SPS Energies Research Articles

Factorial Correlators and Oscillatory Multiplicity Moments at the CERN SPS Energy for Ring-Like and Jet-Like Events

We present an analysis of ring-like and jet-like events in terms of factorial correlations and oscillatory multiplicity moments of 32S-Ag/Br interactions at 200 A GeV. The investigation reveals that the correlated moments increase with decrease in bin-bin separation D, following the power law, which suggests the presence of an intermittent nature of self-similar dynamical fluctuations pattern for ring-like and jet-like events. The analysis further shows that the strength of the non-statistical fluctuations is larger for jet-like events than those of ring-like events and total events. However, ring-like and jet-like events are not to be consistent with the total events of the α model of intermittency. To go beyond the lower order correlation, the oscillatory multiplicity moments are used to study the higher order correlation. The ratios Hq (cumulant over factorial moments, Kq/Fq) are determined for ring-like, jet-like and the total events. The presence of few-particle short range correlation is established. It is extremely interesting to observe that the oscillations of ring-like events are different from the jet-like events and the total events. However, in almost all the cases, the simulated interactions fail to replicate the experimental results.

The non-extensivity parameter of a thermodynamical model of hadronic interactions at LHC energies

The LHC measurements above SPS and Tevatron energies give an opportunity to test predictions of the non-extensive thermodynamical picture of hadronic interaction to examine the measured transverse momentum distributions for a new interaction energy range. We determined the Tsallis model non-extensivity parameter for the hadronization process before short-lived particles decay and distort the initial p⊥ distribution. We have shown that it follows exactly the smooth rise determined at lower energies below the present LHC record. The energy dependence of the q parameter is consistent with expectations and the evidence of the asymptotic limit may be seen.

Radial flow from electromagnetic probes and signal of quark gluon plasma

A first attempt has been made to extract the evolution of radial flow from the analysis of the experimental data on electromagnetic probes experimentally measured at SPS and RHIC energies. The $p_T$ spectra of photons and dileptons measured by WA98 and NA60 collaborations respectively at CERN-SPS and the photon spectra obtained by PHENIX collaboration at BNL-RHIC have been used to constrain the theoretical models, rendering the outcome of the analysis largely model independent. We argue that the variation of the radial velocity with invariant mass is indicative of a phase transition from initially produced partons to hadrons at SPS and RHIC energies.

Λ−pfemtoscopy in collisions ofAr+KClat1.76A GeV

Results on $\Lambda$p femtoscopy are reported at the lowest energy so far. At a beam energy of 1.76A~GeV, the reaction Ar+KCl was studied with the High Acceptance Di-Electron Spectrometer (HADES) at SIS18/GSI. A high-statistics and high-purity $\Lambda$ sample was collected, allowing for the investigation of $\Lambda$ p correlations at small relative momenta. The experimental correlation function is compared to corresponding model calculations allowing the determination of the space-time extent of the $\Lambda$p emission source. The $\Lambda$p source radius is found slightly smaller than the pp correlation radius for a similar collis ion system. The present $\Lambda$p radius is significantly smaller than that found for Au+Au/Pb+Pb collisio ns in the AGS, SPS and RHIC energy domains, but larger than that observed for electroproduction from He. Taking into account all available data, we find the $\Lambda$p source radius to increase almost linearly with the number of participants to the power of one-third.

Strangeness production at SPS energies in a (3+1)-dimensional Boltzmann+hydrodynamics approach

Results on strange particle production at energies from –200 A GeV from a (3+1)-dimensional Boltzmann+hydrodynamics hybrid model are presented and compared to results from the ultra-relativistic quantum molecular dynamics transport approach and existing experimental data. The hybrid model with an intermediate hydrodynamical evolution for the hot and dense stage of the collision allows us to explore the centrality and system size dependence of (multi-strange) hyperon production in A+A collisions. The transition from a system which is not fully thermalized, such as in C+C or Si+Si collisions, to supposedly fully thermalized systems, e.g. created in Pb+Pb reactions, is investigated. The hyperon yields are enhanced in the hybrid approach due to the local equilibrium assumption and therefore in line with the experimental data. The excitation functions of strange to non-strange particle yields (e.g. K+/π+) calculated in the hybrid approach are able to reproduce the famous ‘horn’ structure at SPS energies with a purely hadronic equation of state in contrast to the pure transport approach, which failed to explain the experimental data.

