Kaon production in S+nucleus and Pb+Pb collisions at CERN SPS

Charged and neutral kaon production in S-nucleus and Pb-Pb collisions has been measured at CERN in the NA35 and NA49 experiments, close to mid-rapidity. The production ofK ^± in S+S, Ag, Au at 200 GeV/nucleon was measured via their one-prong (kink) topology inside a Time Projection Chamber (TPC). The resulting charged kaon yields are in agreement with other measurements done in the same experiment. TheK ⁺/K ^? ratio was found to be 1.30±0.15 in central S+Au interactions. The production ofK _S ⁰ in Pb+Pb collisions was measured using the NA49’s large (Main) TPCs outside the magnetic field. The resulting yields are compatible with other measurements performed in the same experiment, within the quoted systematic errors. The preliminary rapidity distribution in Pb+Pb scales with the number of participants if compared to S+S collisions. From this we conclude that no further strangeness enhancement (relative to nucleon-nucleon collisions) is observed in going from S+S to Pb+Pb.     

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.     

Quasielastic transfer reactions in 238U + 197Au and 197Au + 197Au collisions near the Coulomb barrier

Differential cross sections as a function of cm angle were measured for 1n- and 2n-transfer reactions in ²³⁸U + ¹⁹⁷ Au and¹⁹⁷ Au + ¹⁹⁷ Au collisions in the energy range from 0.881 V_c to 1.093 V_c and 0.825 V_c to 0.964 V_c, respectively. For ¹⁹⁸Au and ¹⁹⁹Au from the ²³⁸U + ¹⁹⁷Au collisions, for reduced distances of closest approach d _o 1.55 fm, the angular distributions at all bombarding energies are well described by the semiclassical theory. Equivalently, the transfer probabilities show the expected exponential decrease with increasing d _o over many orders of magnitude. For all other transfer products from ²³⁸U + ¹⁹⁷Au collisions, and for all transfer products from ¹⁹⁷Au + ¹⁹⁷Au collisions, markedly reduced cross sections relative to the semiclassical theory are observed for central collisions at all bombarding energies, even for values of d _o that are well outside the region where absorption is known to set in. Only for the more peripheral collisions, one observes agreement of the angular distributions (transfer probabilities) with the semiclassical expectations. The deviations for central collisions are absent for reactions with positive Q _gg values and scale roughly with increasingly negative values of Q _gg, i.e. with increasing Q-value mismatch. Channel coupling is proposed as the relevant mechanism.     

How dense does parton matter get in Pb + Pb collisions at the CERN SPS?

We examine the qualitative features of parton production through materialization in heavy-ion collisions within perturbative QCD, and estimate the magnitude of the resulting parton density created during the early stage of the collisions. The implications for “anomalous”J/? suppression observed in Pb+Pb collisions at the CERN SPS are discussed. We argue that theA-dependence of absorption ofJ/? by (partonic) comovers is steeper than assumed in most phenomenological models, because the absorption process is dominated by quasi-perturbative QCD interactions. Our argument is supported by results recently obtained in the framework of the parton cascade model. We predict significant “anomalous” suppression for Pb+Pb collisions at the CERN-SPS, but not for S+U collisions.     

Observation of Au + Au → Au + Au + ρ 0 and Au + Au → Au* + Au* + ρ 0 with STAR

We report the first observation of the reactions Au + Au → Au + Au + ρ ⁰ and Au + Au → Au* + Au* + ρ ⁰ with the STAR detector. The ρ are produced at small perpendicular momentum, as expected if they couple coherently to both nuclei. We discuss models of vector meson production and the correlation with nuclear breakup, and present a fundamental test of quantum mechanics that is possible with the system.     

Charged hadron suppression in Au+Au collisions at high p T from STAR

We report high statistics measurements of inclusive charged hadron production in Au+Au collisions at $\sqrt {s_{NN} } = 200$ GeV and 130 GeV. A large, approximately constant hadron suppression is observed in central Au+Au collisions for 5 ‖p _T ‖ 12 GeV/c. The collision energy dependence of the yields and the centrality and p _T dependence of the suppression provide stringent constraints on theoretical models of suppression. Models incorporating initial-state gluon saturation or partonic energy loss in dense matter are largely consistent with observations. The p _T -dependent suppression expected from models incorporating jet attenuation in cold nuclear matter or absorption of fragmentation hadrons is not observed.     

