Probing longitudinal dynamics at RHIC

Particle production of charged hadrons in Au+Au collisions at $\sqrt {s_{NN} } = 200$ GeV bas been studied as a function of rapidity by the BRAHMS Collaboration at RHIC. Selected recent results are presented with emphasis on longitudinal dynamics of particle production. The rapidity dependence of particle production imposes more stringent constraints on theoretical models describing dynamics of nuclear matter created by high-energy heavy ion collisions.     

Highlights from the STAR experiment at RHIC

Ultra-relativistic heavy-ion collisions produced at RHIC differ significantly from a superposition of proton-proton collisions. Evidence of collective expansion has been gathered. The yield of high transverse momentum particles has been found to be lower in head-on Au?Au collisions than is expected by scaling p-p collisions. Di-jet processes, which are frequent in p-p collisions, are almost absent in head-on Au?Au collisions. The current results from RHIC indicate that Au?Au collisions at $\sqrt {S_{NN} } = 130$ GeV and $\sqrt {S_{NN} } = 200$ GeV yield an expanding system that is opaque to high momentum partons.     

Balance functions at RHIC

The balance function is based on the principle that charge is locally conserved when particles are pair produced. Balance functions have been measured for all charged pairs, identified pion pairs, and identified charged kaon pairs in Au+Au collisions at $\sqrt {s_{NN} } = 200$ GeV and p+p collisions at $\sqrt {s_{NN} } = 200$ GeV at the Relativistic Heavy Ion Collider using STAR. Balance functions for all charged particles from Au+Au scale smoothly with centrality to the p+p value. Balance functions for charged particles and pions are narrower in central collisions than in peripheral collisions consistent with trends predicted by models incorporating the concept of late hadronization. Balance functions for kaon pairs represent a strangeness balance. Balance functions for kaons are narrower than those for pion pairs and may show less dependence on centrality.     

Identified particle production in p+p, d+Au and Au+Au collisions at RHIC

Measurements of identified particle production with the PHENIX experiment at RHIC have reached a mature state, where a multitude of nuclear systems at different colliding energies have been studied. The discovery configurations of $\sqrt {s_{NN} } = 130$ and 200 GeV Au+Au collisions have now been supplemented by additional Au+Au and Cu+Cu configurations at various energies, along with baseline p+p and d+Au runs at $\sqrt {s_{NN} } = 130$ GeV. In this work we present a systematic study of the Cronin effect in d+Au collisions and recent results from p+p collisions. We then proceed to make a critical comparison of pion, kaon and proton production in heavy ion and baseline systems, and discuss the observed nuclear effects on hadron production.     

Open charm production at RHIC: Recent results from STAR

We report recent measurements on the open charm production in d+Au and p+p collisions at $\sqrt {s_{NN} } = 200$ GeV from the STAR detector at RHIC. The two independent measurements — direct open charm hadron (D⁰, D^* etc.) reconstruction and non-photonic single electron spectrum — provide consistent results. The mid-rapidity charm differential cross section per nucleon-nucleon collision from d+Au collisions at RHIC is $d\sigma _{c\bar c}^{NN} /dy = 0.30 \pm 0.04(stat.) \pm 0.09(syst.)$ mb, which is higher than predictions from most of the NLO pQCD calculations. Implications for charmonium production in Au+Au collisions will also be discussed.     

Resonance production in STAR

The recent results from resonance production in central Au+Au and p+p collisions at $\sqrt {s_{NN} } = 200$ GeV from the STAR experiment at RHIC are presented and discussed.     

Characteristics of charged particle production in relativistic heavy-ion collisions

Inclusive spectra of charged particles at midrapidity in Au+Au collisions at $\sqrt {s_{NN} } = 130$ GeV and 200 GeV were measured with the STAR detector at RHIC. The measured mean transverse momentum 〈p _T 〉 shows a characteristic dependence on charged particle multiplicity and beam energy in Au+Au collisions that is distinctly different from pp, $p\bar p$ and e⁺e^? collisions. A 32%±3%(syst) increase in 〈p _T 〉 from pp to Au+Au collisions was observed at 200 GeV. While the charged multiplicity was found to increase by 19%±5%(syst) from $\sqrt {s_{NN} } = 130$ GeV to 200 GeV, no significant difference in 〈p _T 〉 was found between the two energies. A comparison with model predictions is discussed.     

