Di-Electron Spectrum at Mid-Rapidity in p+p Collisions at √s=200 GeV

We report on the mid-rapidity mass spectrum of di-electrons and cross sections of pseudoscalar and vector mesons via e+e- decays, from √s = 200 GeV p + p collisions, measured by the large-acceptance experiment STAR at the Relativistic Heavy Ion Collider. The ratio of the di-electron continuum to the combinatorial background is larger than 10% over the entire mass range. Simulations of di-electrons from light-meson decays and heavy-flavor decays (charmonium and open charm correlation) are found to describe the data. The extracted ω → e+e- invariant yields are consistent with previous measurements. The mid-rapidity yields (dN/dy) of Φ and J/Ψ are extracted through their di-electron decay channels and are consistent with the previous measurements of Φ → K+ K- and J/Ψ → e+ e-. Our results suggest a new upper limit of the branching ratio of the eta -\u3e e(+) e(-) of 1.7 x 10-5 at the 90% confidence level

Event Plane Dependent Dihadron Correlations with Harmonic Vⁿ Subtraction in Au + Au Collisions at √ˢᴺᴺ = 200 GeV

STAR measurements of dihadron azimuthal correlations (ΔΦ) are reported in midcentral (20-60%) Au + Au collisions at √ˢᴺᴺ = 200 GeV as a function of the trigger particle\u27s azimuthal angle relative to the event plane, Φs = | Φt- ΨEP|. The elliptic (v2), triangular (v3), and quadratic (v4) flow harmonic backgrounds are subtracted using the zero yield at minimum (ZYAM) method. It is found that a finite near-side (|ΔΦ| \u3c π/2) long-range pseudorapidity correlation (ridge) is present in the in-plane direction (Φs ~ 0). The away-side (|ΔΦ| \u3e π/2) correlation shows a modification from d+ Au data, varying with Φs. The modification may be a consequence of path-length-dependent jet quenching and may lead to a better understanding of high-density QCD

Directed and Elliptic Flow of Charged Particles in Cu + Cu Collisions at √ˢᶰᶰ =22.4 GeV

This paper reports results for directed flow v1 and elliptic flow v2 of charged particles in Cu + Cu collisions at √ˢᶰᶰ = 22.4 GeV at the Relativistic Heavy Ion Collider. The measurements are for the 0-60% most central collisions, using charged particles observed in the STAR detector. Our measurements extend to 22.4-GeV Cu + Cu collisions the prior observation that v1 is independent of the system size at 62.4 and 200 GeV and also extend the scaling of v1 with η/ybeam to this system. The measured v2pT in Cu + Cu collisions is similar for root √ˢᶰᶰ throughout the range 22.4 to 200 GeV. We also report a comparison with results from transport model (ultrarelativistic quantum molecular dynamics and multiphase transport model) calculations. The model results do not agree quantitatively with the measured v1(η), v2pT, and v2(η)

Anomalous Centrality Evolution of Two-Particle Angular Correlations from Au-Au Collisions at √ˢᴺᴺ = 62 and 200 GeV

We present two-dimensional (2D) two-particle angular correlations measured with the STAR detector on relative pseudorapidity η and ϕ for charged particles from Au-Au collisions at √ˢᴺᴺ = 62 and 200 GeV with transverse momentum pt ≥ 0.15 GeV/c, │η│ ≤ 1, and 2 π in azimuth. Observed correlations include a same-side (relative azimuth \u3cπ/2) 2D peak, a closely related away-side azimuth dipole, and an azimuth quadrupole conventionally associated with elliptic flow. The same-side 2D peak and away-side dipole are explained by semihard parton scattering and fragmentation (minijets) in proton-proton and peripheral nucleus-nucleus collisions. Those structures follow N-N binary-collision scaling in Au-Au collisions until midcentrality, where a transition to a qualitatively different centrality trend occurs within one 10% centrality bin. Above the transition point the number of same-side and away-side correlated pairs increases rapidly relative to binary-collision scaling, the η width of the same-side 2D peak also increases rapidly (η elongation), and the ϕ width actually decreases significantly. Those centrality trends are in marked contrast with conventional expectations for jet quenching in a dense medium. The observed centrality trends are compared to perturbative QCD predictions computed in HIJING, which serve as a theoretical baseline, and to the expected trends for semihard parton scattering and fragmentation in a thermalized opaque medium predicted by theoretical calculations and phenomenological models. We are unable to reconcile a semihard parton scattering and fragmentation origin for the observed correlation structure and centrality trends with heavy-ion collision scenarios that invoke rapid parton thermalization. If the collision system turns out to be effectively opaque to few-GeV partons the present observations would be inconsistent with the minijet picture discussed here

