Jet-Like Correlations with Direct-Photon and Neutral-Pion Triggers at √\u3cem\u3e\u3csup\u3eS\u3c/sup\u3e\u3csub\u3eNN\u3c/sub\u3e\u3c/em\u3e = 200 GeV

Azimuthal correlations of charged hadrons with direct-photon (γdir) and neutral-pion (π0) trigger particles are analyzed in central Au+Au and minimum-bias p+p collisions at in the STAR experiment. The charged-hadron per-trigger yields at mid-rapidity from central Au+Au collisions are compared with p+p collisions to quantify the suppression in Au+Au collisions. The suppression of the away-side associated-particle yields per γdir trigger is independent of the transverse momentum of the trigger particle (ptrigT), whereas the suppression is smaller at low transverse momentum of the associated charged hadrons (passocT). Within uncertainty, similar levels of suppression are observed for γdir and π0 triggers as a function of zT (passocT / ptrigT). The results are compared with energy-loss-inspired theoretical model predictions. Our studies support previous conclusions that the lost energy reappears predominantly at low transverse momentum, regardless of the trigger energy

Jet-Hadron Correlations in √\u3csup\u3es\u3c/sup\u3eNN=200 GeV \u3cem\u3ep\u3c/em\u3e+\u3cem\u3ep\u3c/em\u3e and Central Au+Au Collisions

Azimuthal angular correlations of charged hadrons with respect to the axis of a reconstructed (trigger) jet in Au+Au and p+p collisions at √sNN=200  GeV in STAR are presented. The trigger jet population in Au+Au collisions is biased toward jets that have not interacted with the medium, allowing easier matching of jet energies between Au+Au and p+p collisions while enhancing medium effects on the recoil jet. The associated hadron yield of the recoil jet is significantly suppressed at high transverse momentum (passocT) and enhanced at low passocT in 0%–20% central Au+Au collisions compared to p+p collisions, which is indicative of medium-induced parton energy loss in ultrarelativistic heavy-ion collisions

Observation of Transverse Spin-Dependent Azimuthal Correlations of Charged Pion Pairs in \u3cem\u3ep\u3c/em\u3e\u3csup\u3e↑\u3c/sup\u3e+\u3cem\u3ep\u3c/em\u3e at √\u3cem\u3es\u3c/em\u3e=200 GeV

We report the observation of transverse polarization-dependent azimuthal correlations in charged pion pair production with the STAR experiment in p↑+p collisions at RHIC. These correlations directly probe quark transversity distributions. We measure signals in excess of 5 standard deviations at high transverse momenta, at high pseudorapidities η\u3e0.5, and for pair masses around the mass of the ρ meson. This is the first direct transversity measurement in p+p collisions

Energy Dependence of \u3cem\u3eKπ\u3c/em\u3e, \u3cem\u3epπ\u3c/em\u3e, and \u3cem\u3eKp\u3c/em\u3e Fluctuations in Au + Au Collisions from √\u3cem\u3e\u3csup\u3es\u3c/sup\u3eNN\u3c/em\u3e=7.7 to 200 GeV

A search for the quantum chromodynamics (QCD) critical point was performed by the STAR experiment at the BNL Relativistic Heavy Ion Collider, using dynamical fluctuations of unlike particle pairs. Heavy ion collisions were studied over a large range of collision energies with homogeneous acceptance and excellent particle identification, covering a significant range in the QCD phase diagram where a critical point may be located. Dynamical Kπ, pπ, and Kp fluctuations as measured by the STAR experiment in central 0–5% Au + Au collisions from center-of-mass collision energies √sNN=7.7 to 200 GeV are presented. The observable νdyn was used to quantify the magnitude of the dynamical fluctuations in event-by-event measurements of the Kπ, pπ, and Kp pairs. The energy dependences of these fluctuations from central 0–5% Au + Au collisions all demonstrate a smooth evolution with collision energy

Measurements of Dielectron Production in Au + Au Collisions at √\u3cem\u3es\u3csub\u3eNN\u3c/sub\u3e\u3c/em\u3e = 200 GeV from the STAR Experiment

We report on measurements of dielectron (e+e−) production in Au + Au collisions at a center-of-mass energy of 200 GeV per nucleon-nucleon pair using the STAR detector at BNL Relativistic Heavy Ion Collider. Systematic measurements of the dielectron yield as a function of transverse momentum (pT) and collision centrality show an enhancement compared to a cocktail simulation of hadronic sources in the low invariant-mass region (Mee \u3c 1 GeV/c2). This enhancement cannot be reproduced by the ρ-meson vacuum spectral function. In minimumbias collisions, in the invariant-mass range of 0.30–0.76 GeV/c2, integrated over the full pT acceptance, the enhancement factor is 1.76 ± 0.06 (stat.) ± 0.26 (sys.) ± 0.29 (cocktail). The enhancement factor exhibits weak centrality and pT dependence in STAR’s accessible kinematic regions,while the excess yield in this invariant-mass region as a function of the number of participating nucleons follows a power-law shape with a power of 1.44 ± 0.10. Models that assume an in-medium broadening of the ρ-meson spectral function consistently describe the observed excess in these measurements. Additionally, we report on measurements of ω- and φ-meson production through their e+e− decay channel. These measurements show good agreement with Tsallis blast-wave model predictions, as well as, in the case of the φ meson, results through its K+K− decay channel. In the intermediate invariant-mass region (1.1\u3cMee \u3c 3 GeV/c2), we investigate the spectral shapes from different collision centralities. Physics implications for possible in-medium modification of charmed hadron production and other physics sources are discussed

