SPS Energies Research Articles

Micro-canonical pentaquark production in [formula omitted] and pp collisions

The existence of pentaquarks became questionable, because the Θ + peak is observed in some p + p collisions, but not in e + e − annihilations. People think initial baryon number is necessary to produce pentaquarks. In this paper, we estimate and compare the production of Θ + ( 1540 ) and Ξ − − ( 1860 ) in e + e − and pp collisions at different energies using Fermi statistical model as originally proposed in its microcanonical form. We find that both Θ + ( 1540 ) and Ξ − − ( 1860 ) yield more in e + e − at LEP energies than in pp collisions at SPS and RHIC energies, if pentaquarks do exist. Initial baryon number is not necessary for pentaquark production.

Void analysis of pions and its multiplicity dependence at Dubna and SPS energies

This paper investigates the multiplicity dependence of void probability scaling of fluctuation of pions (i.e. classifying fluctuation patterns of pions on the basis of their number, ns) for a wide range of projectile energy from 4.5 to 200 A GeV. The study reveals that void probability scaling is dependent on pion multiplicity at all energies. At comparatively lower projectile energy the scaling becomes poor with increase in average multiplicity, however at high energy the scaling is found to be independent of multiplicity.

Dynamical freeze-out in three-fluid hydrodynamics

Freeze-out procedure accepted in the model of 3-fluid dynamics (3FD) is analyzed. This procedure is formulated in terms of drain terms in hydrodynamic equations. Dynamics of the freeze-out is illustrated by 1-dimensional simulations. It is demonstrated that the resulting freeze-out reveals a nontrivial dynamics depending on initial conditions in the expanding ``fireball''. The freeze-out front is not defined just ``geometrically'' on the condition of the freeze-out criterion met but rather is a subject the fluid evolution. It competes with the fluid flow and not always reaches the place where the freeze-out criterion is met. Dynamics of the freeze-out in 3D simulations is analyzed. It is demonstrated that the late stage of central nuclear collisions at top SPS energies is of the form of three (two baryon-rich and one baryon-free) fireballs separated from each other.

RECENT FEMTOSCOPY RESULTS FROM NA49: EVIDENCE FOR NON-GAUSSIAN TAIL IN 3-D 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. The new results indicate that the pion source exhibits significant non-Gaussian tails in the longitudinal direction at 40 and 158 AGeV and in the outward direction at 158 AGeV. The ratio of the spatial extent in the outward compared to the sideward directions indicates a finite non-zero duration for pion emission. The spatial extent in the longidutional direction suggests that pions decouple from the system at ~10 fm/c at 158 AGeV and at ~8 fm/c at 40 AGeV.

LEE-YANG CIRCLE ANALYSIS FOR GENERALIZED MULTIPLICITY DISTRIBUTION

In this work, we study the Lee–Yang solutions of multiplicty distribution (MD) generating function for SPS energies range data. In particular, weighted generalized multiplicity distribution (GMD) function is used to describe the existing shoulder structure in the experimental data. Our study suggest that the existence of the shoulder structure in the KNO plot is accompanied by the forming of an ear structure in the Lee–Yang zeros plot. Prediction for the LHC energy range is also made.

Charmonium dynamics in Au+Au collisions ats=200GeV

The formation and suppression dynamics of $J/\ensuremath{\Psi},{\ensuremath{\chi}}_{c}$, and ${\ensuremath{\Psi}}^{'}$ mesons is studied within the hadron-string-dynamics (HSD) transport approach for Au+Au reactions at the top energy currently available at the BNL Relativistic Heavy Ion Collider (RHIC) of $\sqrt{s}=200$ GeV. Two prominent models, which have been discussed for more than a decade, are incorporated, i.e., the hadronic comover absorption and reformation model as well as the quark-gluon plasma (QGP) threshold scenario, and compared with available experimental data. Our studies demonstrate that both scenarios, which are compatible with experimental observation at SPS energies, fail severely at RHIC energies. This combined analysis, together with the underestimation of charm elliptic flow, proves that the dynamics of $c,\overline{c}$ quarks are dominated by partonic interactions in the strong QGP and can be neither modeled by hadronic interactions nor described appropriately by color screening alone.

