Central Pb Research Articles

STAR's measurement of energy and system size dependence of K/π fluctuations at RHIC

Strangeness enhancement has been predicted to be one of the signatures of the formation of the quark-gluon plasma (QGP) and event-by-event fluctuations of strangeness may reveal the nature of such a phase transition. We report results for K/π fluctuations from Cu+Cu at = 62.4 and 200 GeV and Au+Au collisions at = 62.4, 130 and 200 GeV using the STAR detector at the relativistic heavy-ion collider. We compare our results with the results observed in central Pb+Pb collisions at SPS energies at = 6.3, 7.6, 8.8, 12.3 and 17.3 GeV. We observe that the fluctuation strength measured at the highest SPS energy point by the NA49 Collaboration in Pb+Pb collisions is similar to our results in Cu+Cu and Au+Au collisions. We also compare our results with HIJING, a statistical hadronization model and a multi-phase transport model.

Photon-tagged jet measurements in Pb+Pb collisions at TeV with the CMS detector

Presented are the results of a detailed study for a complete simulation of the CMS detectors at the LHC in view of the expected modification of jet fragmentation functions in central Pb + Pb collisions at TeV compared to the vacuum (p+p) case. The study is based on γ-jet events, using the correlation between isolated high transverse energy (ET > 70 GeV) photons and fully reconstructed jets, based on the information provided by the CMS calorimeters and silicon tracker.

ϕ and ω − ρ production in d–C, d–U, S–U and Pb–Pb at SPS energies

Nuclear dependence of ϕ and ω − ρ production is studied from heavy nuclei and deuteron-induced reactions. In addition to the ratio ϕ/(ω + ρ), closely related to the strangeness saturation factor γS in transverse mass (MT) domains, absolute cross-sections and multiplicities are also presented. The ϕ multiplicities for central Pb–Pb collisions, as obtained in the most recent NA50 measurement, lead to an updated version of the ϕ puzzle, where reduced discrepancy between experiments opens a new space for a systematic difference between the leptonic and hadronic decay channels.

Modification of the ρ meson detected by low-mass electron–positron pairs in central Pb[sbnd]Au collisions at [formula omitted

We present a measurement of e+e− pair production in central PbAu collisions at 158A GeV/c. As reported earlier, a significant excess of the e+e− pair yield over the expectation from hadron decays is observed. The improved mass resolution of the present data set, recorded with the upgraded CERES experiment at the CERN-SPS, allows for a comparison of the data with different theoretical approaches. The data clearly favor a substantial in-medium broadening of the ρ spectral function over a density-dependent shift of the ρ pole mass. The in-medium broadening model implies that baryon induced interactions are the key mechanism to the observed modifications of the ρ meson at SPS energy.

Pattern recognition techniques of charged hadron identification with HMPID at LHC

The High Momentum Particle Identification Detector (HMPID), already installed in the ALICE setup at LHC, is ready to identify charged hadrons with momentum up to 5 GeV/ c. It consists of seven modules of a RICH detector with a CsI photocathode. The HMPID will operate to identify charged hadrons both in Pb–Pb collisions and in p–p collisions. The strategy to identify charged hadrons will differ according to the colliding system. In the case of central Pb–Pb collisions, the identification of Cherenkov photons requires efficient pattern recognition algorithm (based on a modified Hough transform, that maps the pad coordinate space directly to the Cherenkov angle parameter space). In the first p–p collisions, instead, where the tracking has to be optimized (or even missing, as sometimes in the cosmic runs), a first reconstruction step based on a new algorithm (Hidden Track Algorithm—HTA) will be used. The tracking parameters are not used in the pattern recognition, at the end an estimate of the track direction as well as the mean 〈 θ C 〉 will be performed. The HTA will be here described and preliminary results on its performances will be presented.

Bose-Einstein correlations ofπ−π−pairs in central Pb+Pb collisions at20A,30A,40A,80A, and158AGeV

Measurements of Bose-Einstein correlations of pion pairs in central Pb+Pb collisions were performed with the NA49 detector at the CERN SPS for beam energies of 20A, 30A, 40A, 80A, and 158A GeV. Correlation functions were measured in the longitudinally co-moving ``out-side-long'' reference frame as a function of rapidity and transverse momentum in the forward hemisphere of the reaction. Radius and correlation strength parameters were obtained from fits of a Gaussian parametrization. The results show a decrease of the radius parameters with increasing transverse momentum characteristic of strong radial flow in the pion source. No striking dependence on pion-pair rapidity or beam energy is observed. Static and dynamic properties of the pion source are obtained from simultaneous fits with a blast-wave model to radius parameters and midrapidity transverse momentum spectra. Predictions of hydrodynamic and microscopic models of Pb+Pb collisions are discussed.

