Strangeness enhancement in sulphur—Nucleus collisions at 200 A GeV/c

The NA35 experiment used several independent methods to determine the strange particle production in p+S and S+A collisions. The different techniques show consistent results. Strangeness conservation in full phase space is used as an additional check of the consistency of the data. On the base of the analysis in full phase space it could be shown that strangeness conservation is fullfilled. The NA35K _S ⁰ in S+S and S+Ag are consistent with the NA44 results forK ⁺ andK ^?. The results of the NA36 collaboration for S+Pb collisions were extrapolated to full phase space. The comparison with the NA35 results shows more than two times lower yields. The ratio of Λ to $\bar \Lambda $ at midrapidity of NA36 is inconsistent with the high baryon density determind by NA35. The strange particle production is compared to the abundance of non strange particles, especially negatively charged pions which are measured in full phase space in the same experiment. A clear enhanced strange hadron production relative to σ^? is observed in S+Ag collisions compared to p+S reactions at the same energy. TheK _S ⁰ multiplicity in full phase space per negative hadron (h ^?) in S+S, S+Ag and Pb+Pb is enhanced by about a factor 1.6 compared to N+N and p+S collisions. The NA36 result for theK _S ⁰ multiplicity perh ^? in S+Pb is below the N+N value.     

Identified hadron compositions in p+p and Au+Au collisions at high transverse momenta at √S(NN)=200 GeV

We report transverse momentum (p(T)≤15 GeV/c) spectra of π(±), K(±), p, p[over ˉ], K(S)(0), and ρ(0) at midrapidity in p+p and Au+Au collisions at √S(NN)=200 GeV. Perturbative QCD calculations are consistent with π(±) spectra in p+p collisions but do not reproduce K and p(p[over ˉ]) spectra. The observed decreasing antiparticle-to-particle ratios with increasing p(T) provide experimental evidence for varying quark and gluon jet contributions to high-p(T) hadron yields. The relative hadron abundances in Au+Au at p(T)?8 GeV/c are measured to be similar to the p+p results, despite the expected Casimir effect for parton energy loss.     

Measurements of higher order flow harmonics in Au+Au collisions at √s(NN)=200 GeV

Flow coefficients ν(n) for n=2, 3, 4, characterizing the anisotropic collective flow in Au+Au collisions at √s(NN)=200 GeV, are measured relative to event planes Ψ(n), determined at large rapidity. We report ν(n) as a function of transverse momentum and collision centrality, and study the correlations among the event planes of different order n. The ν(n) are well described by hydrodynamic models which employ a Glauber Monte Carlo initial state geometry with fluctuations, providing additional constraining power on the interplay between initial conditions and the effects of viscosity as the system evolves. This new constraint can serve to improve the precision of the extracted shear viscosity to entropy density ratio η/s.     

Suppression of back-to-back hadron pairs at forward rapidity in d+Au collisions at √s(NN)=200 GeV

Back-to-back hadron pair yields in d+Au and p+p collisions at √s(NN)=200 GeV were measured with the PHENIX detector at the Relativistic Heavy Ion Collider. Rapidity separated hadron pairs were detected with the trigger hadron at pseudorapidity |η|<0.35 and the associated hadron at forward rapidity (deuteron direction, 3.0<η<3.8). Pairs were also detected with both hadrons measured at forward rapidity; in this case, the yield of back-to-back hadron pairs in d+Au collisions with small impact parameters is observed to be suppressed by a factor of 10 relative to p+p collisions. The kinematics of these pairs is expected to probe partons in the Au nucleus with a low fraction x of the nucleon momenta, where the gluon densities rise sharply. The observed suppression as a function of nuclear thickness, p(T), and η points to cold nuclear matter effects arising at high parton densities.     

Particle-yield modification in jetlike azimuthal dihadron correlations in Pb-Pb collisions at √s(NN)=2.76 TeV

The yield of charged particles associated with high-p(t) trigger particles (83 GeV/c on the away side drops to about 60% of that observed in pp collisions, while on the near side a moderate enhancement of 20%-30% is found.     

