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Register a new userSystem Size and Centrality Dependence of the Electric Charge Correlations in A+A and p+p Collisions at the SPS Energies
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NA49 Collaboration
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The Balance Function analysis method was developed in order to study the long range correlations in pseudo-rapidity of charged particle. The final results on p+p, C+C, Si+Si and centrality selected Pb+Pb collisions at $sqrt{s_{NN}} = 17.2$ GeV and the preliminary data at $sqrt{s_{NN}} = 8.8$ GeV are presented. The width of the Balance Function decreases with increasing system size and centrality of the collisions. This could suggest a delayed hadronization scenario.
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Results from electric charge correlations studied with the Balance Function method in A+A collisions from 20emph{A} to 158emph{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 BFs width with centrality. This confirms the proposed sensitivity of the Balance Function analysis to the time of hadronization.
Electric charge correlations are studied with the Balance Function method for central Pb + Pb collisions at the CERN - SPS. The results on centrality selected Pb + Pb interactions at 40 and 158 AGeV are presented for the first time for two different rapidity intervals. In the mid-rapidity region a decrease of the width with increasing centrality of the collision is observed whereas in the forward rapidity region this effect vanishes. This could suggest a delayed hadronization scenario. In addition, the results from a first attempt to study the energy dependence of the Balance Function throughout the whole SPS energy range, are presented. The suitably scaled decrease of the width is approximately constant for the intermediate energies (30 to 80 AGeV) and gets stronger for the highest SPS and RHIC energies. On the other hand, both URQMD and HSD simulation results show no dependence on the collision energy.
Results are presented of a two-pion interferometry (HBT) analysis in Pb+Au collisions at 40, 80, and 158 AGeV. A detailed study of the Bertsch-Pratt HBT radius parameters has been performed as function of the mean pair transverse momentum $k_t$ and in bins of the centrality of the collision. From these results we extract model dependent information about the space-time evolution of the reaction. An investigation of the effective volume of the pion emitting system provides an important tool to study the properties of thermal pion freeze-out.
The elliptic and triangular flow coefficients ($mathrm{v_n, , n=2,3}$) measured in Pb+Pb ($sqrt{s_{_{rm NN}}} = 2.76$ TeV) and p+Pb ($sqrt{s_{_{rm NN}}} = 5.02$ TeV) collisions, are studied as a function of initial-state eccentricity ($varepsilon_n$), and dimensionless size characterized by the cube root of the mid-rapidity charged hadron multiplicity density $mathrm{left< N_{ch} right>^{1/3}}$. The results indicate that the influence of eccentricity ($mathrm{v_n} propto varepsilon_n$) observed for large $mathrm{left< N_{ch} right>}$, is superseded by the effects of viscous attenuation for small $mathrm{left< N_{ch} right>}$, irrespective of the colliding species. Strikingly similar acoustic scaling patterns of exponential viscous modulation, with a damping rate proportional to $mathrm{n^2}$ and inversely proportional to the dimensionless size, are observed for the eccentricity-scaled coefficients for the two sets of colliding species. The resulting scaling parameters suggest that, contrary to current predilections, the patterns of viscous attenuation, as well as the specific shear viscosity $left<frac{eta}{s}(text{T})right>$ for the matter created in p+Pb and Pb+Pb collisions, are comparable.
First results are presented from BNL experiment E910 on pion production and stopping in proton-Be, Cu, and Au collisions at a beam momentum of 18 GeV/c. We characterize the centrality of the collisions using the measured number of grey tracks, Ngrey,and a derived quantity, nu, the number of inelastic nucleon-nucleon scatterings suffered by the projectile during the collision. We find that for the three targets the average backward rapidity shift of the leading proton follows a common trend versus nu with the projectile losing, on average, 2 units of rapidity in the first 2-3 scatterings. The average rapidity shift increases more slowly with subsequent scatterings reaching a maximum of 2.5 units. The pi- multiplicity measured within the E910 acceptance saturates with increasing nu in p-Au collisions while the pi- multiplicity in p-Be collisions increases faster with nu than expected from the wounded-nucleon model. Comparisons of our data with the RQMD cascade model suggest that in very central p-Au collisions most of the pions are produced near zero rapidity in the lab.
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