A first look at open charm production in Indium-Indium collisions at SPS energies

NA60 is an experiment at the CERN-SPS devoted to the study of dimuon production in heavy-ion and proton-nucleus collisions. The main topics under study are low mass vector meson production, J/ production and suppression, and the sources of the dimuon continuum in the mass range 1.2-2.7 GeV/c ². In 2003, NA60 collected million dimuon events from Indium-Indium collisions. We present preliminary results of the analysis of this data sample in view of measuring the open charm contribution to the dimuon spectrum. Although we are still working on the final background subtraction procedure, we can already demonstrate that the detector performance is good enough to allow the separation of prompt dimuons from muon pairs originating in distant D decays. Arrival of the final proofs: 1 July 2005 PACS: 25.75.Dw, 25.75.Nq     

Evidence for radial flow of thermal dileptons in high-energy nuclear collisions

The NA60 experiment at the CERN SPS has studied low-mass dimuon production in 158A GeV In-In collisions. An excess of pairs above the known meson decays has been reported before. We now present precision results on the associated transverse momentum spectra. The slope parameter Teff extracted from the spectra rises with dimuon mass up to the rho, followed by a sudden decline above. While the initial rise is consistent with the expectations for radial flow of a hadronic decay source, the decline signals a transition to an emission source with much smaller flow. This may well represent the first direct evidence for thermal radiation of partonic origin in nuclear collisions.     

Searching for quark matter with dileptons and photons: From SPS to relativistic heavy-ion collider

The heavy-ion programme at the CERN SPS, which started back in ’86, has produced a wealth of very interesting and intriguing results in the quest for the quark-gluon plasma. The highlights of the programme on dilepton and direct photon measurements are reviewed emphasizing the most recent results obtained in Pb-Pb collisions at 158 A GeV. Prospects from RHIC are discussed.     

Measuring Displaced muon Pairs in Indium-Indium collisions at the CERN SPS

The NA60 experiment at the CERN-SPS studies dimuon production in proton-nucleus and heavy-ion collisions. We report on the effort towards measuring open charm in Indium-Indium collisions. Although work on the final background subtraction procedure is still under way, we can already demonstrate that the detector performance is adequate to separate prompt dimuous from muon pairs originating in distant $D\bar D$ decays.     

φ meson production in NA60

NA60 is a fixed-target experiment at the CERN SPS which measured dimuon production in nucleus–nucleus and proton–nucleus collisions. The experiment collected muon pair samples of unprecedented quality in heavy-ion experiments. This paper presents a high quality measurement of the p_T distribution of the φ meson, covering a broad p_T window. The data were collected in 2003 in In-In collisions at 158 GeV per nucleon. The results, presented as a function of centrality, were studied against several possible sources of systematic effects and proved to be fairly stable. We show that the inverse m_T slope measured in In-In collisions, in the φ→μμ decay channel, depends significantly on the range used to perform the fit. When the fit is performed at low transverse momentum, the effective inverse slope increases from peripheral to central collisions, as measured by other experiments. We finally show that our measurement for In-In is compatible with the overall systematics of T slope versus mass, measured in different collision systems by the NA49 experiment PACS 25.75.Nq; 25.75.-q; 25.75.Dw; 14.40.Cs; 12.38.Mh     

Transverse-momentum distribution of <Emphasis Type="Italic">J</Emphasis>/<Emphasis Type="Italic">ψ</Emphasis> mesons produced in proton-nucleus and lead-lead interactions

The transverse-momentum and transverse-mass distributions of J/ψ mesons produced in proton-nucleus and lead-lead interactions at 400 GeV and 158 GeV/nucleon, respectively, were measured in the NA50 experiment. The change in these distributions with increasing centrality is studied for various interaction energies. Data obtained in the NA38 experiment for collisions of light nuclei at 200 GeV per nucleon is also used in the present analysis. Investigation of the transverse-momentum and transverse-mass distributions of J/ψ particles can provide additional information about the properties of the phase transition of ordinary nuclear matter to quark-gluon plasma.     

Charmonium production in Pb−Pb collisions at 158 <Emphasis Type="Italic">A</Emphasis> GeV

The NA50 experiment at CERN SPS studies charmonia and Drell-Yan production in the μ⁺ μ^? decay channel at 158A GeV. The data collected in Pb?Pb collisions during 1995, 1996 and 1998 run periods show that the J/ψ is anomalously suppressed in central collisions. This observed suppression pattern can be considered as a strong indication of the production of Quark-Gluon Plasma.     

