pi-/pi+ ratio in heavy ions collisions: Coulomb effect or chemical equilibration?

We calculate the pi-/pi+ ratio for Pb+Pb at CERN/SPS energies and for Au+Au at BNL/AGS energies using a (3+1) dimensional hydrodynamical model. Without consideration of Coulomb effect an enhancement of this ratio at low mt is found compatible with that observed in these experiments. Our calculations are based on previous (3+1) dimensional hydrodynamical simulations (HYLANDER), which described many other aspects of experimental data. In this model the observed enhancement is a consequence of baryon and strangeness conservation and of chemical equilibration of the system and is caused by the decay of produced hyperons, which leads to a difference in the total number of positive and negative pions as well. Based on the same approach, we also present results for the pi-/pi+ ratio for S+S (CERN/SPS) collisions, where we find a similar effect. The absence of the enhancement of the pi-/pi+ ratio in the S+S data presented by the NA44 Collaboration, if confirmed, could indicate that chemical equilibration has not yet been estabilished in this reaction.Comment: 8 pages and 2 figures, submmited to Phys. Lett. B. This reviewed version (Nov.29,1996) contains more details about the model simulated efficiency considering the experimental detection conditions. Other small modifications were mad

Two- and Three-Pion Interferometry for a Nonchaotic Source in Relativistic Nuclear Collisions

Two- and three-pion correlation functions are investigated for a source that is not fully chaotic. Various models are examined to describe the source. The chaoticity and weight factor are evaluated in each model as measures of the strength of correlations and compared to experimental results. A new measure of three-pion correlation is also suggested.Comment: 19 pages, 6 figure

All-graphene edge contacts: Electrical resistance of graphene T-junctions

Using ab-initio methods we investigate the possibility of three-terminal graphene "T-junction" devices and show that these all-graphene edge contacts are energetically feasible when the 1D interface itself is free from foreign atoms. We examine the energetics of various junction structures as a function of the atomic scale geometry. Three-terminal equilibrium Green's functions are used to determine the transmission spectrum and contact resistance of the system. We find that the most symmetric structures have a significant binding energy, and we determine the contact resistances in the junction to be in the range of 1-10 kOhm which is comparable to the best contact resistance reported for edge-contacted graphene-metal contacts. We conclude that conducting all-carbon T-junctions should be feasible

Dynamics of Ultra-Relativistic Nuclear Collisions with Heavy Beams: An Experimental Overview

We review, from an experimental point of view, the current status of ultra-relativistic nuclear collisions with heavy beams.Comment: 16 pages, 9 figure

Freeze-out from HBT and Coulomb Effects

The freeze-out of hot and dense hadronic matter formed in relativistic nuclear collisions is probed by HBT interferometry of identical pions, kaons, etc. Coulomb repulsion/attraction of positive/negative particles show up at small particle momenta and is also very sensitive to the freeze-out conditions. The source sizes and times freeze-out are extracted from $\pi^-/\pi^+$ spectra and HBT radii and compared.Comment: 4 pages, proc. of QM'97, Tsukuba, Japa

Beam Energy Evolution of HBT Systematics at the AGS

We present preliminary results of the first pion interferometry (HBT) excitation function at intermediate AGS energies. The beam energy evolution of the correlations' dependence on mT, centrality, and emission angle with respect to the reaction plane are discussed. Comparisons with predictions of the RQMD cascade model are made.Comment: to appear in proceedings of Quark Matter '9

Transverse Momentum Spectra in Au+Au and d+Au Collisions at sNN\sqrt{s_{NN}}=200 GeV and the Pseudorapidity Dependence of High pT_T Suppression

We present spectra of charged hadrons from Au+Au and d+Au collisions at $\sqrt{s_{NN}}=200$ GeV measured with the BRAHMS experiment at RHIC. The spectra for different collision centralities are compared to spectra from ${\rm p}+\bar{{\rm p}}$ collisions at the same energy scaled by the number of binary collisions. The resulting ratios (nuclear modification factors) for central Au+Au collisions at $\eta=0$ and $\eta=2.2$ evidence a strong suppression in the high $p_{T}$ region ($>$2 GeV/c). In contrast, the d+Au nuclear modification factor (at $\eta=0$) exhibits an enhancement of the high $p_T$ yields. These measurements indicate a high energy loss of the high $p_T$ particles in the medium created in the central Au+Au collisions. The lack of suppression in d+Au collisions makes it unlikely that initial state effects can explain the suppression in the central Au+Au collisions.Comment: 4 pages, 4 figures, submitted to Phys. Rev. Let