Central Pb Research Articles

Momentum imbalance of isolated photon-tagged jet production at RHIC and LHC.

In collisions of ultrarelativistic nuclei, photon-tagged jets provide a unique opportunity to compare jet production and modification due to parton shower formation and propagation in strongly interacting matter at vastly different center-of-mass energies. We present first results for the cross sections of jets tagged by an isolated photon to O(α(em)α(s)(2)) in central Au+Au reactions with sqrt[s(NN)] = 200 GeV at RHIC and central Pb + Pb reactions with sqrt[s(NN)] = 2.76 TeV at LHC. We evaluate the increase in the transverse momentum imbalance of the observed γ + jet state, induced by the dissipation of the parton shower energy due to strong final-state interactions. Theoretical predictions to help interpret recent and upcoming experimental data are presented.

Recent results from NA61/SHINE and NA49

Preliminary results on particle spectra and fluctuations in inelastic p+p interactions measured by NA61/SHINE at the CERN SPS are presented and its future ion program is discussed. We also show results on the excitation functions of event-by-event particle ratio fluctuations in central Pb+Pb collisions from NA49 using a novel approach. We found a dependence of the results on phase space coverage which appears to explain the reported difference between measurements of NA49 and those of STAR in central Au+Au collisions.

UrQMD calculations of two-pion HBT correlations in p+p and Pb+Pb collisions at LHC energies

Two-pion Hanbury-Brown-Twiss (HBT) correlations for p+p and central Pb+Pb collisions at the Large-Hadron-Collider (LHC) energies are investigated with the ultrarelativistic quantum molecular dynamics model combined with a correlation afterburner. The transverse momentum dependence of the Pratt-Bertsch HBT radii Rlong, Rout, and Rside is extracted from a three-dimensional Gaussian fit to the correlator in the longitudinal co-moving system. In the p+p case, the dependence of correlations on the charged particle multiplicity and formation time is explored and the data allows to constrain the formation time in the string fragmentation to τf ≤ 0.8 fm/c. In the Pb+Pb case, it is found that Rout is overpredicted by nearly 50%. The LHC results are also compared to data from the STAR experiment at RHIC. For both energies we find that the calculated Rout/Rside ratio is always larger than data, indicating that the emission in the model is less explosive than observed in the data.

Dependence of Charged Particle Pseudorapidity Distributions on Centrality in Pb—Pb Collisions at √sNN = 2.76 TeV

The dependence of charged particle pseudorapidity distributions on centrality in Pb—Pb collisions at √sNN = 2.76 TeV is studied by using a multi-source thermal model (previously the two-cylinder model or the multi-source ideal gas model). The rapidity shifts of the interacting system are almost independent of the centrality class. The contribution of leading nucleons increases with the increasing centrality class (percent). The calculated results are compared and found to be in agreement with the experimental data of the ATLAS, ALICE, and CMS collaborations.

Centrality dependence of charged particle production at large transverse momentum in Pb–Pb collisions at [formula omitted

The inclusive transverse momentum (pT) distributions of primary charged particles are measured in the pseudo-rapidity range |η|<0.8 as a function of event centrality in Pb–Pb collisions at sNN=2.76 TeV with ALICE at the LHC. The data are presented in the pT range 0.15<pT<50 GeV/c for nine centrality intervals from 70–80% to 0–5%. The results in Pb–Pb are presented in terms of the nuclear modification factor RAA using a pp reference spectrum measured at the same collision energy. We observe that the suppression of high-pT particles strongly depends on event centrality. The yield is most suppressed in central collisions (0–5%) with RAA≈0.13 at pT=6–7 GeV/c. Above pT=7 GeV/c, there is a significant rise in the nuclear modification factor, which reaches RAA≈0.4 for pT>30 GeV/c. In peripheral collisions (70–80%), only moderate suppression (RAA=0.6–0.7) and a weak pT dependence is observed. The measured nuclear modification factors are compared to other measurements and model calculations.

