SPS Energies Research Articles

Evidence for non-gaussian tail in 3-dimensional pion emission source at SPS

The NA49 experiment at CERN SPS has acquired a huge data set of Pb+Pb events over a broad range of energy and centrality during the last several years. This high statistics data set, coupled with a state-of-the-art analysis technique, allows for the first model-independent extraction and energy scan of 3D emission sources for pion pairs at SPS energies. These 3D pion emission sources provide new insights into the nature of a long-range source previously reported by PHENIX at RHIC. Preliminary results indicate that the pion source function is essentially Gaussian from 20 AGeV to 80 AGeV but it displays significant non-Gaussian tails at 158 AGeV.

Rapidity-dependent spectra from a single-freeze-out model of relativistic heavy-ion collisions

An extension of the single-freeze-out model with thermal and geometric parameters dependent on the spatial rapidity, ${\ensuremath{\alpha}}_{\ensuremath{\parallel}}$, is used to describe the rapidity and transverse-momentum spectra of pions, kaons, protons, and antiprotons measured at the Relativistic Heavy Ion Collider at $\sqrt{{s}_{\mathit{NN}}}=200\phantom{\rule{0.3em}{0ex}}\mathrm{GeV}$ by the BRAHMS Collaboration. $\mathrm{THERMINATOR}$ is used to perform the necessary simulation, which includes all resonance decays. The result of the fit to the rapidity spectra in the range of the BRAHMS data is the expected growth of the baryon and strange chemical potentials with the magnitude of ${\ensuremath{\alpha}}_{\ensuremath{\parallel}}$, whereas the freeze-out temperature is kept fixed. The value of the baryon chemical potential at ${\ensuremath{\alpha}}_{\ensuremath{\parallel}}~3$, which is the relevant region for particles detected at the BRAHMS forward rapidity $y~3$, is about $200\phantom{\rule{0.3em}{0ex}}\mathrm{GeV}$, i.e., lies in the range of the values obtained for the highest SPS energy. The chosen geometry of the fireball has a decreasing transverse size as the magnitude of ${\ensuremath{\alpha}}_{\ensuremath{\parallel}}$ is increased, which also corresponds to decreasing transverse flow. This feature is verified by reproducing the transverse momentum spectra of pions and kaons at various rapidities. The strange chemical potential obtained from the fit to the ${K}^{+}/{K}^{\ensuremath{-}}$ ratio is such that the local strangeness density in the fireball is compatible with zero. The resulting rapidity spectra of net protons are described qualitatively in the model. As a result of the study, the knowledge of the ``topography'' of the fireball is achieved, making other calculations possible. As an example, we give predictions for the rapidity spectra of hyperons.

Statistical hadronization of heavy quarks in ultra-relativistic nucleus–nucleus collisions

We present new results on the statistical hadronization of heavy quarks at SPS, RHIC and LHC energies. Several new aspects are considered, among them a separation of the collision geometry into a “core” and a “corona” part and an estimate of the annihilation rate of charm quark in a hot plasma, together with a critical assessment of its influence on the results. For RHIC energies we investigate the centrality dependence of J / ψ production focusing on the model results for different values of the charm production cross section, including its theoretical and experimental uncertainty. We also study, within this model, the rapidity dependence of the J / ψ yield. Recent RHIC data from the PHENIX experiment are well reproduced. At LHC energy, we update our model predictions for the centrality dependence of the J / ψ yield and investigate as well the rapidity dependence. We also discuss the transverse momentum distributions of J / ψ mesons expected from the model and provide predictions for a range of values of the expansion velocity at chemical freeze-out. Finally, we extend the model to predict ϒ yields in Pb + Pb collisions at LHC energy.

Charmonium dynamics in nucleus–nucleus collisions at SPS and FAIR energies

Charmonium production and suppression in In + In and Pb + Pb reactions at SPS energies is investigated with the HSD transport approach within the ‘hadronic comover model’ as well as the ‘QGP threshold scenario’. The results of the transport calculations for J / Ψ suppression and the Ψ ′ to J / Ψ ratio are compared with the recent data of the NA50 and NA60 Collaborations. We find that the comover absorption model—with a single parameter | M 0 | 2 for the matrix element squared for charmonium-meson dissociation—performs best with respect to all data sets. The ‘threshold scenario’—within different assumptions for the melting energy densities—yields a reasonable suppression for J / Ψ but fails in reproducing the Ψ ′ to J / Ψ ratio for Pb + Pb at 158 A GeV . Predictions for Au + Au reactions are presented for a bombarding energy of 25 A GeV in the different scenarios which will allow for a clear distinction between the models from the experimental side at the future FAIR facility.