K/π, K/p and p/π ratio fluctuations within the HSD transport approach

Particle number fluctuations and correlations in nucleus–nucleus collisions at SPS and RHIC energies have been studied within the hadron-string-dynamics (HSD) transport approach. Event-by-event fluctuations of kaon-to-pion, proton-to-pion and kaon-to-proton number ratios are calculated for the samples of most central collision events and compared with the available experimental data. It has been found that the HSD model can qualitatively reproduce the measured excitation function for the K/π ratio fluctuations in central Au+Au (or Pb+Pb) collisions from low SPS up to the top RHIC energy. Predictions for the K/p and p/π ratios impose a challenge for future experiments.

Study of dynamical K/π and p/π fluctuations in = 22.4 GeV Cu+Cu collisions from the STAR experiment

Results from the measurement of dynamical K/π and p/π ratio fluctuations are presented. Dynamical fluctuations in global conserved quantities such as the baryon number, strangeness or charge may be observed near a QCD critical point. The first measurement results of K/π and p/π fluctuations from Cu+Cu collisions at an energy = 22.4 GeV are shown. This energy range is close to the top SPS energy and has garnered additional interest due to the proposed QCD critical point search at RHIC. The new Cu+Cu data are compared to previous results from Au+Au collisions at the energies = 19.6, 62.4, 130 and 200 GeV, and to measurements from the NA49 experiment. The dynamical K/π fluctuation results from Cu+Cu at = 22.4 GeV are in good agreement with previous measurements from the STAR and NA49 experiments near that energy. Results are compared to theoretical predictions from several models including the HIJING and UrQMD models.

Transport model study of nuclear stopping in heavy-ion collisions over the energy range from0.09Ato160A GeV

Nuclear stopping in heavy-ion collisions over a beam energy range from SIS and AGS up to SPS is studied in the framework of the modified Ultrarelativistic Quantum Molecular Dynamics transport model, in which mean field potentials of both formed and ``preformed'' hadrons (from string fragmentation) and medium-modified nucleon-nucleon elastic cross sections are considered. It is found that nuclear stopping is influenced by both the stiffness of the equation of state and medium modifications of nucleon-nucleon cross sections at SIS energies. At high SPS energies, a two-bump structure is shown in the experimental rapidity distribution of free protons, which can be understood by considering the preformed hadron potentials.

Energy dependence of pion interferometry scales in ultra-relativistic heavy ion collisions

A study of energy behavior of the pion spectra and interferometry scales is carried out for the top SPS, RHIC and LHC energies within the hydrokinetic approach. The latter allows one to describe evolution of quark–gluon and hadron matter as well as continuous particle emission from the fluid in agreement with the underlying kinetic equations. The main mechanisms that lead to the paradoxical, at first sight, behavior of the interferometry scales, are exposed. In particular, a slow decrease and apparent saturation of Rout/Rside ratio with an energy growth happens due to a strengthening of positive correlations between space and time positions of pions emitted at the radial periphery of the system. Such en effect is a consequence of the two factors: a developing of the pre-thermal collective transverse flows and an increase of the initial energy density in the fireball.