Study of Z Boson Production in PbPb Collisions at √S(NN)=2.76 TeV

A search for Z bosons in the μ(+)μ(-) decay channel has been performed in PbPb collisions at √S(NN)=2.76 TeV with the CMS detector at the LHC, in a 7.2 μb(-1) data sample. The number of opposite-sign muon pairs observed in the 60-120 GeV/c(2) invariant mass range is 39, corresponding to a yield per unit of rapidity (y) and per minimum bias event of [33.8±5.5(stat)±4.4(syst)]×10(-8), in the |y|<2.0 range. Rapidity, transverse momentum, and centrality dependencies are also measured. The results agree with next-to-leading order QCD calculations, scaled by the number of incoherent nucleon-nucleon collisions.     

Strangeness enhancement at midrapidity in Pb–Pb collisions at 158 A GeV/c: A comparison with VENUS and RQMD models

Recently the WA97 Collaboration has measured , , , and negative particle yields and transverse mass spectra at central rapidity in Pb–Pb and p–Pb collisions at 158 A GeV/c. These results are compared with the predictions of two of the most widely used event generators for heavy-ion collisions: VENUS 4.12 and RQMD 2.3. Both models predict that enhancements increase with the strangeness content of the particle. They fail, however, to reproduce completely the measured values of yields at central rapidity. In particular, for multistrange particles, VENUS fails to reproduce both the p–Pb and the Pb–Pb data, while RQMD works for p–Pb collisions but seems to be unable to reproduce the data in Pb–Pb collisions. Moreover, the predicted behavior for strangeness production as a function of the centrality of the collision appears to be different from the observed behavior. Received: 29 April 1999 / Published online: 14 October 1999     

Azimuthal anisotropy of electrons from heavy flavor decays in \sqrt{s_{NN}} =200 GeV Au-Au collisions at PHENIX

The PHENIX experiment has measured the azimuthal anisotropy parameter v₂, the second harmonic of the azimuthal distribution, for electrons at mid-rapidity (|η|<0.35) as a function of transverse momentum (0.5<p_T (GeV/c)<5.0) in Au+Au collisions at  =200 GeV. From the result we have calculated the non-photonic electron v₂, which is expected to reflect charm quark azimuthal anisotropy, by subtracting the v₂ of electrons from other sources such as photon conversions and Dalitz decays.     

Energy and system size dependence of charged hadron transverse momentum spectra from Cu+Cu and Au+Au collisions at √S NN =62.4 and 200 GeV

The PHOBOS Collaboration has measured transverse momentum distributions of charged hadrons produced in Cu+Cu collisions at √s _NN =200 and 62.4 GeV. The nuclear modification factor R _AA ^N _part is calculated relative to p+p data at both collision energies as a function of collision centrality. For the same number of participating nucleons, R _AA ^N _part is essentially the same in both systems over the full range of p Tthat is measured. In addition, we observe that within experimental uncertainties, the ratio of 200 GeV to 62.4 GeV Cu+Cu yields has only a moderate centrality dependence and is consistent with the value previously measured in Au+Au collisions for a broad range of p _T.     

Longitudinal spin transfer to Λ and \bar \Lambda in polarized proton-proton collisions at √s = 200 GeV at STAR

The longitudinal spin transfer D _LL of Λ and $\bar \Lambda$ hyperons is expected to be sensitive to the helicity distribution function of strange (anti-)quarks, and to the polarized fragmentation functions. We report new results on D _LL , extracted from √s = 200 GeV polarized pp collisions recorded at the STAR detector during the 2009 RHIC run. These new results cover mid-rapidity |η| < 1.2, with extended p _T up to 6 GeV/c.     