Results on Total and Elastic Cross Sections in Proton–Proton Collisions at √s = 200 GeV

We report results on the total and elastic cross sections in proton-proton collisions at √s = 200 GeV obtained with the Roman Pot setup of the STAR experiment at the Relativistic Heavy Ion Collider (RHIC). The elastic differential cross section was measured in the squared four-momentum transfer range 0.045 ≤ −t ≤ 0.135 GeV². The value of the exponential slope parameter B of the elastic differential cross section dσ/dt ∼ e−Bt in the measured −t range was found to be B = 14.32 ± 0.09 (stat.)+0.13−0.28(syst.) GeV⁻². The total cross section σtot, obtained from extrapolation of the dσ/dt to the optical point at −t = 0, is σtot = 54.67 ± 0.21(stat.)+1.28−1.38(syst.) mb. We also present the values of the elastic cross section σel = 10.85±0.03 (stat.)+0.49−0.41 (syst.) mb, the elastic cross section integrated within the STAR t-range σeldet = 4.05 ± 0.01(stat.)−0.17+0.18(syst.) mb, and the inelastic cross section σinel= 43.82 ± 0.21(stat.)−1.44+1.37(syst.) mb. The results are compared with the world data

Measurements of Dihadron Correlations Relative to the Event Plane in Au Plus Au Collisions at √SNN= 200 GeV

Dihadron azimuthal correlations containing a high transverse momentum (pT) trigger particle are sensitive to the properties of the nuclear medium created at RHIC through the strong interactions occurring between the traversing parton and the medium, i.e. jet-quenching. Previous measurements revealed a strong modification to dihadron azimuthal correlations in Au+Au collisions with respect to p+p and d+Au collisions. The modification increases with the collision centrality, suggesting a path-length or energy density dependence to the jet-quenching effect. This paper reports STAR measurements of dihadron azimuthal correlations in mid-central (20%-60%) Au+Au collisions at √sNN = 200 GeV as a function of the trigger particle\u27s azimuthal angle relative to the event plane, ϕs = |ϕt - ψEP| .The azimuthal correlation is studied as a function of both the trigger and associated particle pT. The subtractions of the combinatorial background and anisotropic flow, assuming Zero Yield At Minimum (ZYAM), are described. The correlation results are first discussed with subtraction of the even harmonic (elliptic and quadrangular) flow backgrounds. The away-side correlation is strongly modified, and the modification varies with ϕs, with a double-peak structure for out-of-plane trigger particles. The near-side ridge (long range pseudo-rapidity Δη correlation) appears to drop with increasing ϕs while the jet-like component remains approximately constant. The correlation functions are further studied with the subtraction of odd harmonic triangular flow background arising from fluctuations. It is found that the triangular flow, while responsible for the majority of the amplitudes, is not sufficient to explain the ϕs-dependence of the ridge or the away-side double-peak structure. The dropping ridge with ϕs, could be attributed to a ϕs-dependent elliptic anisotropy; however, the physics mechanism of the ridge remains an open question. Even with a ϕs-dependent elliptic flow, the away-side correlation structure is robust. These results, with extensive systematic studies of the dihadron correlations as a function of ϕs, trigger and associated particle pT, and the pseudo-rapidity range Δη, should provide stringent inputs to help understand the underlying physics mechanisms of jet-medium interactions in high energy nuclear collisions

