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Register a new userRapidity and energy dependence of the electric charge correlations in A+A collisions at the SPS energies
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NA49 Collaboration
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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.
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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.
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.
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 STAR Collaboration at RHIC presents a systematic study of high transverse momentum charged di-hadron correlations at small azimuthal pair separation dphino, in d+Au and central Au+Au collisions at $rts = 200$ GeV. Significant correlated yield for pairs with large longitudinal separation deta is observed in central Au+Au, in contrast to d+Au collisions. The associated yield distribution in detano$times$dphi can be decomposed into a narrow jet-like peak at small angular separation which has a similar shape to that found in d+Au collisions, and a component which is narrow in dphi and textcolor{black}{depends only weakly on} $deta$, the ridge. Using two systematically independent analyses, textcolor{black}{finite ridge yield} is found to persist for trigger $pt > 6$ GeVc, indicating that it is correlated with jet production. The transverse momentum spectrum of hadrons comprising the ridge is found to be similar to that of bulk particle production in the measured range ($2 < pt < 4 GeVc$).
New measurements of directed flow for charged hadrons, characterized by the Fourier coefficient vone, are presented for transverse momenta $mathrm{p_T}$, and centrality intervals in Au+Au collisions recorded by the STAR experiment for the center-of-mass energy range $mathrm{sqrt{s_{_{NN}}}} = 7.7 - 200$ GeV. The measurements underscore the importance of momentum conservation and the characteristic dependencies on $mathrm{sqrt{s_{_{NN}}}}$, centrality and $mathrm{p_T}$ are consistent with the expectations of geometric fluctuations generated in the initial stages of the collision, acting in concert with a hydrodynamic-like expansion. The centrality and $mathrm{p_T}$ dependencies of $mathrm{v^{even}_{1}}$, as well as an observed similarity between its excitation function and that for $mathrm{v_3}$, could serve as constraints for initial-state models. The $mathrm{v^{even}_{1}}$ excitation function could also provide an important supplement to the flow measurements employed for precision extraction of the temperature dependence of the specific shear viscosity.
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