Jet-Hadron Correlations in √ˢᴺᴺ = 200 GeV p + p 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 √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 (passoc/T) and enhanced at low passoc/T 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

Precision Measurement of the Longitudinal Double-Spin Asymmetry for Inclusive Jet Production in Polarized Proton Collisions at √s= 200GeV

We report a new measurement of the midrapidity inclusive jet longitudinal double-spin asymmetry, ALL, in polarized pp collisions at center-of-mass energy √s = 200 GeV. The STAR data place stringent constraints on polarized parton distribution functions extracted at next-to-leading order from global analyses of inclusive deep-inelastic scattering (DIS), semi-inclusive DIS, and RHIC pp data. The measured asymmetries provide evidence at the 3σ level for positive gluon polarization in the Bjorken-x region x \u3e 0.05

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)

J/ψ Production at Low Pᵀ in Au + Au and Cu + Cu Collisions at √ˢᴺᴺ = 200 GeV With the STAR Detector

The J/ψ pT spectrum and nuclear modification factor (RAA) are reported for pT \u3c 5 GeV/c and |y| \u3c 1 from 0% to 60% central Au + Au and Cu + Cu collisions at √ˢᴺᴺ = 200 GeV at STAR. A significant suppression of pT - integrated J/ψ production is observed in central Au + Au events. The Cu + Cu data are consistent with no suppression, although the precision is limited by the available statistics. RAA in Au + Au collisions exhibits a strong suppression at low transverse momentum and gradually increases with pT. The data are compared to high-pT STAR results and previously published BNL Relativistic Heavy Ion Collider results. Comparing with model calculations, it is found that the invariant yields at low pT are significantly above hydrodynamic flow predictions but are consistent with models that include color screening and regeneration

Inclusive Charged Hadron Elliptic Flow in Au + Au Collisions at √ˢᴺᴺ = 7.7-39 GeV

A systematic study is presented for centrality, transverse momentum pT, and pseudorapidity (η) dependence of the inclusive charged hadron elliptic flow v2 at midrapidity (|η| \u3c 1.0) in Au + Au collisions at √ˢᴺᴺ = 7.7, 11.5, 19.6, 27, and 39 GeV. The results obtained with different methods, including correlations with the event plane reconstructed in a region separated by a large pseudorapidity gap and four-particle cumulants (v2{4}), are presented to investigate nonflow correlations and v2 fluctuations. We observe that the difference between v2{2} and v2{4} is smaller at the lower collision energies. Values of v2, scaled by the initial coordinate space eccentricity, v2/ϵ, as a function of pT are larger in more central collisions, suggesting stronger collective flow develops in more central collisions, similar to the results at higher collision energies. These results are compared to measurements at higher energies at the Relativistic Heavy Ion Collider √ˢᴺᴺ = 62.4 and 200 GeV) and at the Large Hadron Collider (Pb + Pb collisions at √ˢᴺᴺ = 2.76 TeV). The v2(pT) values for fixed pT rise with increasing collision energy within the pT range studied (\u3c2 GeV/c). A comparison to viscous hydrodynamic simulations is made to potentially help understand the energy dependence of v2(pT). We also compare the v2 results to UrQMD and AMPT transport model calculations, and physics implications on the dominance of partonic versus hadronic phases in the system created at beam energy scan energies are discussed

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

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

Dielectron Azimuthal Anisotrophy at Mid-Rapidity in Au+Au Collisions at √ˢᶰᶰ = 200 GeV

We report on the first measurement of the azimuthal anisotropy v2 of dielectrons (e+e- pairs) at mid-rapidity from √ˢᶰᶰ = 200 GeV Au + Au collisions with the STAR detector at the Relativistic Heavy Ion Collider (RHIC), presented as a function of transverse momentum pT for different invariant-mass regions. In the mass region Mee \u3c 1.1 GeV/c2 the dielectron v2measurements are found to be consistent with expectations from π0, η, ω, and Φ decay contributions. In the mass region 1.1 \u3c Mee \u3c 2.9 GeV/c2, the measured dielectron v2is consistent, within experimental uncertainties, with that from the c¯c contributions

Experimental Studies of Di-Jets in Au + Au Collisions Using Angular Correlations With Respect to Back-to-Back Leading Hadrons

Jet-medium interactions are studied via a multihadron correlation technique ( called 2 + 1 ), where a pair of back-to-back hadron triggers with large transverse momentum is used as a proxy for a di-jet. This work extends the previous analysis for nearly symmetric trigger pairs with the highest momentum threshold of trigger hadron of 5 GeV/c with the new calorimeter-based triggers with energy thresholds of up to 10 GeV and above. The distributions of associated hadrons are studied in terms of correlation shapes and per-trigger yields on each trigger side. In contrast with di-hadron correlation results with single triggers, the associated hadron distributions for back-to-back triggers from central Au + Au data at √ˢᴺᴺ = 200 GeV show no strong modifications compared to d + Au data at the same energy. An imbalance in the total transverse momentum between hadrons attributed to the near-side and away-side of jetlike peaks is observed. The relative imbalance in the Au + Au measurement with respect to d + Au reference is found to increase with the asymmetry of the trigger pair, consistent with the expectation from medium-induced energy-loss effects. In addition, this relative total transverse momentum imbalance is found to decrease for softer associated hadrons. Such evolution indicates that the energy missing at higher associated momenta is converted into softer hadrons

Energy Dependence of Moments of Net-Proton Multiplicity Distributions at RHIC

We report the beam energy (√ˢᴺᴺ = 7.7-200 GeV) and collision centrality dependence of the mean (M), standard deviation (σ), skewness (S), and kurtosis (κ) of the net-proton multiplicity distributions in Au + Au collisions. The measurements are carried out by the STAR experiment at midrapidity (|y| \u3c 0.5) and within the transverse momentum range 0.4 \u3c pT \u3c 0.8 GeV/c in the first phase of the Beam Energy Scan program at the Relativistic Heavy Ion Collider. These measurements are important for understanding the quantum chromodynamic phase diagram. The products of the moments, Sσ and Kσ2, are sensitive to the correlation length of the hot and dense medium created in the collisions and are related to the ratios of baryon number susceptibilities of corresponding orders. The products of moments are found to have values significantly below the Skellam expectation and close to expectations based on independent proton and antiproton production. The measurements are compared to a transport model calculation to understand the effect of acceptance and baryon number conservation and also to a hadron resonance gas model