Scaling properties of azimuthal anisotropy of mesons and baryons at RHIC

Detailed measurements of the azimuthal anisotropy (v ₂) for identified charged particles are reported as a function of transverse momentum (pT) and centrality for Au+Au collisions at $\sqrt {s_{NN} } = 200$ GeV. The measurements indicate clear evidence for eccentricity and particle flavor scaling over a broad range of centralities and transverse rapidity yT, indicating a hydrodynamical origin of the fine structure of azimuthal anisotropy at RHIC. The observed scaling supports the picture of a suddenly hadronizing (recombining) fluid of quarks. An apparent breaking of flavor scaling at relatively large values of yT points to an important change in the mechanism for particle emission.     

Analysis of multi-strange hyperon production in p+p and Au+Au collisions at RHIC

The analysis of multi-strange hyperon production (Ξ^?, Ω^? and their anti-particles) at mid-rapidity is part of the program covered by the STAR experiment at RHIC. We report on preliminary results in Au+Au and p+p collisions at $\sqrt {s_{NN} } = 130$ and 200 GeV. Measurements of particle ratios are presented and production yields and spectra are discussed for both collision systems.     

Balance functions in a thermal model with resonances

The π ⁺ π ^? balance function in rapidity is computed in a thermal model with resonances. It is found that the correlations from the neutral-resonance decays are important, yielding about a half of the total contribution, which in general consist of resonance and non-resonance parts. The model yields the pionic balance function a few per cent wider that what follows from the recent data for the Au+Au collisions at $\sqrt {s_{NN} } = 130$ .     

Global observables at PHENIX

We present PHENIX recent results on charged particle and transverse energy densities measured at mid-rapidity in Au?Au collisions at $\sqrt {s_{NN} } = 130$ GeV and 200 GeV over a broad range of centralities. The mean transverse energy per charged particle is derived. The first PHENIX measurements at $\sqrt {s_{NN} } = 19.6$ GeV are also presented. A comparison with calculations from various theoretical models is performed.     

Mid-rapidity ϕ meson production at \sqrt {s_{NN} } = 200 GeV Au+Au and pp collisions from STAR

We present the results for the measurement of ? meson production in $\sqrt {s_{NN} } = 200$ GeV Au+Au and pp collisions at the Relativistic Heavy Ion Collider (RHIC). Using the event mixing technique, spectra and yields are obrained from the ?→K⁺K^? decay channel for five centrality bins in Au+Au collisions and in pp collisions. We observe that the spectrum shape in Au+Au collisions depends weakly on the centrality and the shape of the spectrum in pp collisions is significantly different from that in Au+Au collisions.     

Recent results from PHOBOS

The Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory has delivered the collisions between Au nuclei at the highest center of mass energies achieved in the laboratory to date. PHOBOS is one of four experiments studying the these interactions. In this paper we will briefly describe the PHOBOS experiment, and discuss some of the current physics results obtained at $\sqrt {s_{NN} } = 56$ , 130 and 200 GeV: the systematic study of charged particle multiplicity as a function of pseudorapidity and centrality, measurements of antiparticle to particle ratios, and the hadron spectra as function of transverse momentum for both high and low pt.     

Hadron production at forward and backward rapidities in \sqrt {s_{NN} } = 200 GeV d+Au collisions

Understanding normal nuclear medium effects present in heavy ion collisions is essential for understanding the following dynamics of the high density matter produced in the collision. Asymmetric collisions, such as deuteron + gold, provide a key tool for studying these effects since particles produced in the forward and backward directions may be subject to different phenomena. Particle production has been studied in d+Au collisions for various kinematic regions at the RHIC facility. PHENIX has measured charged hadron production as a function of pT for different centrality classes using the PHENIX muon spectrometers for d+Au collisions at $\sqrt {s_{NN} } = 200$ GeV. The PHENIX muon spectrometers have coverage in both forward and backward directions in the rapidity range 1.2 |η| < 2.4. The R _cp measurement, the ratio of central to peripheral production, is presented and discussed. Comparisons are also made with some relevant theoretical calculations.     

