Energy Dependence of Acceptance-Corrected Dielectron Excess Mass Spectrum at Mid-Rapidity in Au + Au Collisions at √sNN = 19.6 and 200 GeV

The acceptance-corrected dielectron excess mass spectra, where the known hadronic sources have been subtracted from the inclusive dielectron mass spectra, are reported for the first time at mid-rapidity |yee|<1 in minimum-bias Au+Au collisions at √sNN=19.6 and 200 GeV. The excess mass spectra are consistently described by a model calculation with a broadened ρ spectral function for Mee<1.1 GeV/c2. The integrated dielectron excess yield at √sNN=19.6 GeV for 0.4<Mee<0.75 GeV/c2, normalized to the charged particle multiplicity at mid-rapidity, has a value similar to that in In+In collisions at √sNN=17.3 GeV. For √sNN=200 GeV, the normalized excess yield in central collisions is higher than that at √sNN=17.3 GeV and increases from peripheral to central collisions. These measurements indicate that the lifetime of the hot, dense medium created in central Au+Au collisions at √sNN=200 GeV is longer than those in peripheral collisions and at lower energies

Nucleon-Gold Collisions at 200 AGeV Using Tagged d+Au Interactions in PHOBOS

Forward calorimetry in the PHOBOS detector has been used to study charged hadron production in d+Au, p+Au and n+Au collisions at sqrt(s_nn) = 200 GeV. The forward proton calorimeter detectors are described and a procedure for determining collision centrality with these detectors is detailed. The deposition of energy by deuteron spectator nucleons in the forward calorimeters is used to identify p+Au and n+Au collisions in the data. A weighted combination of the yield of p+Au and n+Au is constructed to build a reference for Au+Au collisions that better matches the isospin composition of the gold nucleus. The p_T and centrality dependence of the yield of this improved reference system is found to match that of d+Au. The shape of the charged particle transverse momentum distribution is observed to extrapolate smoothly from pbar+p to central d+Au as a function of the charged particle pseudorapidity density. The asymmetry of positively- and negatively-charged hadron production in p+Au is compared to that of n+Au. No significant asymmetry is observed at mid-rapidity. These studies augment recent results from experiments at the LHC and RHIC facilities to give a more complete description of particle production in p+A and d+A collisions, essential for the understanding the medium produced in high energy nucleus-nucleus collisions.Comment: 17 pages, 18 figure

System size dependence of cluster properties from two-particle angular correlations in Cu+Cu and Au+Au collisions at sNN\sqrt{s_{_{NN}}} = 200 GeV

We present results on two-particle angular correlations in Cu+Cu and Au+Au collisions at a center of mass energy per nucleon pair of 200 GeV over a broad range of pseudorapidity ($\eta$) and azimuthal angle ($\phi$) as a function of collision centrality. The PHOBOS detector at RHIC has a uniquely-large angular coverage for inclusive charged particles, which allows for the study of correlations on both long- and short-range scales. A complex two-dimensional correlation structure in $\Delta \eta$ and $\Delta \phi$ emerges, which is interpreted in the context of a cluster model. The effective cluster size and decay width are extracted from the two-particle pseudorapidity correlation functions. The effective cluster size found in semi-central Cu+Cu and Au+Au collisions is comparable to that found in proton-proton collisions but a non-trivial decrease of the size with increasing centrality is observed. Moreover, a comparison between results from Cu+Cu and Au+Au collisions shows an interesting scaling of the effective cluster size with the measured fraction of total cross section (which is related to the ratio of the impact parameter to the nuclear radius, $b/2R$), suggesting a geometric origin. Further analysis for pairs from restricted azimuthal regions shows that the effective cluster size at $\Delta\phi \sim 180^{\circ}$ drops more rapidly toward central collisions than the size at $\Delta\phi \sim 0^{\circ}$. The effect of limited $\eta$ acceptance on the cluster parameters is also addressed, and a correction is applied to present cluster parameters for full $\eta$ coverage, leading to much larger effective cluster sizes and widths than previously noted in the literature. These results should provide insight into the hot and dense medium created in heavy ion collisions.Comment: 9 pages, 8 figures, Published in Phys. Rev.

Participant and spectator scaling of spectator fragments in Au + Au and Cu + Cu collisions at √sNN = 19.6 and 22.4 GeV

Spectator fragments resulting from relativistic heavy ion collisions, consisting of single protons and neutrons along with groups of stable nuclear fragments up to nitrogen (Z=7), are measured in PHOBOS. These fragments are observed in Au+Au (√sNN =19.6GeV) and Cu+Cu (22.4 GeV) collisions at high pseudorapidity (η). The dominant multiply-charged fragment is the tightly bound helium (α), with lithium, beryllium, and boron all clearly seen as a function of collision centrality and pseudorapidity. We observe that in Cu+Cu collisions, it becomes much more favorable for the α fragments to be released than lithium. The yields of fragments approximately scale with the number of spectator nucleons, independent of the colliding ion. The shapes of the pseudorapidity distributions of fragments indicate that the average deflection of the fragments away from the beam direction increases for more central collisions. A detailed comparison of the shapes for α and lithium fragments indicates that the centrality dependence of the deflections favors a scaling with the number of participants in the collision.United States. Department of Energy (Grant DE-AC02-98CH10886)United States. Department of Energy (Grant DE-FG02-93ER40802)United States. Department of Energy (Grant DE-FG02-94ER40818)United States. Department of Energy (Grant DE-FG02-94ER40865)United States. Department of Energy (Grant DE-FG02- 99ER41099)United States. Department of Energy (Grant DE-AC02-06CH11357)National Science Foundation (U.S.) (Grant 9603486)National Science Foundation (U.S.) (Grant 0072204)National Science Foundation (U.S.) (Grant 0245011