ϕ-meson production in In–In collisions at [formula omitted]: Evidence for relics of a thermal phase

Yields and transverse mass distributions of the ϕ-mesons reconstructed in the ϕ→μ+μ− channel in In+In collisions at Elab=158A GeV are calculated within an integrated Boltzmann+hydrodynamics hybrid approach based on the Ultrarelativistic Quantum Molecular Dynamics (UrQMD) transport model with an intermediate hydrodynamic stage. The analysis is performed for various centralities and a comparison with the corresponding NA60 data in the muon channel is presented. We find that the hybrid model, that embeds an intermediate locally equilibrated phase subsequently mapped into the transport dynamics according to thermal phase-space distributions, gives a good description of the experimental data, both in yield and slope. On the contrary, the pure transport model calculations tend to fail in catching the general properties of the ϕ meson production: not only the yield, but also the slope of the mT spectra, compare poorly with the experimental observations at top SPS energies.

The horn, the hadron mass spectrum and the QCD phase diagram – the statistical model of hadron production in central nucleus-nucleus collisions

We present the status of the description of hadron production in central nucleus-nucleus collisions within the statistical model . The recent inclusion of very high-mass resonances and the sigma meson leads to an improved description of the data, in particular the energy dependence of the K+/pi+ ratio at SPS energies (``the horn''). The connection to the QCD phase diagram is discussed.

Formula omitted] elliptic flow in relativistic heavy ion collisions

The J / ψ elliptic flow in high energy nuclear collisions is calculated in a transport model. While the flow is very small at SPS and RHIC energies, it is strongly enhanced at LHC energy due to the dominance of the regeneration mechanism.

ESTIMATE OF THE MAGNETIC FIELD STRENGTH IN HEAVY-ION COLLISIONS

Magnetic fields created in the noncentral heavy-ion collision are studied within a microscopic transport model, namely the Ultrarelativistic Quantum Molecular Dynamics model (UrQMD). Simulations were carried out for different impact parameters within the SPS energy range (E lab = 10-158A GeV ) and for highest energies accessible for RHIC. We show that the magnetic field emerging in heavy-ion collisions has the magnitude of the order of [Formula: see text] for the SPS energy range and [Formula: see text] for the RHIC energies. The estimated value of the magnetic field strength for the LHC energy amounts to [Formula: see text].

Inclusive production of protons, anti-protons and neutrons in p+p collisions at 158 GeV/c beam momentum

New data on the production of protons, anti-protons and neutrons in p+p interactions are presented. The data come from a sample of 4.8 million inelastic events obtained with the NA49 detector at the CERN SPS at 158 GeV/c beam momentum. The charged baryons are identified by energy loss measurement in a large TPC tracking system. Neutrons are detected in a forward hadronic calorimeter. Inclusive invariant cross sections are obtained in intervals from 0 to 1.9 GeV/c (0 to 1.5 GeV/c) in transverse momentum and from -0.05 to 0.95 (-0.05 to 0.4) in Feynman x for protons (anti-protons), respectively. pT integrated neutron cross sections are given in the interval from 0.1 to 0.9 in Feynman x. The data are compared to a wide sample of existing results in the SPS and ISR energy ranges as well as to proton and neutron measurements from HERA and RHIC.

Particle number fluctuations and correlations in nucleus–nucleus collisions

Particle number fluctuations and correlations in nucleus–nucleus collisions at SPS and RHIC energies are studied within the statistical hadron-resonance gas model in different statistical ensembles and in the Hadron-String-Dynamics (HSD) transport approach. Event-by-event fluctuations of the proton to pion and kaon to proton number ratios are calculated in the HSD model for samples of the most central collision events and compared with the available experimental data. The role of the experimental acceptance and centrality selection is discussed.

SPS energy scan results and physics prospects at FAIR

Experimental studies of nucleus-nucleus collisions in the whole SPS energy range are reviewed. Selected topics such as statistical properties of the hadronic phase, strangeness production, fluctuations and correlations are discussed with regard to information on the onset of deconfinement and the critical point of strongly interacting matter. In spite of the very interesting results obtained in particular at the low SPS energies, additional data including rare probes such as charmed particles and di-leptons are required for a precise understanding of the underlying physics. An outlook about prospects and capabilities of upcoming experiments in this interesting energy region at RHIC, SPS, and in particular with CBM at FAIR, is given.