On energy densities reached in heavy-ion collisions at the CERN SPS

We present a few estimates of energy densities reached in heavy-ion collisions at the CERN SPS. The estimates are based on data and models of proton-nucleus and nucleus-nucleus interactions. In all of these estimates the maximum energy density in central Pb+Pb interactions is larger than the critical energy density GeV/fm³ following from lattice gauge theory computations. In estimates which we consider as realistic the maximum energy density is about . In this way our analysis gives some support to claims that deconfined matter has been produced at the CERN SPS. Any definite statement requires a deeper understanding of formation times of partons and hadrons in nuclear collisions. We also compare our results with implicit energy estimates contained in earlier models of anomalous suppression in nuclear collisions. Received: 3 February 2003 / Revised version: 5 March 2003 / Published online: 5 May 2003     

Re-hardening of hadron transverse mass spectra in relativistic heavy-ion collisions

We analyze the spectra of pions and protons in heavy-ion collisions at relativistic energies from 2 A GeV to 65+65 A GeV by using a jet-implemented hadron-string cascade model. In this energy region, hadron transverse mass spectra first show softening until SPS energies, and re-hardening may emerge at RHIC energies. Since hadronic matter is expected to show only softening at higher energy densities, this re-hardening of spectra can be interpreted as a good signature of the quark-gluon plasma formation     

Mass spectrum of the dimuons produced in relativistic heavy-ion collisions

The mass spectrum of dimuons produced from the matter in the central region of rapidity in ultra-relativistic heavy-ion collisions is calculated in accordance with Bjorken’s recently proposed model for relativistic heavy-ion collisions. The matter in this central region is assumed to consist of a deconfined quark-gluon plasma phase and a pionized phase. Distinct enhancements of the dimuon mass spectrum below 500 MeV, due to the quark-gluon phase, are predicted for a deconfinement phase-transition temperatureT _c<200 MeV.     

Diphoton rates from thermalized matter resulting in ultra-relativistic heavyion collisions

Diphoton radiation off strongly interacting matter resulting from ultrarelativistic heavy-ion collisions is estimated for SPS and RHIC conditions. At SPS energies the thermal diphoton signal competes strongly with the Drell — Yan like background. For RHIC energies we find that radiation from initially undersaturated (but gluon-rich) deconfined matter overwhelms the hadron gas at invariant mass M > 1 GeV; the Drell — Yan like diphotons dominate at M > 2.5 GeV. Due to this the so-called M_⊥ scaling is approximately obtained in a narrow window and can serve as additional tool for discriminating radiation from quark-gluon matter.     

Evidence for the production of thermal muon pairs with masses above 1 GeV/c 2 in 158 A GeV indium-indium collisions

The yield of muon pairs in the invariant mass region 1<M<2.5 GeV/c ² produced in heavy-ion collisions significantly exceeds the sum of the two expected contributions, Drell-Yan dimuons and muon pairs from the decays of D meson pairs. These sources properly account for the dimuons produced in proton-nucleus collisions. In this paper, we show that dimuons are also produced in excess in 158 A GeV In-In collisions. We furthermore observe, by tagging the dimuon vertices, that this excess is not due to enhanced D meson production, but made of prompt muon pairs, as expected from a source of thermal dimuons specific to high-energy nucleus-nucleus collisions. The yield of this excess increases significantly from peripheral to central collisions, both with respect to the Drell-Yan yield and to the number of nucleons participating in the collisions. Furthermore, the transverse mass distributions of the excess dimuons are well described by an exponential function, with inverse slope values around 190 MeV. The values are independent of mass and significantly lower than those found at masses below 1 GeV/c ², rising there up to 250 MeV due to radial flow. This suggests the emission source of thermal dimuons above 1 GeV/c ² to be of largely partonic origin, when radial flow has not yet built up.     

Production of secondaries in high-energy heavy-ion collisions

In the framework of the quark-gluon string model, we calculate the inclusive spectra of secondaries produced in heavy-ion collisions at intermediate (CERN SPS) and at much higher (RHIC) energies. We demonstrate that the mechanism of secondary production changed drastically in the energy interval √s = 20–60 GeV and that it is in agreement with qualitative estimates of Glauber-Gribov theory. The results of numerical calculations at intermediate energies are in reasonable agreement with the data without change of the model parameters. At RHIC energies, numerically large inelastic screening correlations should be accounted for in calculations. The text was submitted by the authors in English.     

Strong-coupling diffusion in relativistic systems

Different from the early universe, heavy-ion collisions at very high energies do not reach statistical equilibrium, although thermal models explain many of their features. To account for nonequilibrium strong-coupling effects, a Fokker-Planck equation with time-dependent diffusion coefficient is proposed. A schematic model for rapidity distributions of participant baryons is set up and solved analytically. The evolution from SIS via AGS and SPS to RHIC energies is discussed. Strong-coupling diffusion produces double-peaked spectra in central collisions at the higher SPS momentum of 158 A.GeV/c and beyond.     

