with respect to the reaction plane are presented for Au+Au collisions at $\sqrt{s_{_{NN}}}$= 200 GeV. Two- and four-particle correlations results are presented as well as a comparison of azimuthal correlations in Au+Au collisions to those in $p+p$ at the same energy. Elliptic anisotropy, $v_2$, is found to reach its maximum at $p_t \sim 3$ GeV/c, then decrease slowly and remain significant up to $p_t\approx 7$ -- 10 GeV/c. Stronger suppression is found in the back-to-back high-$p_t$ particle correlations for particles emitted out-of-plane compared to those emitted in-plane. The centrality dependence of $v_2$ at intermediate $p_t$ is compared to simple models based on jet quenching.

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arXiv2004

Multi-strange baryon correlations in p+p and d+Au collisions at $\sqrt{s}=200$ GeV

Multi-strange baryon correlations with charged hadrons have been measured in the p+p and d+Au reference systems at s^(0.5) = 200 GeV per nucleon pair, for future comparison with correlations in Au+Au collisions. Weak correlation peaks are observed on both the same side as the high pT (pT > 2 GeV/c) Xi (the trigger particle), and on the away side (pi radians away in azimuth) in the p+p data. A quantitative analysis requires better statistics than is presently available. Distinct peaks are also seen in PYTHIA simulations of p+p collisions, but spectra comparison between PYTHIA and data shows that PYTHIA does not reproduce the data. A clear correlation is also present in d+Au data, establishing a reference foundation for a future Au+Au study.

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arXiv2004

Azimuthally sensitive Hanbury Brown-Twiss interferometry in Au+Au collisions at sqrt(s_{NN}) = 200 GeV

We present the results of a systematic study of the shape of the pion distribution in coordinate space at freeze-out in Au+Au collisions at RHIC using two-pion Hanbury Brown-Twiss (HBT) interferometry. Oscillations of the extracted HBT radii vs. emission angle indicate sources elongated perpendicular to the reaction plane. The results indicate that the pressure and expansion time of the collision system are not sufficient to completely quench its initial shape.

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arXiv2004

Measurements of transverse energy distributions in Au+Au collisions at $\sqrt{s_{NN}}= 200$ GeV

Transverse energy ($E_T$) distributions have been measured for Au+Au collisions at $\sqrt{s_{NN}}= 200$ GeV by the STAR collaboration at RHIC. $E_T$ is constructed from its hadronic and electromagnetic components, which have been measured separately. $E_T$ production for the most central collisions is well described by several theoretical models whose common feature is large energy density achieved early in the fireball evolution. The magnitude and centrality dependence of $E_T$ per charged particle agrees well with measurements at lower collision energy, indicating that the growth in $E_T$ for larger collision energy results from the growth in particle production. The electromagnetic fraction of the total $E_T$ is consistent with a final state dominated by mesons and independent of centrality.

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