Pseudorapidity distribution of multiplicity in Au+Au collisions at √sNN=200GeV

Using the Glauber model, we discuss the number of binary nucleon-nucleon collisions in heavy-ion collisions. Based on the latter, after considering the effect of energy loss of the nucleons in multiple collisions, we derive the pseudorapidity distribution of the multiplicity as a function of the impact parameter in nucleus-nucleus collisions. Using this, we analyze the experimental measurements carried out by the BRAHMS Collaboration in Au+Au collisions at √s_NN=200GeV. The results are in good agreement with the experimental observations.     

Indirect observation of an in-medium η' mass reduction in √s NN =200 GeV Au+Au collisions

PHENIX Collaboration and STAR Collaboration data on the intercept parameter of the two-pion Bose-Einstein correlation functions in √ s{NN}=200 GeV Au+Au collisions are analyzed in terms of different models of hadronic multiplicities. To describe this combined PHENIX and STAR data set, an in-medium η' mass reduction of at least 200 MeV is needed, at the 99.9% confidence level in the considered model class. Such a significant η' mass modification may indicate the restoration of the U{A}(1) symmetry in a hot and dense hadronic matter and the return of the 9th "prodigal" Goldstone boson.     

Rapidity dependence of hadron PT spectra in central Au＋Au collisions at √^SNN = 200 GeV

The transverse momentum spectra for identified hadrons at different rapidities in central Au＋Au collisions at √^SNN= 200 GeV are studied in a quark combination model. The results for PT spectra of π^±, K^±, p（p^-） and for the p/π ratios in a broader PT range at midrapidity agree well with the data. The transverse momentum spectra of pions, protons and antiprotons at various rapidities y - 1, η= 2.2 and y ≈ 3.2 are calculated and compared with the data.     

Properties of Dihadron Correlations for p＋p Collisions at √8NN = 200 GeV

The dihadron azimuthal angular correlations for p＋p collisions at √8NN = 200 GeV are simulated by a multiphase transport model. The dispersions of near-side and away-side peaks, indicated by the width of Gaussian fit functions and the rms width, decrease with the transverse momentum of associated particles. This trend is consistent with the experimental results. Conditional-yields are also calculated to obtain distributions of the associated particle transverse momentum for both away-side and near-side. Furthermore, the hadronic rescattering effects make the conditional-yield distributions softer.     

Scaling and fragmentation distributions of beam remnants in high Et proton-proton collisions at √s =27.4 GeV

Charged particle trajectories have been reconstructed in pp collisions triggered by transverse energies (E_t) ranging from 1 GeV to 20 GeV. The forward fragmentation function of negative particles in the beam jet is found to scale with E_t in variable x=2p_L/√s−E_t, and the slope is near to that found for small jet triggers in an earlier experiment. Above about E_t=8 GeV, positive particles scale and for 0.5<x<1.0 the ratio of positives to negatives is 1.73±0.2. The forward fragmentation functions at high E_t do not show the behavior expected from the fragmentation of diquarks and are more like that from quarks.Multiplicity and energy flow in various forward polar angle regions are also presented; they are compared to the beam jet predictions of ISAJET and found to be greater at wide angles and less at small.     

Measurements of dihadron correlations relative to the event plane in Au+Au collisions at √sNN=200 GeV

Dihadron azimuthal correlations containing a high transverse momentum (PT) trigger particle are sensitive to the properties of the nuclear medium created at RHI...     

A second look at single-photon production in S + Au collisions at 200 A GeV and implications for quark–hadron phase transition

We reanalyze the production of single photons in S + Au collisions at CERN SPS to investigate: (i) the consequences of using a much richer equation of state for hadrons than the one used in an earlier study by us, and (ii) to see if the recent estimates of photon production in quark matter (at two-loop level) by Aurenche, et al. are consistent with the upper limit of the photon production measured by the WA80 group. We find that the data are consistent with a quark–hadron phase transition. The data are also consistent with a scenario where no phase transition takes place, but where the hadronic matter reaches a density of several hadrons per unit volume, which is rather unphysical. Received: 21 June 1999 / Revised version: 25 August 1999 / Published online: 16 November 1999