DIRECT PHOTON PRODUCTION IN pp AND $p {\bar p}$ COLLISIONS AT HIGH ENERGIES

The invariant cross sections for direct photon production in hadron-hadron collisions are calculated for several initial energies (SPS, ISR, [Formula: see text], RHIC, Tevatron, LHC) including initial parton transverse momenta within the formalism of unintegrated parton distributions (UPDF). Kwieciński UPDFs provide very good description of all world data, especially at SPS and ISR energies. Inclusion of the QCD evolution effects and especially their effect on initial parton transverse momenta allowed to solve the long-standing problem of understanding the low energy and low transverse momentum data.

Formula omitted] Production Relative to the Reaction Plane in Pb-Pb Collisions at 158 GeV/n

J / ψ production in Pb-Pb collisions at 158 A GeV/c as a function of the J / ψ azimuthal angle relative to the reaction plane has been studied for the first time. Different c c ¯ absorption mechanisms are expected to introduce anisotropy in the J / ψ azimuthal distributions at SPS energies. The present analysis has been performed on the data sample collected in year 2000 by the NA50 Collaboration. The reaction plane is estimated from the azimuthal distribution of the neutral transverse energy seen in the electromagnetic calorimeter. The results of this study are presented here expressed as number of J / ψ 's in different bins of azimuthal angle relative to the reaction plane as well as Fourier coefficients v 1 ′ and v 2 ′ (uncorrected for event plane resolution).

Two Particle Azimuthal Correlations at High Transverse Momentum in Pb-Au at 158 AGeV/c

The analysis of two-particle azimuthal angular correlations at high transverse momentum from Pb+Au collisions at 158 AGeV/c at SPS reveals substantial modifications of the away-side peak as compared to the distributions from p+p reactions. The data recorded with the CERES Time-Projection Chamber, which provides excellent tracking efficiency, suggest that the observed modification of the back-to-back structure implies significant re-interactions of the scattered partons within the medium. These findings at top SPS energy show features qualitatively similar to the results obtained at RHIC from Au+Au collisions at s = 200 GeV / c and Cu+Cu collisions at s = 62 GeV / c [S.S. Adler et al., PHENIX Collaboration, arXiv:nucl-ex/0507004, M. McCumber and J. Frantz, arXiv:nucl-ex/0511048]. We present the centrality and charge dependent conditional di-jet yields for similar transverse momentum windows as investigated by the RHIC experiments [S.S. Adler et al., PHENIX Collaboration, arXiv:nucl-ex/0507004, M. McCumber and J. Frantz, arXiv:nucl-ex/0511048, J. Adams et al., STAR Collaboration, Phys. Rev. Lett. 95 (2005) 152301, arXiv:nucl-ex/0501016].

High pT Measurements at the CERN SPS

The current experimental situation concerning high p t observables at the CERN SPS is reviewed. Recent data from the NA45, NA49 and NA57 collaborations are discussed and compared to earlier measurements by WA98 and NA45 at the same center-of-mass energies, as well as to measurements at the higher energies by the RHIC experiments. The observables include new p+p, A+A spectra, nuclear modification factors ( R AA, R CP), two particle azimuthal correlations, and baryon to meson ratios at moderately high p t. Generally, the interpretation of the SPS data suffers from the lack of reliable baseline measurements (p+p and p+A). However, the overall picture that is emerging suggests that already at SPS energies medium effects similar to those observed at RHIC are present.

Transport model analysis of the transverse momentum and rapidity dependence of pion interferometry at SPS energies

Based on the UrQMD transport model, the transverse momentum and the rapidity dependence of the Hanbury–Brown–Twiss (HBT) radii RL, RO, RS as well as the cross term ROL at SPS energies are investigated and compared with the experimental NA49 and CERES data. The rapidity dependence of RL, RO, RS is weak while ROL is significantly increased at large rapidities and small transverse momenta. The HBT ‘lifetime’ issue (the phenomenon that the calculated value is larger than the correspondingly extracted experimental data) is also present at SPS energies.