Collective flows in high-energy heavy-ion collisions at AGS and SPS energies

Proton collective flows in heavy-ion collisions from AGS ((2–11) A GeV) to SPS ((40,158) A GeV) energies are investigated in a nonequilibrium transport model with nuclear mean-field (MF). Sideward (p _x), directedv ₁, and ellipticv ₂ flows are systematically studied with different assumptions on the nuclear equation of state (EoS). We find that momentum dependence in the nuclear MF is important for understanding the proton collective flows at AGS and SPS energies. Calculated results with momentum-dependent MF qualitatively reproduce the experimental data of proton sideward, directed, and elliptic flows in an incident energy range of (2–158) A GeV This talk is based on ref. [1]     

Search for deconfinement in NA49 at the CERN SPS

Experiment NA49 at the CERN SPS uses a large acceptance detector for a systematic study of particle yields and correlations in nucleus-nucleus, nucleon-nucleus and nucleon-nucleon collisions. Preliminary results for Pb+Pb collisions at 40, 80 and 158 A⋅GeV beam energy shown and compared to measurements at lower and higher energies.     

粲夸克对在质子-原子核碰撞J/ψ 产生过程中的能量损失

高能重离子碰撞热核环境中J/ψ产额的压低被认为是夸克胶子等离子体生成的重要信号。研究冷核物质中各种核效应对J/ψ 形成过程的影响,是准确理解热核环境中核效应对J/ψ产额压低机制的重要途径。研究了质子-原子核碰撞J/ψ 产生过程中初态部分子分布函数的核效应、入射质子的能量损失效应、以及末态粲夸克对能量损失效应对J/ψ 粒子产额的压低作用,对质子-原子核碰撞J/ψ产生过程的微分截面比R_Fe/Be(xF)进行了领头阶的唯象分析,并与E866 实验数据中代表J/ψ粒子在靶核外产生的实验数据进行比较,获取了粲夸克对的能量损失值(3.81±1.71) GeV/fm。研究结果表明,部分子分布函数的核效应对截面比RFe=Be(xF) 的压低作用在xF > 0区域是随着xF 的增大而增大的;末态粲夸克对的能量损失效应是影响J= 粒子产额压低的重要核效应;当xF较大时(0:65≤xF≤0.92),初态入射质子的能量损失效应在对微分截面比R_Fe/Be(xF)的压低中起了主要作用。J/ψ suppression observed in hot nuclear matter from heavy-ion collisions is considered as a most reliable signature for the formation of Quark-Gluon Plasma. The study about the nuclear effects on J/ψ production in cold nuclear matter provides an important tool for clarifying the conventional nuclear suppression mechanism in heavy-ion collisions. J/ψ suppression in p-A collisions is studied by considering the nuclear effects on parton distribution, the energy loss of beam proton in initial state, and the finial state energy loss of c¯c pair.The leading-order computations for J/ψ production cross-section ratios RFe=Be(xF) are presented and compared with the selected E866 experimental data with the J/ψ production occurring outside the nucleus, the obtained energy loss of the color octet is (3.81±1.71) GeV/fm. It is found that the nuclear suppression from the nuclear effects on the parton distribution functions becomes larger as the increase of xF in the range xF > 0,the J/ψ suppression on R_Fe/Be(xF) induced by the energy loss of color octet c¯c is an important nuclear effect, in the region 0:656xF 60:92, the energy loss of beam proton in initial state is the dominant mechanism which causes a reduction of the J/ψ yield.     

Physics of collisions between ultrarelativistic nuclei: Manifestation of collective effects

Experimental results concerning heavy-ion collisions studied by means of the Relativistic Heavy-Ion Collider (RHIC) at an energy of 100 GeV per nucleon and showing manifestations of collective effects are surveyed. These effects are interpreted as a consequence of the formation of a dense thermalized medium referred to as quark-gluon plasma (strongly interacting quark-gluon plasma, quark-gluon matter, parton medium). An azimuthal anisotropy of particles, the suppression of the particle yield at high transverse momenta in relation to proton-proton collisions, and a change in the shape of the peak from a hadron jet in nucleus-nucleus collisions are spectacular manifestations of collective effects.     

PARTICLE DENSITIES AT THE CENTER REGION IN HIGH ENERGY HEAVY-ION COLLISIONS

Based on the two-chain model by A.Capella et al. the rapidity distributions of the multiplicity in high energy heavy-ion collisions, especially in head-on collisions, are discussed. The energy density at the center region in head-on collisions is estimated for different incident energies and different heavy ions. It is shown that the condition of the phase transition to the quark-gluon plasma can be realized in heavy-ion collisions at several tens of GeV.     

Particle production in Pb+Pb collisions at the SPS

Experiment NA49 at the CERN SPS employs a large acceptance detector to study particle spectra, yields and correlations in nucleus-nucleus, nucleon-nucleus and nucleon-nucleon collisions. Preliminary results on pion, kaon, A and ā production in central Pb+Pb collisions at 40, 80 and 158 A·GeV beam energy are shown and compared to measurements at lower and higher energies.