Strange particle measurements at the CERN SPS – NA49 and NA61/SHINE experiments

Abstract Strangeness production in heavy-ion collisions has always been an important observable for probing the state of matter created in these collisions. In particular, relative strangeness production, its event-by-event fluctuations as well as their dependence on the collision energy and centrality are sensitive probes of the deconfinement phase transition. This contribution presents selected NA49 and NA61/SHINE results on strangeness production in central Pb + Pb collisions and p + p interactions in the SPS energy range. Special attention is given to data on particle ratios and ratio fluctuations as well as K ⁎ , K ¯ ⁎ resonance production.

Polaris as a guide to northern exploration: Ore textures, paragenesis and the origin of the carbonate-hosted Polaris Zn–Pb Mine, Nunavut, Canada

The Polaris Zn–Pb Mine in Nunavut, Canada was one of the largest single Mississippi Valley-type ore deposits in the world. Over 20Mt of sphalerite (ZnS) and galena (PbS) was hosted in brecciated carbonate rocks of the Upper Ordovician Thumb Mountain Formation. Three paragenetic stages are recognized: 1) early dolomite and marcasite; 2) main stage sulphide and dolomite; and 3) late calcite, marcasite and barite. Ore mineral textures range from discrete crystals to massive crystal aggregates and formed as replacements of the dolomite host rock or as fracture- and open space-filling mineralization. Zinc concentration is highest in the core of the deposit where botryoidal aggregates predominate, whereas iron is concentrated in the upper part. Observations of temperature and in situ sulphur isotope fractionation support a genetic model for the Polaris deposit in which thermochemical sulphate reduction occurred within the deposit, with locally generated hydrocarbons acting as a reducing agent. Information from the Polaris Mine indicates that hydrothermal alteration including dolomite, marcasite and barite; complex paragenesis with numerous ore textures; Th values >100°C associated with organic-rich strata; and a geochemical signature that includes in situ sulphur fractionation are effective predictors for determining which showings are prospective in the vast central Arctic Pb–Zn district.

QGP flow fluctuations and the characteristics of higher moments

The dynamical development of expanding Quark-gluon Plasma (QGP) flow is studied in a 3+1D fluid dynamical model with a globally symmetric, initial condition. We minimize fluctuations arising from complex dynamical processes at finite impact parameters and from fluctuating random initial conditions to have a conservative fluid dynamical background estimate for the statistical distributions of the thermodynamical parameters. We also avoid a phase transition in the equation of state, and we let the matter supercool during the expansion. Then central Pb + Pb collisions at \( \sqrt{s_{NN}}=2.76\) TeV are studied in an almost perfect fluid dynamical model, with azimuthally symmetric initial state generated in a dynamical flux-tube model. The general development of thermodynamical extensives are also shown for lower energies. We observe considerable deviations from a thermal equilibrium source, changing skewness and kurtosis by time depending on beam energy as a consequence of the fluid dynamical expansion arising from a least fluctuating initial state.

Net-baryon-, net-proton-, and net-charge kurtosis in heavy-ion collisions within a relativistic transport approach

We explore the potential of net-baryon, net-proton and net-charge kurtosis measurements to investigate the properties of hot and dense matter created in relativistic heavy-ion collisions. Contrary to calculations in a grand canonical ensemble we explicitly take into account exact electric and baryon charge conservation on an event-by-event basis. This drastically limits the width of baryon fluctuations. A simple model to account for this is to assume a grand-canonical distribution with a sharp cut-off at the tails. We present baseline predictions of the energy dependence of the net-baryon, net-proton and net-charge kurtosis for central ($b\leq 2.75$ fm) Pb+Pb/Au+Au collisions from $E_{lab}=2A$ GeV to $\sqrt{s_{NN}}=200$ GeV from the UrQMD model. While the net-charge kurtosis is compatible with values around zero, the net-baryon number decreases to large negative values with decreasing beam energy. The net-proton kurtosis becomes only slightly negative for low $\sqrt{s_{NN}}$.

Gluon Saturation Model with Geometric Scaling for Net-Baryon Distributions in Relativistic Heavy Ion Collisions

The net-baryon number is essentially transported by valence quarks that probe the saturation regime in the target by multiple scattering. The net-baryon distributions, nuclear stopping power and gluon saturation features in the SPS and RHIC energy regions are investigated by taking advantage of the gluon saturation model with geometric scaling. Predications are made for the net-baryon rapidity distributions, mean rapidity loss and gluon saturation features in central Pb + Pb collisions at LHC.