The Heavy Photon Search Test Detector

The Heavy Photon Search (HPS), an experiment to search for a hidden sector photon in fixed target electroproduction, is preparing for installation at the Thomas Jefferson National Accelerator Facility (JLab) in the Fall of 2014. As the first stage of this project, the HPS Test Run apparatus was constructed and operated in 2012 to demonstrate the experiment׳s technical feasibility and to confirm that the trigger rates and occupancies are as expected. This paper describes the HPS Test Run apparatus and readout electronics and its performance. In this setting, a heavy photon can be identified as a narrow peak in the e+ e− invariant mass spectrum above the trident background or as a narrow invariant mass peak with a decay vertex displaced from the production target, so charged particle tracking and vertexing are needed for its detection. In the HPS Test Run, charged particles are measured with a compact forward silicon microstrip tracker inside a dipole magnet. Electromagnetic showers are detected in a PbW04 crystal calorimeter situated behind the magnet, and are used to trigger the experiment and identify electrons and positrons. Both detectors are placed close to the beam line and split top-bottom. This arrangement provides sensitivity to low-mass heavy photons, allows clear passage of the unscattered beam, and avoids the spray of degraded electrons coming from the target. The discrimination between prompt and displaced e+ e− pairs requires the first layer of silicon sensors be placed only 10 cm downstream of the target. The expected signal is small, and the trident background huge, so the experiment requires very large statistics. Accordingly, the HPS Test Run utilizes high-rate readout and data acquisition electronics and a fast trigger to exploit the essentially 100% duty cycle of the CEBAF accelerator at JLab

Scaling Properties at Freeze-Out in Relativistic Heavy-Ion Collisions

Identified charged pion, kaon, and proton spectra are used to explore the system size dependence of bulk freeze-out properties in Cu+Cu collisions at √sNN=200 and 62.4 GeV. The data are studied with hydrodynamically motivated blast-wave and statistical model frameworks in order to characterize the freeze-out properties of the system. The dependence of freeze-out parameters on beam energy and collision centrality is discussed. Using the existing results from Au + Au and pp collisions, the dependence of freeze-out parameters on the system size is also explored. This multidimensional systematic study furthers our understanding of the QCD phase diagram revealing the importance of the initial geometrical overlap of the colliding ions. The analysis of Cu+Cu collisions expands the system size dependence studies from Au+Au data with detailed measurements in the smaller system. The systematic trends of the bulk freeze-out properties of charged particles is studied with respect to the total charged particle multiplicity at midrapidity, exploring the influence of initial state effects

Beam-Energy and System-Size Dependence of Dynamical Net Charge Fluctuations

We present measurements of net charge fluctuations in Au+Au collisions at √sNN=19.6, 62.4, 130, and 200 GeV, Cu+Cu collisions at √sNN=62.4 and 200 GeV, and p+p collisions at √s=200GeV using the dynamical net charge fluctuations measure ν+−,dyn. We observe that the dynamical fluctuations are nonzero at all energies and exhibit a modest dependence on beam energy. A weak system size dependence is also observed. We examine the collision centrality dependence of the net charge fluctuations and find that dynamical net charge fluctuations violate 1/Nch scaling but display approximate 1/Npart scaling. We also study the azimuthal and rapidity dependence of the net charge correlation strength and observe strong dependence on the azimuthal angular range and pseudorapidity widths integrated to measure the correlation

Parton Energy Loss in Heavy-Ion Collisions Via Direct-Photon and Charged-Particle Azimuthal Correlations

Charged-particle spectra associated with direct photon Ydir) and π0 are measured in p+p and Au+Au collisions at center-of-mass energy √sNN=200 GeV with the STAR detector at the Relativistic Heavy Ion Collider. A shower-shape analysis is used to partially discriminate between Ydirand π0. Assuming no associated charged particles in the γdir direction (near side) and small contribution from fragmentation photons (Yfrag), the associated charged-particle yields opposite to Ydir(away side) are extracted. In central Au+Au collisions, the charged-particle yields at midrapidity (|η|\u3c1) and high transverse momentum (3 \u3c PTassoc \u3c 16 GeV/c) associated with γdir and π0 (|η|\u3c0.9, 8 \u3c PTtrig \u3c16 GeV/c) are suppressed by a factor of 3–5 compared with p+p collisions. The observed suppression of the associated charged particles is similar for Ydir and π0 and independent of the γdirenergy within uncertainties. These measurements indicate that, in the kinematic range covered and within our current experimental uncertainties, the parton energy loss shows no sensitivity to the parton initial energy, path length, or color charge