The ABC130 barrel module prototyping programme for the ATLAS strip tracker

For the Phase-II Upgrade of the ATLAS Detector, its Inner Detector, consisting of silicon pixel, silicon strip and transition radiation sub-detectors, will be replaced with an all new 100 % silicon tracker, composed of a pixel tracker at inner radii and a strip tracker at outer radii. The future ATLAS strip tracker will include 11,000 silicon sensor modules in the central region (barrel) and 7,000 modules in the forward region (end-caps), which are foreseen to be constructed over a period of 3.5 years. The construction of each module consists of a series of assembly and quality control steps, which were engineered to be identical for all production sites. In order to develop the tooling and procedures for assembly and testing of these modules, two series of major prototyping programs were conducted: an early program using readout chips designed using a 250 nm fabrication process (ABCN-25) and a subsequent program using a follow-up chip set made using 130 nm processing (ABC130 and HCC130 chips). This second generation of readout chips was used for an extensive prototyping program that produced around 100 barrel-type modules and contributed significantly to the development of the final module layout. This paper gives an overview of the components used in ABC130 barrel modules, their assembly procedure and findings resulting from their tests.Comment: 82 pages, 66 figure

Photon and η production in p+Pb and p+C collisions at

Measurements of direct photon production in p+Pb and p+C collisions at √sNN=17.4 GeV are presented. Upper limits on the direct photon yield as a function of pT are derived and compared to the results for Pb+Pb collisions at √sNN=17.3 GeV. The production of the η meson, which is an important input to the direct photon signal extraction, has been determined in the η → 2γ channel for p+C collisions at √sNN=17.4 GeV. © 2012 Elsevier B.V

J/ψ Production at High Transverse Momenta in P+p and Au + Au Collisions at √SNN=200 GeV

We report J/ψ spectra for transverse momenta p[subscript T]> 5GeV/c at mid-rapidity in p+p and Au+Au collisions at √sNN=200GeV. The inclusive J/ψ spectrum and the extracted B-hadron feed-down are compared to models incorporating different production mechanisms. We observe significant suppression of the J/ψ yields for p[subscript T] > 5GeV/c in 0-30% central Au+Au collisions relative to the p+p yield scaled by the number of binary nucleon-nucleon collisions in Au+Au collisions. In 30-60% mid-central collisions, no such suppression is observed. The level of suppression is consistently less than that of high-p[subscript T]π[superscript ±] and low-p[subscript T]J/ψ at RHIC and high-p[subscript T]J/ψ at the LHC. Keywords: J/ψ, suppression, Color-screening, Quarkonium, Heavy-ion collisions, STARNational Science Foundation (U.S.)Alfred P. Sloan Foundatio

Measurement of charge multiplicity asymmetry correlations in high-energy nucleus-nucleus collisions at √s[subscript NN] = 200 GeV

A study is reported of the same- and opposite-sign charge-dependent azimuthal correlations with respect to the event plane in Au + Au collisions at √s[subscript NN] = 200 GeV. The charge multiplicity asymmetries between the up/down and left/right hemispheres relative to the event plane are utilized. The contributions from statistical fluctuations and detector effects were subtracted from the (co-)variance of the observed charge multiplicity asymmetries. In the mid- to most-central collisions, the same- (opposite-) sign pairs are preferentially emitted in back-to-back (aligned on the same-side) directions. The charge separation across the event plane, measured by the difference, Δ, between the like- and unlike-sign up/down-left/right correlations, is largest near the event plane. The difference is found to be proportional to the event-by-event final-state particle ellipticity (via the observed second-order harmonic v[obs over 2]), where Δ = [1.3 ± 1.4(stat)[+4.0 over −1.0](syst)] × 10[superscript −5] + [3.2 ± 0.2(stat)[+0.4 over −0.3](syst)] × 10[superscript −3]v[obs over 2] for 20–40% Au + Au collisions. The implications for the proposed chiral magnetic effect are discussed.United States. Dept. of Energy. Office of Nuclear PhysicsUnited States. Dept. of Energy. Office of High Energy PhysicsNational Science Foundation (U.S.)Alfred P. Sloan Foundatio

Transverse single-spin asymmetry and cross section for π[superscript 0] and η mesons at large Feynman x in p[superscript ↑]+p collisions at √s=200 GeV

Measurements of the differential cross section and the transverse single-spin asymmetry, A[subscript N], vs x[subscript F] for π[superscript 0] and η mesons are reported for 0.40.55, is consistent with a perturbative QCD prediction, and the η/π[superscript 0] cross-section ratio agrees with existing midrapidity measurements. For 0.55<x[subscript F]<0.75, the average A[subscript N] for η is 0.210±0.056, and that for π[superscript 0] is 0.081±0.016. The probability that these two asymmetries are equal is ∼3%