Net-Baryon and Net-Kaon Rapidity Distributions in Ultrarelativistic Heavy-Ion Collisions

In this paper, the gluon distribution is extracted from the KLR-AdS/CFT saturation model and used to investigate net-baryon and net-kaon rapidity distributions in ultrarelativistic heavy-ion collisions. With the same parameters of the saturation model fitting to HERA data and an χ2 analysis of the overall constant C, the theoretical results are in good agreement with RHIC data in Au+Au collisions at √s = 0.2 TeV. Then, we present the predictive results for net-baryon rapidity distributions in central Pb+Pb collisions at LHC energies of √s = 2.76, 3.94, and 5.52 TeV, and give the corresponding values of dN/dy for net-baryon at y = 0.

Formula omitted] analysis of inclusive jet and di-jet production in heavy ion reactions at the Large Hadron Collider

Jets physics in heavy ion reactions is an important new area of active research at the Relativistic Heavy Ion Collider (RHIC) and at the Large Hadron Collider (LHC) that paves the way for novel tests of QCD multi-parton dynamics in dense nuclear matter. At present, perturbative QCD calculations of hard probes in elementary nucleon–nucleon reactions can be consistently combined with the effects of the nuclear medium up to O(αs3). While such accuracy is desirable but not necessary for leading particle tomography, it is absolutely essential for the new jet observables. With this motivation, we present first results and predictions to O(αs3) for the recent LHC lead–lead (Pb+Pb) run at a center-of-mass energy of 2.76 TeV per nucleon–nucleon pair. Specifically, we focus on the suppression of the single and double inclusive jet cross sections. Our analysis includes not only final-state inelastic parton interactions in the QGP, but also initial-state cold nuclear matter effects and an estimate of the non-perturbative hadronization corrections. We demonstrate how an enhanced di-jet asymmetry in central Pb+Pb reactions at the LHC, recently measured by the ATLAS and CMS experiments, can be derived from these results. We show quantitatively that a fraction of this enhancement may be related to the ambiguity in the separation between the jet and the soft background medium and/or the diffusion of the parton shower energy away from the jet axis through collisional processes. We point to a suite of measurements that can help build a consistent picture of parton shower modification in heavy ion collisions at the LHC.

Hadronization and hadronic freeze-out in relativistic nuclear collisions

We analyze hadrochemical freeze-out in central Pb+Pb collisions at CERN SPS energies, employing the hybrid version of the Ultrarelativistic Quantum Molecular Dynamics model, which describes the transition from a hydrodynamic stage to hadrons by the Cooper-Frye mechanism, and matches to a final hadron-resonance cascade. We fit the results both before and after the cascade stage using the Statistical Model, to assess the effect of the cascade phase. We observe a strong effect on antibaryon yields except anti-Omega, resulting in a shift in T and mu_B of the freeze-out curve. We discuss indications of a similar effect in SPS and RHIC data, and propose a method to recover the bulk hadron freeze-out conditions.

Antideuteron and deuteron production in midcentral Pb+Pb collisions at 158AGeV

Production of deuterons and antideuterons was studied by the NA49 experiment in the 23.5% most central Pb+Pb collisions at the top SPS energy of $\sqrt{s_{NN}}$=17.3 GeV. Invariant yields for $\bar{d}$ and $d$ were measured as a function of centrality in the center-of-mass rapidity range $-1.2<y<-0.6$. Results for $\bar{d}(d)$ together with previously published $\bar{p}(p)$ measurements are discussed in the context of the coalescence model. The coalescence parameters $B_2$ were deduced as a function of transverse momentum $p_t$ and collision centrality.

Gluon saturation and baryon stopping in the SPS, RHIC, and LHC energy regions

A new geometrical scaling method with a gluon saturation rapidity limit is proposed to study the gluon saturation feature of the central rapidity region of relativistic nuclear collisions. The net-baryon number is essentially transported by valence quarks that probe the saturation regime in the target by multiple scattering. We take advantage of the gluon saturation model with geometric scaling of the rapidity limit to investigate net baryon distributions, nuclear stopping power and gluon saturation features in the SPS and RHIC energy regions. Predictions for net-baryon rapidity distributions, mean rapidity loss and gluon saturation feature in central Pb+Pb collisions at the LHC are made in this paper.

