identified hadrons π ± , K ± , p, andp produced at midrapidity in Cu+Au collisions at

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Bent crystal channeling has been observed with protons and fully stripped gold ions in the Relativistic Heavy Ion Collider (RHIC). Prior to 2003, a bent crystal was installed in one ring of RHIC as the first stage of a two stage collimation system. The observed channeling efficiency was approximately 25%, less than half of original predictions. We show that this is due to a difference between the model and real Twiss parameters at the crystal location and our improved understanding of the beam halo. Collimation using the crystal was unsuccessful and raised background at the STAR detector by as much as a factor of 2 because of the low channeling efficiency. We give a report of our channeling studies in RHIC and describe our experience using the bent crystal as a collimator. The results are discussed and compared to simulations and theoretical predictions.

Based on an evaluation of data on pion interferometry and on particle yields at mid-rapidity, we propose a universal condition for thermal freeze-out of pions in heavy-ion collisions. We show that freeze-out occurs when the mean free path of pions λ f reaches a value of about 1 fm, which is much smaller than the spatial extent of the system at freeze-out. This critical mean free path is independent of the centrality of the collision and beam energy from AGS to RHIC. A systematic study of the space-time extent and the dynamical behavior of the pion source in relativistic heavy ion collisions at thermal freeze-out can be obtained from analysis of pion interferometry (HBT) data. Understanding these aspects is vital for interpretation of the data in terms of formation of the quark-gluon plasma. Indeed recent HBT results from RHIC and how they fit into the systematics have been noted as a major puzzle [1]. In this letter we present an investigation of the freezeout conditions at beam energies from AGS to RHIC. In particular, the recently published CERES HBT data at 40, 80, and 158 A GeV [2] provide an important link between the existing results from AGS, SPS, and RHIC, thereby shedding light on the RHIC puzzle.Thermal freeze-out of pions and its connection to the mean free path has been discussed previously (see e.g. [3,4,5,6,7,8]). The mean free path of pions at freeze-out is defined aswhere σ is the total cross section of pions with the surrounding medium and ρ f is the freeze-out density which can be replaced by the number of particles N contained in the freeze-out volume V f . The pion freeze-out volume V f can be accessed experimentally by pion interferometry. Mid-rapidity pion HBT data have been published from central collisions of lead and gold nuclei over a wide range of beam energies. Here, we focus on recent HBT results from three experiments which have kinematical access to the region of low transverse pair momentum k t = 1 2 | p t,1 + p t,2 |: Experiment E895 at the AGS [9], the CERES/NA45 experiment at the SPS [2,10], and the STAR experiment at RHIC [11]. All three experiments employ large volume Time Projection Chambers (TPCs), thereby applying comparable analysis methods with similar sources of systematic uncertainties.For the calculation of the freeze-out volume V f we use the following expression:assuming a density distribution of Gaussian shape in all three dimensions. The longitudinal and sideward radius parameters R long and R side are measured in the longitudinal co-moving system of the pion pair, using the cartesian

Two particle azimuthal correlation functions are presented for charged hadrons produced in Au + Au collisions at RHIC ( √ s N N = 130 GeV). The measurements permit determination of elliptic flow without event-by-event estimation of the reaction plane. The extracted elliptic flow values (v2) show significant sensitivity to both the collision centrality and the transverse momenta of emitted hadrons, suggesting rapid thermalization and relatively strong velocity fields. When scaled by the eccentricity of the collision zone ε, the scaled elliptic flow shows little or no dependence on centrality for charged hadrons with relatively low pT . A breakdown of this ε scaling is observed for charged hadrons with pT > 1.0 GeV/c for the most central collisions.