Suppression of High-pTNeutral Pion Production in CentralPb+PbCollisions atsNN=17.3 GeVRelative top+Candp+PbCollisions

Neutral pion transverse momentum spectra were measured in p+C and p+Pb collisions at sqrt[S{NN}]=17.4 GeV at midrapidity (2.3 less than or approximately equal eta{lab} less than or approximately equal 3.0) over the range 0.7 less than or approximately equal p{T} less than or approximately equal 3.5 GeV/c. The spectra are compared to pi{0} spectra measured in Pb+Pb collisions at sqrt[S{NN}]=17.3 GeV in the same experiment. For a wide range of Pb+Pb centralities (N{part} less than or approximately equal 300), the yield of pi{0}'s with p{T} greater than or approximately equal 2 GeV/c is larger than or consistent with the p+C or p+Pb yields scaled with the number of nucleon-nucleon collisions (N{coll}), while for central Pb+Pb collisions with N{part}greater than or approximately equal 350, the pi{0} yield is suppressed.

Anti-nuclei and nuclei production in Pb+Pb collisions at CERN SPS energies

We present new results on production of 3He and t obtained by the NA49 experiment in 20A, 30A, 40A and 80A GeV central Pb+Pb collisions at the CERN SPS. Transverse mass spectra and rapidity distributions for clusters measured over a large phase space domain are discussed. We observe a weak dependence of the mid-rapidity t/3He ratio on collision energy at SPS. The energy dependence of the total yield for 3He is remarkably reproduced by a statistical hadron gas model. A gradual decrease of the coalescence parameters B3 for 3He with s√SNN is observed. In addition, a measurement of the yield of antideuterons in Pb+Pb reactions at SPS's top energy of s√SNN = 17.2 GeV is presented.

J/ψ suppression in the threshold model at RHIC and LHC energy

In the QGP-based threshold model (Blaizot et al 2000 Phys. Rev. Lett. 85 4012, Blaizot and Ollitrault 1996 Phys. Rev. Lett. 77 1703), in addition to the normal nuclear absorption, J/ψ's are subjected to an ‘anomalous’ suppression such that above a threshold density nJ/ψ, all the J/ψ's are melted. In the threshold model we have analyzed the recent PHENIX data on the centrality dependence of J/ψ suppression in Au+Au collisions at RHIC. Feedback from decay of the state χ is accounted for. J/ψ's are anomalously suppressed above a threshold density, nJ/ψ = 3.57 ± 0.17 fm−2. Threshold density for anomalous suppression of the state χ is uncertain to a large extent, nχ = 0.32 ± 0.32 fm−2. The fraction F of the state χ cannot be determined unambiguously, depending on the nuclear absorption, it can vary from 20% to 40%. We have also predicted for the suppression in Pb+Pb collisions at LHC energy. In central Pb+Pb collisions, J/ψ's are suppressed by a factor of 3–4. The suppression pattern is rather similar to that in Au+Au collisions, if not slighty less in central collisions. Using the PHENIX data on the participant number dependence of the Bjorken energy density, we have also estimated the QGP formation time. For critical temperature Tc = 192 MeV, the estimated QGP formation time ranges between 0.07 and 0.09 fm c−1.

Multiplicity fluctuations in relativistic nuclear collisions: statistical model versus experimental data

Multiplicity distributions of hadrons produced in central nucleus–nucleus collisions are studied within the hadron-resonance gas model in the large volume limit. In the canonical ensemble conservation of three charges (baryon number, electric charge and strangeness) is enforced. In addition, in the micro-canonical ensemble, energy conservation is included. An analytical method is used to account for resonance decays. Multiplicity distributions and scaled variances for negatively charged hadrons are presented along the chemical freeze-out line of central Pb+Pb (Au+Au) collisions from SIS to LHC energies. Predictions obtained within different statistical ensembles are compared with preliminary NA49 experimental results on central Pb+Pb collisions in the SPS energy range. The measured fluctuations are significantly narrower than a Poisson reference distribution, and clearly favor expectations for the micro-canonical ensemble.