On multiplicity and transverse-momentum correlations in collisions of ultrarelativistic ions

A theoretical description of long-range correlations between observables measured in separated rapidity intervals for nucleus-nucleus collisions at high energies is given. With allowance for an actual distribution of nucleons in nuclei, the ptn and nn correlation functions and respective correlation coefficients are calculated at various values of the impact parameter. It is shown that fluctuations of the impact parameter at a level of 1 fm, which are irremovable in experiments, affect the correlation coefficients substantially. It is shown that ptn correlations become much more pronounced upon going over from the SPS to the RHIC and LHC energies. The ptn and nn correlation functions calculated for the minimumbias case (that is, without selection in centrality) are compared with experimental data on PbPb collisions at the SPS energies.

Long-range correlations between transverse momenta of charged particles produced in relativistic nucleus-nucleus collisions

A theoretical description of long-range correlations between observables measured in separated rapidity intervals is given. It is shown that, in contrast to two-particle correlations-specifically, correlations between the transverse momenta of charged particles produced in different rapidity intervals-the correlations between the average transverse momenta of all particles in an event that are produced in two rapidity intervals do not tend to zero with increasing number of strings. The energy dependence of the correlations is analyzed. At a fixed degree of collision centrality, the correlations are found to grow upon going over from the SPS energy to the RHIC and LHC energies; they are compared with correlations of the p{sub t}n type. Correlation functions calculated for the minimum-bias case (that is, without any selection in the degree of collision centrality) are contrasted against relevant experimental data on PbPb collisions at the SPS energies.

Low-energy thermal photons from meson–meson bremsstrahlung

Within an effective hadronic model including electromagnetic interactions via a U em ( 1 ) gauge, we re-investigate photon bremsstrahlung from a hot hadronic gas as expected to be formed in relativistic heavy-ion collisions at SPS energies. We calculate photon emission from the reactions π π → π π γ and π K → π K γ by an explicit (numerical) evaluation of the multi-dimensional phase space integral. This, in particular, allows to avoid the commonly employed soft photon approximation (SPA), as well as to incorporate final-state thermal enhancement factors. Both improvements are shown to result in an appreciable increase of the photon production rate over previous hadronic calculations. Upon convolution over a thermal fireball we find an improvement in the description of recent low transverse-momentum WA98 data at SPS. The influence of both Landau–Pomeranchuk–Migdal and in-medium effects on “ σ” and ρ-meson exchanges are briefly discussed.

Recent femtoscopy results from NA49: evidence for a non-Gaussian tail in the 3-dimensional two-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 two-pion emission sources provide new insights into the nature of a long-range source previously reported by PHENIX at RHIC. The new results indicate that the two-pion source function is essentially Gaussian from 20 AGeV to 80 AGeV but it displays significant non-Gaussian tails at 158 AGeV.

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.

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

The multiplicity distributions of hadrons produced in central nucleus-nucleus collisions are studied within the hadron-resonance gas model in the large-volume limit. The microscopic correlator method is used to enforce conservation of three charges---baryon number, electric charge, and strangeness---in the canonical ensemble. In addition, in the microcanonical ensemble energy conservation is included. An analytical method is used to account for resonance decays. The multiplicity distributions and the scaled variances for negatively, positively, and all charged hadrons are calculated along the chemical freeze-out line of central $\mathrm{Pb}+\mathrm{Pb}$ ($\mathrm{Au}+\mathrm{Au}$) collisions from GSI Schwerionen Synchrotron to CERN Large Hadron Collider energies. Predictions obtained within different statistical ensembles are compared with the preliminary NA49 experimental results on central $\mathrm{Pb}+\mathrm{Pb}$ collisions in the SPS energy range. The measured fluctuations are significantly narrower than the Poisson ones and clearly favor expectations for the microcanonical ensemble. Thus this is the first observation of the recently predicted suppression of the multiplicity fluctuations in relativistic gases in the thermodynamical limit owing to conservation laws.