Report from NA49

The most recent data of NA49 on hadron production in nuclear collisions at CERN SPS energies are presented. Anomalies in the energy dependence of pion and kaon production in central Pb+Pb collisions are observed. They suggest that the onset of deconfinement is located at about 30 AGeV. Large multiplicity and transverse momentum fluctuations are measured for collisions of intermediate mass systems at 158 AGeV. The need for a new experimental programme at the CERN SPS is underlined.

Baryon Stopping Probes Partonic and Hadronic Processes

Stopping and baryon transport in central relativistic Pb + Pb and Au + Au collisions are reconsidered with the aim to find indications for the transition from hadronic to partonic processes. At energies reached at the CERN Super Proton Synchrotron ( √ sNN = 6.3– 17.3 GeV) and at RHIC (62.4 GeV) the fragmentation-peak positions as obtained from the data depend linearly on the beam rapidity and are in agreement with earlier results from a QCD-based approach that accounts for gluon saturation. No discontinuities in the netproton fragmentation peak positions occur in the expected transition region from partons to hadrons at 6–10 GeV. In contrast, the mean rapidity loss depends linearly on the beam rapidity only at high energies beyond the RHIC scale. The combination of both results offers a clue for the transition from hard partonic to soft hadronic processes in baryon stopping.

K*(892)0andK¯*(892)0production in central Pb+Pb, Si+Si, C+C, and inelasticp+pcollisions at 158AGeV

Production of the $K^{\ast}(892)^0$ and $\bar{K}^{\ast}(892)^0$ resonances was studied via their $K^+ \pi^-$ and $K^- \pi^+$ decay modes in central Pb+Pb, Si+Si, C+C and inelastic p+p collisions at 158\agev ($\sqrt{s_{NN}}$ = 17.3 GeV) with the NA49 detector at the CERN SPS. Transverse momentum and rapidity distributions were measured and total yields were estimated. The yield of $K^{\ast}$ exceeds that of $\bar{K}^{\ast}$ by about a factor of two in nucleus-nucleus reactions. The total yield ratios $< K^{\ast} >/< K^+ >$ and $< \bar{K}^{\ast} >/< K^->$ are strongly suppressed in central Pb+Pb compared to p+p, C+C and Si+Si collisions in agreement with the expected attenuation of these short-lived resonance states in the hadronic phase of the expanding fireball. The UrQMD model, although incorporating such a scenario, does not provide a quantitative description of the experimental results. The statistical hadron gas model assuming the same freeze-out parameters for stable hadrons and resonances overestimates the $< K^{\ast} >/< K >$ ratios in central Pb+Pb collisions by about a factor of 2.5.

Suppression of high pT hadrons in Pb + Pb collisions at 2.76 TeV

The nuclear modification factor RAA(pT) for large pT hadrons in central Pb + Pb collisions at TeV/n is calculated within the next-to-leading order perturbative QCD parton model with medium-modified fragmentation functions and agree well with the new data. The jet transport parameter that controls medium modification is assumed to be proportional to the initial parton density and the coefficient is fixed by the RHIC data. The charged hadron multiplicity dNch/dη = 1584 ± 80 in central Pb + Pb collisions from the ALICE experiment at the LHC is used to determine both the jet transport parameter and the initial condition for (3+1)D ideal hydrodynamic evolution of the bulk matter that is employed for the calculation of RAA(pT).

Charged particle production at large transverse momentum in Pb–Pb collisions at TeV measured with ALICE at the LHC

Transverse momentum (pT) spectra of charged particles are measured as a function of event centrality in Pb−Pb collisions at TeV with ALICE at the LHC. The spectra are compared to those measured in pp collisions at the same collision energy in terms of the nuclear modification factor RAA. The high-pT charge particle production in central Pb−Pb collisions (0–5%) is strongly suppressed by a factor ≈6 at transverse momenta pT = 6–7 GeV/c as compared to expectation from independent superposition of nucleon–nucleon collisions. Above pT = 7 GeV/c there is a significant rise in the nuclear modification factor, which reaches RAA ≈ 0.4 at pT = 50 GeV/c. The measured suppression of high-pT particles is stronger than that measured at the RHIC.