Onset of deconfinement and critical point: NA49 and NA61/SHINE at the CERN SPS

This paper is dedicated to the memory of Jozsef Zimanyi one of the founders of the experiment NA49 at the CERN SPS. Firstly, the paper summarizes the main results of NA49 concerning observation of the onset of deconfinement in central Pb+Pb collisions at the low SPS energies. Secondly, it sketches the physics program of NA61 at the CERN SPS, the successor of NA49, which in particular aims to discover the critical point of strongly interacting matter. Finaly, a brief review of the future experimental programs in the CERN SPS energy range is given.

Pion and kaon production in centralPb+Pbcollisions at20Aand30AGeV: Evidence for the onset of deconfinement

Results on charged pion and kaon production in central Pb+Pb collisions at 20A and 30A GeV are presented and compared to data at lower and higher energies. A rapid change of the energy dependence is observed around 30A GeV for the yields of pions and kaons as well as for the shape of the transverse mass spectra. The change is compatible with the prediction that the threshold for production of a state of deconfined matter at the early stage of the collisions is located at low SPS energies.

Searching for the critical point of QCD: Theoretical benchmark calculations

We present a comprehensive study of event-by-event multiplicity fluctuations in nucleon-nucleon and nucleus-nucleus interactions from the BNL Alternating Gradient Synchrotron/GSI Facility for Antiproton and Ion Research to BNL Relativistic Heavy Ion Collider energies within the ultrarelativistic quantum molecular dynamics transport approach. The scaled variances of negative, positive, and all charged hadrons are analyzed. The scaled variance in central Pb+Pb collisions increases with energy and behaves similar to inelastic p+p interactions. We find a nontrivial dependence of multiplicity fluctuations on the rapidity and transverse-momentum interval used for the analysis and on the centrality selection procedure. Quantitative predictions for the NA49 experiment are given, taking into account the acceptance of the detector and the selection procedure of central events.

Pion-pion and pion-proton correlations: new results from CERES

Results of a new two-particle correlation analysis of central Pb+Au collision data at 158 GeV per nucleon are presented. The emphasis is put on pion-proton correlations and on the dependence of the two-pion correlation radii on the azimuthal emission angle with respect to the reaction plane.

Rapidity and energy dependence of the electric charge correlations inA+Acollisions from20Ato158AGeV

Results from electric charge correlations studied with the Balance Function method in A+A collisions from 20\emph{A} to 158\emph{A} GeV are presented in two different rapidity intervals: In the mid-rapidity region we observe a decrease of the width of the Balance Function distribution with increasing centrality of the collision, whereas this effect vanishes in the forward rapidity region. Results from the energy dependence study in central Pb+Pb collisions show that the narrowing of the Balance Function expressed by the normalised width parameter \textit{W} increases with energy towards the highest SPS and RHIC energies. Finally we compare our experimental data points with predictions of several models. The hadronic string models UrQMD and HIJING do not reproduce the observed narrowing of the Balance Function. However, AMPT which contains a quark-parton transport phase before hadronization can reproduce the narrowing of the BF's width with centrality. This confirms the proposed sensitivity of the Balance Function analysis to the time of hadronization.

Are high-pT pions suppressed in Pb+Pb collisions at ?

Transverse momentum spectra of neutral pions in the range 0.7 < pT < 3.2 GeV/c have been measured at 2.3 ≲ ηlab ≲ 3.0 by the WA98 experiment in p+C and p+Pb collisions at . Scaled by the number of nucleon–nucleon collisions (Ncoll), the π0 yields in p+C and p+Pb at pT ≈ 2.0 − 2.5 GeV/c are higher than the respective yields in central Pb+Pb collisions with Npart ≳ 300. This observation is qualitatively consistent with expectations from parton energy loss.

Two- and three-particle azimuthal correlations of high-pt charged hadrons in Pb–Au collisions at 158 A GeV/c

Azimuthal correlations of hadrons with high transverse momenta serve as a measure to study the energy loss and the fragmentation pattern of jets emerging from hard parton–parton interactions in heavy-ion collisions. Preliminary results from the CERES experiment on two- and three-particle correlations in central Pb–Au collisions are presented. A strongly non-Gaussian shape on the away-side of the two-particle correlation function is observed, indicating significant interactions of the emerging partons with the medium. Mechanisms like deflection of the initial partons or the evolution of a mach cone in the medium can lead to similar modifications of the jet structure on the away-side. An analysis based on three-particle correlations is presented which helps us to shed light on the origin of the observed away-side pattern.