Di-jet correlation in Au+Au and Cu+Cu Collisions from PHENIX

PHENIX has measured the two-particle azimuth correlation in Au+Au at √s=200 GeV. Jet shape and yield at the away side are found to be strongly modified at intermediate and low p _T, and the modifications vary dramatically with p _T and centrality. At high p _T, away side jet peak reappears but the yield is suppressed. Similar jet strength is found for Au+Au and Cu+Cu collisions with similar number of participant nucleons.     

Local thermalization in d+Au collisions

The extent of a locally equilibrated parton plasma in d+Au collisions at √s _NN =200 GeV is investigated as a function of centrality in a nonequilibrium-statistical framework. Based on a three-sources model, analytical solutions of a relativistic diffusion equation are in precise agreement with recent data for charged-particle pseudorapidity distributions. The moving midrapidity source indicates the size of the local thermal equilibrium region after hadronization. In central d+Au collisions it contains 19% of the produced particles.     

Forward a production and nuclear stopping power in d+Au collisions at RHIC

Using the forward time projection chambers of STAR we measure the centrality dependent A and ā yields in d+Au collisions at √s _NN =200 GeV at forward and backward rapidities. The contributions of different processes to particle production and baryon transport are probed exploiting the inherent asymmetry of the d+Au system. While the d side appears to be dominated by multiple independent nucleon-nucleon collisions, nuclear effects contribute significantly on the Au side. Using the constraint of baryon number conservation, the rapidity loss of baryons in the incoming deuteron can be estimated as a function of centrality. This is compared to a model and to similar measurements in Au+Au, which gives insights into the nuclear stopping power at relativistic energies.     

Comparison between the fragmentation processes in central Pb + Ag and Pb + Au collisions

The fragmentation processes of a medium mass system and of a very massive one formed in central collisions are compared within the Landau-Vlasov model taking into account both the isospin dependence and the two-body residual interactions. The simulations predict the formation of a roughly ellipsoidal source in the central Pb + Ag reactions while, for the Pb + Au system, the fragmentation occurs from an hollow source, the configuration of which being intermediate between bubble-like and toroidal shapes. This different shapes explain and allow to reproduce semi-quantitatively the two different profiles of the experimental kinetic energy spectra.     

Quarkonium production from d + Au to Au + Au collisions

The PHENIX experiment measured J/ production in pp, d + Au and Au + Au reactions at = 200 GeV over a wide range of rapidity and transverse momentum. The nuclear modification factor obtained by comparing the d + Au and pp cross sections as a function of rapidity, is consistent with shadowing of the gluon distribution functions. J/ production in Au + Au collisions was compared to the production in pp collisions and it was found to be inconsistent with models that predict strong enhancement relative to binary collision scaling. Arrival of the final proofs: 29 June 2005 PACS: 25.75-q, 25.75-Dw, ^* Deceased Spokesperson     

Event-by-event hadron ratio fluctuations from Au+Au collisions at RHIC-STAR

We present measurements of event-by-event fluctuations on hadron multiplicity ratios (K/??, p/??, K/p) in Au+Au collisions at $\sqrt {s_{NN} } $ and 200 GeV using the STAR detector at RHIC. The magnitude of dynamical fluctuations ?? _dyn for p/?? and K/p ratios change smoothly from a large negative value at 7.7 GeV to a smaller negative value at 200 GeV while that for the K/?? ratios exhibits no significant beam energy dependence. The dynamical fluctuations related to pair production ?? _dyn ^pair dyn for the p/K, K/p, K ^?/K ⁺ and p/p ratios at 200 GeV all exhibit a maximum at the mid-central collisions and decrease at the most peripheral and most central collisions.     