J/ ψ production in p-A collisions at 158 and 400 GeV: recent results from the NA60 experiment

The NA60 experiment has studied muon pair production in p-A and In-In collisions at the CERN SPS. We present new results on nuclear effects on J/ ψ production in p-A at 158 GeV, the same energy used for collecting A-A data at the SPS. We then compare nuclear effects with previous results from fixed target experiments, and with the results obtained from a p-A data sample taken by NA60 at 400 GeV. Based on the 158 GeV results, we calculate the expected J/ ψ suppression in In-In and Pb-Pb collisions due to cold nuclear matter effects, and we extract a new estimate for the anomalous suppression at SPS energies. Finally, we show preliminary results on J/ ψ polarization in p-A collisions.

Investigating Sources of Angular Correlations at High [formula omitted] in Nucleon–Nucleon and Nucleus–Nucleus Collisions at the CERN SPS

Angular correlations of high- p T hadrons can serve as a probe of interactions of partons with the dense medium produced in high-energy heavy-ion collisions but other effects may also be important at SPS energies. To study the various contributions, NA49 has performed an energy and system-size scan of two-particle azimuthal correlations in Pb+ Pb, Si+ Si and p+ p collisions at s N N = 17.3 GeV , as well as central Pb+ Pb collisions at 6.3, 7.6, 8.8 and 12.3 GeV. These results were compared to UrQMD simulations. NA49 results show a flattened away side of C 2 ( Δ ϕ ) in central Pb+ Pb ( Au+ Au) collisions which depends weakly on collision energy even for low SPS energies. This is at odds with the standard scenario of parton energy loss. On the other hand, the near-side peak amplitude drops visibly with decreasing collision energy, turning into a depletion below s N N = 8.8 GeV . UrQMD describes the correlation functions on the away side but disagrees on the near side.

A Reanalysis of Single Photon Data at CERN SPS

We reanalyze the WA98 single photon data [M. M. Aggarwal et al. [WA98 Collaboration], Phys. Rev. Lett. 85, 3595 (2000)] at CERN SPS by incorporating several recent developments in the study of prompt and thermal photon production from relativistic heavy ion collisions [R. Chatterjee, D. K. Srivastava, and S. Jeon, Phys. Rev. C 79, 034906 (2009)]. Isospin and shadowing corrected NLO pQCD, along with an optimized scale for factorization, fragmentation and renormalization are considered for prompt photon production. Photons from thermal medium are estimated by considering a boost invariant azimuthally anisotropic hydrodynamic expansion of the plasma along with a well tested equation of state and initial conditions. A quantitative explanation of the data is obtained by combining κ× prompt with thermal photons, where κ is an overall scaling factor. We show that, elliptic flow of thermal photons can play a crucial role to distinguish between the ‘with’ and ‘without’ phase transition scenarios at SPS energy.

A transport calculation with an embedded (3+1)d hydrodynamic evolution: Elliptic flow as a function of transverse momentum at SPS energies

The transverse momentum dependence of elliptic flow of pions calculated in a full (3+1)d Boltzmann approach with an intermediate hydrodynamic stage for heavy ion reactions for CERN-SPS energies is discussed in the context of the experimental data. At higher SPS energies, where the pure transport calculation cannot account for the high elliptic flow values, the smaller mean free path in the hydrodynamic evolution leads to larger elliptic flow. Due to the more realistic initial conditions and the incorporated hadronic rescattering the results are in line with the experimental data. Within this integrated dynamical approach different equations of state are applied without adjusting the initial state and the freeze-out conditions. We employ a hadron gas equation of state to investigate the differences in the dynamics and viscosity effects, a chiral equation of state with a moderate first order phase transition and a critical endpoint and a bag model equation of state with a large latent heat. The elliptic flow results are shown to be rather insensitive to changes in the equation of state.