Isolation of Flow and Nonflow Correlations by Two and Four Particle Cumulant Measurements of Azimuthal Harmonics in √Sɴɴ = GeV Au+Au Collisions

A data-driven method was applied to Au+Au collisions at √Sɴɴ = 200 GeV made with the STAR detector at RHIC to isolate pseudorapidity distance Δ η-dependent and Δ η-independent correlations by using two- and four-particle azimuthal cumulant measurements. We identified a Δ η-independent component of the correlation, which is dominated by anisotropic flow and flow fluctuations. It was also found to be independent of η within the measured range of pseudorapidity | η | \u3c 1. In 20-30% central Au+Au collisions, the relative flow fluctuation was found to be 34% ± 2%(stat.) ± 3%(sys.) for particles with transverse momentum pT less than 2 GeV/c. The Δ η-dependent part, attributed to nonflow correlations, is found to be 5% ± 2%(sys.) relative to the flow of the measured second harmonic cumulant at | Δ η | \u3e 0.7

Azimuthal Anisotropy in U+U and Au+Au Collisions at RHIC

Collisions between prolate uranium nuclei are used to study how particle production and azimuthal anisotropies depend on initial geometry in heavy-ion collisions. We report the two- and four-particle cumulants, v2{2} and v2{4}, for charged hadrons from U+U collisions at √sNN=193  GeV and Au+Au collisions at √sNN=200  GeV. Nearly fully overlapping collisions are selected based on the energy deposited by spectators in zero degree calorimeters (ZDCs). Within this sample, the observed dependence of v2{2} on multiplicity demonstrates that ZDC information combined with multiplicity can preferentially select different overlap configurations in U+U collisions. We also show that v2 vs multiplicity can be better described by models, such as gluon saturation or quark participant models, that eliminate the dependence of the multiplicity on the number of binary nucleon-nucleon collisions

Di-Hadron Correlations with Identified Leading Hadrons in 200 GeV Au + Au and d + Au Collisions at STAR

The STAR Collaborationpresents for the first time two-dimensional di-hadron correlations with identified leading hadrons in 200 GeV central Au+Au and minimum-bias d+Au collisions to explore hadronization mechanisms in the quark gluon plasma. The enhancement of the jet-like yield for leading pions in Au + Au data with respect to the d + Au reference and the absence of such an enhancement for leading non-pions (protons and kaons) are discussed within the context of a quark recombination scenario. The correlated yield at large angles, specifically in the ridge region, is found to be significantly higher for leading non-pions than pions. The consistencies of the constituent quark scaling, azimuthal harmonic model and a mini-jet modification model description of the data are tested, providing further constraints on hadronization

Isolation of Flow and Nonflow Correlations by Two- and Four-Particle Cumulant Measurements of Azimuthal Harmonics in √\u3csup\u3es\u3c/sup\u3eNN=200 GeV Au+Au collisions

A data-driven method was applied to Au+Au collisions at √sNN = 200 GeV made with the STAR detector at RHIC to isolate pseudorapidity distance Δη-dependent and Δη-independent correlations by using two- and four-particle azimuthal cumulant measurements. We identified a Δη-independent component of the correlation, which is dominated by anisotropic flow and flow fluctuations. It was also found to be independent of η within the measured range of pseudorapidity |η| \u3c 1. In 20–30% central Au+Au colli- sions, the relative flow fluctuation was found to be 34% ± 2%(stat.) ± 3%(sys.) for particles with transverse momentum PT less than 2 GeV/c. The Δη-dependent part, attributed to nonflow correlations, is found to be 5% ± 2%(sys.) relative to the flow of the measured second harmonic cumulant at |Δη| \u3e 0.7

Long-Range Pseudorapidity Dihadron Correlations in \u3cem\u3ed\u3c/em\u3e + Au Collisions at √\u3csup\u3es\u3c/sup\u3eNN = 200 GeV

Dihadron angular correlations in d + Au collisions at √sNN = 200 GeV are reported as a function of the measured zero-degree calorimeter neutral energy and the forward charged hadron multiplicity in the Au-beam direction. A finite correlated yield is observed at large relative pseudorapidity (Δη) on the near side (i.e. relative azimuth Δφ ~ 0). This correlated yield as a function of Δη appears to scale with the dominant, primarily jet-related, away-side (Δφ ~ π ) yield. The Fourier coefficients of the Δφ correlation, Vn = (cos nΔφ), have a strong Δη dependence. In addition, it is found that V1 is approximately inversely proportional to the mid-rapidity event multiplicity, while V2 is independent of it with similar magnitude in the forward (d-going) and backward (Au-going) directions