Multiplicity fluctuations in proton–proton and nucleus–nucleus collisions

We study the scaled variances of multiplicity fluctuations in nucleus–nucleus collisions at SPS and RHIC energies within the HSD transport model. The HSD results are compared with proton–proton data and with predictions of the hadron-resonance gas statistical model. We find that the HSD scaled variances ωi for negatively, positively, and all charged hadrons in central nucleus–nucleus collisions remain close to the ωi values in proton–proton collisions and increase with collision energy as the corresponding multiplicities per participating nucleon. The statistical model predicts very different behavior of ωi. However, a comparison with preliminary NA49 data for the most central Pb+Pb collisions at SPS energies does not permit to distinguish the HSD and statistical model results. New measurements of the multiplicity fluctuations in nucleus–nucleus collisions in a wide energy region with large acceptance are needed to allow for a proper determination of the underlying dynamics.

Lead biogeochemistry in a central Ontario Forested watershed

To determine the sources and sinks of atmospherically deposited Pb at a forested watershed (Plastic Lake) in central Ontario, Canada, Pb pools and fluxes through upland, wetland and lake compartments were measured during 2002/2003 and compared with previous measurements taken between 1989 and 1991. In 2002/2003, annual bulk deposition of Pb was 0.49 mg m−2 compared with 1.90–1.30 mg m−2 in 1989–1991. Annual Pb concentrations in stream water draining the upland part of the catchment were very low (0.04 μg l−1) and were approximately half those measured in 1989–1991 (0.11–0.08 μg l−1). Leaching losses in stream water were small and mass balance estimates indicate almost complete retention (>95%) of atmospherically deposited Pb in upland soils. In contrast, annual Pb concentrations in stream water draining a wetland were between 0.38 and 0.77 μg l−1, with the highest concentration occurring in 2002/2003 and mass balance calculations indicate that the wetland is a net source of Pb in all measured years. Lead concentrations in the lake outflow were low and the average Pb concentration measured in 2002/2003 (0.09 μg l−1) was approximately half the value recorded in 1989–1991 (0.19 μg l−1 both years). Annual mass balance estimates indicate that the lake retained between 2.47 mg m−2 (1989/1990) and 1.42 mg m−2 (2002/2003) and that in 2002/2003 68% of the Pb input to the lake is derived from the terrestrial catchment. These estimates are higher than sediment core records, which indicate around 18 mg m−2 Pb was retained in sediment during the 1990s. Nevertheless, Pb concentrations decrease with sediment depth and 206Pb/207Pb concentrations increase with depth, a pattern also observed in mineral soils that reflects the substantial contribution of anthropogenic Pb to the watershed. Lead isotope data from soil and sediment indicate a recent anthropogenic Pb signal (206Pb/207Pb ∼ 1.185) in upper soils and sediments and an older anthropogenic signal (206Pb/207Pb ∼ 1.20) in deeper soil and sediment. Lead isotope data in sediment and vegetation indicate that practically all the Pb cycled in the forest at Plastic Lake is anthropogenic in origin.

Multiplicity studies and effective energy in ALICE at the LHC

In this work we explore the possibility to perform “effective energy” studies in very high energy collisions at the CERN large hadron collider (LHC). In particular, we focus on the possibility to measure in pp collisions the average charged multiplicity as a function of the effective energy with the ALICE experiment, using its capability to measure the energy of the leading baryons with the zero degree calorimeters. Analyses of this kind have been done at lower centre-of-mass energies and have shown that, once the appropriate kinematic variables are chosen, particle production is characterized by universal properties: no matter the nature of the interacting particles, the final states have identical features. Assuming that this universality picture can be extended to ion–ion collisions, as suggested by recent results from RHIC experiments, a novel approach based on the scaling hypothesis for limiting fragmentation has been used to derive the expected charged event multiplicity in AA interactions at LHC. This leads to scenarios where the multiplicity is significantly lower compared to most of the predictions from the models currently used to describe high energy AA collisions. A mean charged multiplicity of about 1000–2000 per rapidity unit (at η∼0) is expected for the most central Pb–Pb collisions at $\sqrt{s_{{\text{NN}}}} = 5.5\,\text{TeV}$ .