Ratios of charged antiparticles-to-particles near mid-rapidity in Au + Au collisions at sqrt[s(NN)]=130 GeV

We have measured the ratios of antiparticles to particles for charged pions, kaons, and protons near mid-rapidity in central Au+Au collisions at sqrt[s(NN)] = 130 GeV. We observe / = 1.00+/-0.01(stat)+/-0.02(syst), / = 0.91+/-0.07(stat)+/-0.06(syst), and

/

= 0.60+/-0.04(stat)+/-0.06(syst). The / and

/

ratios give a consistent estimate of the baryo-chemical potential mu(B) of 45 MeV, a factor of 5-6 smaller than in central Pb+Pb collisions at sqrt[s(NN)] = 17.2 GeV.     

PbK-shell ionization probability in high-energy Xe+Pb collisions

Probabilities for PbK-shell ionization in 5.9-, 7.2-, and 8.5-MeV/a.m.u. Xe+Pb collisions for impact parameters between 10 and 60 fm are reported. The measured probabilities are compared with theories of 1sσ-vacancy production.     

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.     

Strangelet search and particle production studies in Pb+Pb collisions at 158 AGeV/c at CERN SPS

We searched for long-lived strange quark matter particles, so-calledstrangelets, and studied particle and antiparticle production in Pb+Pb collisions at 158 GeV/c per nucleon at zero degree production angle. We give upper limits for the production of strangelets covering a mass to charge ratio up to 120 GeV/c ² and lifetimest _lab>1.2 μs and plot invariant differential production cross sections as a function of rapidity for a variety of particles.     

Selective observation of continuum transitions in the Pb+Pb quasimolecule

Cross sections for doubleK shell excitation and for the production of continuum x-rays in coincidence withK x-rays from 4.8 MeV/amu²⁰⁸Pb +²⁰⁸Pb collisions have been measured. A simple statistical model is presented to explain double vacancy contributions to the coincidence cross sections and the relative contributions of MO x-ray,K x-ray cascade and MO x-ray,K x-ray double vacancy coincidences are evaluated. These latter yields are found to be significant enough to make extraction of the cascade contribution to the measured coincidence yields problematic.     

Quarkonia production at forward rapidity in Pb+Pb collisions at {\sqrt{{s}_{\rm NN}}=2.76} TeV with the ALICE detector

The study of formation of heavy quarkonia in relativistic heavy-ion collisions provides important insight into the properties of the produced high-density QCD medium. Lattice QCD studies show sequential suppression of quarkonia states with increasing temperature; which affirms that a full spectroscopy can provide us a thermometer for the matter produced under extreme conditions in relativistic heavy-ion collisions and one of the most direct probes of deconfinement. Muons from the decay of charmonium resonances are detected in ALICE experiment in p?+?p and Pb+Pb collisions with a muon spectrometer, covering the forward rapidity region (2.5?<?y?<?4). The analysis of the inclusive J/?? production in the first Pb+Pb data collected in the fall of 2010 at a centre of mass energy of $\sqrt{s_{\rm NN}}=2.76$ ?TeV is discussed. Preliminary results on the nuclear modification factor (R _AA) and the central to peripheral nuclear modification factor (R _CP) are presented.     

Dynamical intermediate-mass fragment production in197Au+197Au collisions at energies ≤15 MeV/u

We present radiochemical data on the production of intermediate-mass fragments (IMF, 11≤Z<≈25) in collisions of¹⁹⁷Au+¹⁹⁷Au at 9, 11, 13 and 15 MeV/u. We demonstrate that the IMFs are produced in two-step reactions: Highly excited heavy nuclei are formed in a binary deep-inelastic reaction, and the IMFs originate from fast mass-asymmetric sequential fission of these primary reaction products.     

Photon production in √s NN =200 GeV Au+Au collisions measured by the PHENIX experiment at RHIC

Direct photon production in Au+Au collisions at √s _NN =200 GeV has been measured in the p _Trange of 1<p _T<5 GeV/c. The yield as a function of centrality is in agreement with published data of the RHIC 2002 run, and with results from a new method that explores very low mass dileptons. The result is compared to several theoretical calculations, and it is found that the measurement is not inconsistent with calculations including thermal photon contributions.