Direct photon measurement at RHIC-PHENIX

Results on direct photon measurements from the PHENIX experiment at RHIC are presented. The direct photon yields for P _T >6GeV/c as a function of centrality in Au+Au collisions at $\sqrt {s_{NN} } = 200$ GeV are found to be consistent with NLO pQCD calculation scaled by the number of binary collisions. The results suggest that the photons observed are emitted from the initial stage of hard scattering. Comparisons with several theoretical calculations are also presented.     

Proton and antiproton distributions at RHIC

Properties of transverse momentum spectra and rapidity dependence of protons and antiprotons in Au?Au collisions at $\sqrt {s_{NN} } = 200$ GeV are discussed. The net-proton yields are approximately constant at |y| < 1 and increases towards y ≈ 3. The mean rapidity loss is estimated to be in the range of 1.9 < δy <2.4.     

Resonance studies at STAR

Preliminary results from measurements of resonances (K ^*0(892), $\overline {K*^0 } (892)$ , Φ(1020), and ρ(770)) and weakly decaying particles (Λ(1116), $\bar \Lambda (1116)$ , and K _S ⁰ (498)) are presented. The measurements are performed at mid-rapidity by the STAR detector in $\sqrt {s_{NN} } = 130$ GeV Au?Au collisions at RHIC. The ratios K ^*0/h?, $\overline {K*^0 } /K$ , and $\bar \Lambda /\Lambda $ are compared to measurements at different energies and colliding systems. Estimates of thermal parameters, such as temperature and baryon chemical potential, are also presented.     

High p T correlations of γ and charged hadrons at RHIC

Prompt photon production in ultra-relativistic heavy-ion collisions provides a calibrated probe for the study of the properties of high energy density QCD matter. Especially interesting are the measurements of γ-tagged jets where the hard scattering scale is known and can be used to determine the partonic energy loss in the dense matter. We discuss the potential of γ-jet measurements at the Relativistic Heavy Ion Collider (RHIC) and argue that the observed suppression of the away-side correlations for di-jet production in central Au+Au collisions at $\sqrt {s_{NN} } = 200$ GeV should significantly reduce the backgrounds for the γ-jet coincidence measurements.     

Hadron production at forward rapidity in nuclear collisions at RHIC

The main experimental results obtained by the BRAHMS experiment at Relativistic Heavy Ion Collider (RHIC) for Au+Au collisions at \(\sqrt {s_{NN} } \) =62.4, 200 GeV and d+Au collisions at \(\sqrt {s_{NN} } \) = 200GeV are presented. The m _T spectra and the Gaussian-like rapidity densities of produced pions and kaons in Au+Au central collisions at \(\sqrt {s_{NN} } \) = 200 are GeV shown, and their rapidity densities are compared with results from models. The net-proton yield in the same system is compared with that from AGS and SPS energies to study the high energy collision scenario-transparency and stopping. The rapidity, energy and centrality dependence of the nuclear modification factors in both systems are compared with models to differentiate between the initial and final state effect.     

Antiproton and charged kaon production in \sqrt {s_{NN} } = 130 GeV Au+Au collisions at RHICGeV Au+Au collisions at RHIC

Preliminary results on antiproton and charged kaon spectra and the net-proton number at mid-rapidity are reported for Au+Au collisions at $\sqrt {s_{NN} } = 130$ GeV as measured by the STAR experiment at RHIC. Inverse slope parameters of the transverse mass distributions increase with the collision centrality, consistent with a strong radial flow. The antiproton and charged kaon extrapolated yields, normalized to the uncorrected negatively charged hadron yield, increase with the collision centrality. A finite but small number of net-baryons is found to be present at mid-rapidity.