A STATISTICAL MODEL ANALYSIS OF K/π FLUCTUATIONS IN HEAVY ION COLLISIONS

We briefly describe two statistical hadronization models, based respectively on the presence and absence of light quark chemical equilibrium, used to analyze particle yields in heavy ion collisions. We then try to distinguish between these models using K/π fluctuations data. We find that while the non-equilibrium model provides an acceptable description of fluctuations at top SPS and RHIC energies, both models considerably under-estimate fluctuations at low SPS energies.

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. The new results indicate that the pion source displays significant non-Gaussian tails in the longitudinal direction at 40 and 158 AGeV and in the outward direction at 158 AGeV.

DOES THE CRONIN PEAK DISAPPEAR AT LHC ENERGIES?

In this work we compare the nuclear modification factors in proton (deuteron) — nucleus collisions at CERN SPS, FNAL and RHIC energies in a wide pT range. In these experiments the nuclear modification factor has shown an enhancement at pT ≈ 4 GeV /c. The height of this “Cronin peak” depends on the c.m. energy of the collision, as it is subject to stronger shadowing at higher energies. One of the aims of this contribution is to analyze the shadowing phenomenon at lower (2 GeV / c ≲ pT ≲ 4 GeV / c ) and intermediate (4 GeV / c ≲ pT ≲ 8 GeV / c ) transverse momentum. Different shadowing parametrizations are considered and the obtained Cronin peaks are investigated at RHIC and LHC energies.

Evidence for non-Gaussian tail in a three-dimensional pion emission source at SPS

The NA49 experiment has acquired a huge data set of Pb+Pb events over a broad range of energy at CERN SPS. This high statistics data set, coupled with a state-of-the-art analysis technique, allows for the first model-independent extraction of 3D two-pion emission sources at SPS energies. These sources provide new insights into the nature of a long-range source previously reported by PHENIX at RHIC. The new results indicate that the pion source displays significant non-Gaussian tails in the longitudinal direction at 40 and 158 A GeV and in the outward direction at 158 A GeV.

The effect of the spectator charge on the charged pion spectra in peripheral ultrarelativistic heavy-ion collisions

We estimate the electromagnetic effect of the spectator charge on the momentum spectra of charged pions produced in peripheral Pb+Pb collisions at SPS energies. We find a large effect which results in strongly varying structures in the xF dependence of the π+/π− ratio, especially at low transverse momenta where a deep valley in the above ratio is predicted at xF∼ 0.15–0.20. The effect depends on initial conditions. Thus, it provides new information on the space and time evolution of the non-perturbative pion creation process.

Cold nuclear matter effects on J/ψ and ψ′ production

We present an overview of cold nuclear matter effects on charmonium production at SPS and RHIC energies, including nuclear effects on the parton distribution functions and final-state nuclear absorption. A good understanding of these effects, present in proton–nucleus (p + A) and d+Au data, is crucial to establish a robust reference baseline with respect to which the nucleus–nucleus (A + A) data can be compared when searching for modifications signalling new physics.

PSEUDORAPIDITY DISTRIBUTION OF SHOWER PARTICLES IN 28Si-EMULSION INTERACTIONS AT 4.5 A GeV/c PER NUCLEON

Pseudorapidity density distributions of relativistic charged particles produced in 28 Si -induced interactions in nuclear emulsion at JINR Synchrophasotron have been measured. The particle distributions in minimum-bias events exhibit a pronounced forward asymmetry with respect to the peak in the pseudorapidity spectra. The effect of intranuclear collisions on the mean pseudorapidity distribution versus the shower particles (ns) or the grey particles is discussed and compared with similar studies of 28 Si and 32 S at BNL AGS and CERN SPS energies respectively.

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.