Baryon number and electric charge fluctuations in Pb+Pb collisions at relativistic energies

Event-by-event fluctuations of the net baryon number and electric charge in nucleus-nucleus collisions are studied in Pb+Pb at SPS energies within the Hadron-String Dynamics (HSD) transport model. We reveal an important role of the fluctuations in the number of target nucleon participants. They strongly influence all measured fluctuations even in the samples of events with rather rigid centrality trigger. This fact can be used to check different scenarios of nucleus-nucleus collisions by measuring the multiplicity fluctuations as a function of collision centrality in fixed kinematical regions of the projectile and target hemispheres. The HSD results for the event-by-event fluctuations of electric charge in central collisions at $20A,30A,40A,80A$, and $158A$ GeV are in a good agreement with the NA49 experimental data and considerably larger than expected in a quark-gluon plasma. This demonstrates that the distortions of the initial fluctuations by the hadronization phase and, in particular, by the final resonance decays dominate the observable fluctuations.

D and J/ψ Production from Deconfined Matter in Relativistic Heavy Ion Collisions

We study the production of the D and J/ψ mesons from deconfined quark matter at CERN SPS energy. Using the MICOR microscopical coalescence model we determine the transverse momentum spectra of these charm mesons. We predict the slopes of their transverse momentum spectra in Pb+Pb collision at 158 GeV/nucleon beam energy.

What can we learn from the energy dependence of strangeness production at RHIC?

Measurements of strange and multi-strange particles with the STAR detector at center of mass energies per nucleon pair ( $\sqrt{s_{NN}}$ ) of 62.4 GeV and 200 GeV in ultra-relativistic Au+Au collisions at RHIC are presented. The results are discussed in order to get insight into chemical and dynamical properties of the created medium. The former are obtained by comparison of transverse momentum (pT) integrated observables such as yields and particle ratios to statistical models, while the latter use pT dependent quantities such as single particle spectra and elliptic flow measurements to challenge hydrodynamical model calculations and parameterizations. The discussion is orientated towards the energy dependence of these properties by confronting the results at the different RHIC energies but also with the lower SPS energies.

Effects of multiple scattering in cold nuclear matter on J/ψ suppression and 〈pT2〉 in heavy ion collisions

Coherent multiple scatterings of c c ¯ quark pairs in the environment of heavy ion collisions have been used in a previous work by Qiu et al. [J. Qiu, J.P. Vary, X. Zhang, Phys. Rev. Lett. 88 (2002) 232301; J. Qiu, J.P. Vary, X. Zhang, Nucl. Phys. A 698 (2002) 571, nucl-th/0106040] to study J / ψ suppression. That model suggests that heavy quark re-scatterings in a cold nuclear medium can completely explain the centrality dependence of the observed J / ψ suppression in Pb + Pb collisions at the SPS [M.C. Abreu, et al., NA50 Collaboration, Phys. Lett. B 521 (2001) 195]. Their calculations also revealed significant differences under the assumptions of a color singlet or color octet production mechanism. A more recent analytic calculation [H. Fujii, Phys. Rev. C 67 (2003) 031901], which includes incoherent final-state re-scatterings with explicit momentum transfer fluctuations in three dimensions, indicates much less suppression and little sensitivity to the production mechanism. In this Letter, we study simultaneously both the J / ψ suppression and p T modifications, at SPS and RHIC energies. We mainly focus on incoherent momentum transfer fluctuations in two dimensions, which is more appropriate for the heavy-ion collision kinematics. Our analytic and Monte Carlo calculations reinforce the analytic results in [H. Fujii, Phys. Rev. C 67 (2003) 031901]. Additionally, we find that the experimental J / ψ suppression and 〈 p T 2 〉 from nucleus–nucleus collisions at the SPS or RHIC cannot simultaneously be described in this incoherent multiple scattering framework for any value of the fluctuation strength parameter 〈 k T 2 〉 .

Studying the energy dependence of elliptic and directed flow within a relativistic transport approach

The energy excitation functions of directed flow (v1) and elliptic flow (v2) from Ebeam=90 A MeV to Ecm=200 A GeV are explored within the UrQMD framework and discussed in the context of the available data. The radial and the elliptic flow of the particles produced in a relativistic heavy-ion collision are intimately connected to the pressure and its gradients in the early stage of the reaction. Therefore, these observables should also be sensitive to changes in the equation of state. To prove this connection, the temporal evolution of the pressure, pressure gradients and elliptic flow are shown. For the flow excitation functions it is found that, in the energy regime below Ebeam≤10 A GeV, the inclusion of nuclear potentials is necessary to describe the data. Above 40 A GeV beam energy, the UrQMD model starts to underestimate the elliptic flow. Around the same energy the slope of the rapidity spectra of the proton directed flow develops negative values. This effect is known as the third flow component (“antiflow”) and cannot be reproduced by the transport model. The difference between the data and the UrQMD model can possibly be explained by assuming a phase transition from hadron gas to quark–gluon plasma around Elab=40 A GeV. This would be consistent with the model calculations, indicating a transition from hadronic matter to “string matter” in this energy range. Thus, we speculate that the missing pressure might be generated by strong interactions in the early pre-hadronic/partonic phase of central Au + Au (Pb + Pb) collisions already at lower SPS energies.

Event-by-event fluctuation in K/π ratio at RHIC

We present the first results from the measurement of event-by-event fluctuation in the K/π ratio for Au+Au collisions at , 130 and 200 GeV at the Relativistic Heavy Ion Collider. We observe that the strength of dynamical fluctuation decreases with increasing centrality for all centre-of-mass energies of the collisions. The excitation function for the dynamical fluctuation in the event-by-event K/π ratio for central collisions will be presented. The results will be compared with fluctuations obtained at SPS energies.

Charmonia production at SPS energies

J/ψ suppression in ultrarelativistic heavy-ion collisions has been predicted to be a signature of the transition from normal hadronic matter to a quark–gluon plasma. Several experiments have been devoted to charmonia production at the CERN/SPS: NA38, NA50 and NA60. All the results obtained so far in proton–proton, proton–nucleus and nucleus–nucleus collisions are summarized. Particular focus is dedicated to the proton–nucleus studies (and comparison to other experiments) needed to determine the normal nuclear absorption, knowledge of which is essential in order to extract the anomalous suppression obtained in nucleus–nucleus collisions.

Strangeness production at SPS energies from NA49

We present results on strange particle production from experiment NA49 in central and minimum bias Pb+Pb collisions in the beam energy range 20–158 A GeV. New results on Ξ production in central Pb+Pb collisions and on Λ, Ξ production in minimum bias collisions are shown. Transverse mass spectra and rapidity distributions of strange particles at different energies are compared. The energy dependence of the particle yields and ratios is discussed. NA49 measurements of the Λ and Ξ enhancement factors are shown for the first time.

Evidence from identified particles for active quark and gluon degrees of freedom

Measurements of intermediate pT (1.5 < pT < 5.0 GeV/c) identified particle distributions in heavy-ion collisions at SPS and RHIC energies display striking dependences on the number of constituent quarks in the corresponding hadron. One finds that elliptic flow at intermediate pT follows a constituent quark scaling law as predicted by models of hadron formation through coalescence. In addition, baryon production is also found to increase with event multiplicity much faster than meson production. The rate of increase is similar for all baryons and seemingly independent of mass. This indicates that the number of constituent quarks determines the multiplicity dependence of identified hadron production at intermediate pT. We review these measurements and interpret the experimental findings.

Results from NA57

The NA57 experiment has measured strange baryon and antibaryon production in Pb–Pb collisions at 40 A GeV/c and 158 A GeV/c beam momenta. Expansion dynamics has been investigated; transverse dynamics using blast wave fits, longitudinal dynamics obtained from rapidity distributions. HBT analysis gives results compatible with those obtained by the other methods. Similar values are observed for the transverse and longitudinal flow parameters. Particles carrying all levels of strangeness have been studied. Enhancement factors increase with the strangeness content of the particle, reaching above 20 for Ωs in central collisions. Enhancement values are strikingly similar for SPS and RHIC energies. Central-to-peripheral nuclear modification factors have been measured. The baryon/meson pattern displays similarities with that observed at RHIC.

Indications of the onset of deconfinement at low SPS energies

We present evidence for an onset of deconfinement at low SPS energies obtained by the NA49 experiment in central Pb+Pb collisions in an energy scan from 20 to 158 AGeV. As indicators for such an onset, the independence of mean mt on beam energy, a sharp peak in the strangeness-to-pion ratio and dynamical fluctuations in the event-by-event K/π ratio are discussed.