Low-\u3cem\u3ep\u3csub\u3eT\u3c/sub\u3e\u3c/em\u3e \u3cem\u3ee\u3c/em\u3e\u3csup\u3e+\u3c/sup\u3e \u3cem\u3ee\u3c/em\u3e\u3csup\u3e-\u3c/sup\u3e Pair Production in Au + Au Collisions at √\u3cem\u3es\u3c/em\u3e\u3csub\u3eNN\u3c/sub\u3e\u3c/em\u3e = 193 GeV and U + U Collisions at √\u3cem\u3es\u3c/em\u3e\u3csub\u3eNN\u3c/sub\u3e\u3c/em\u3e = 193 GeV at STAR

We report first measurements of e+e− pair production in the mass region 0.4 \u3c Mee \u3c 2.6  GeV/c2 at low transverse momentum (pT \u3c 0.15  GeV/c) in noncentral Au+Au collisions at √sNN = 200  GeV and U+U collisions at √sNN = 193  GeV. Significant enhancement factors, expressed as ratios of data over known hadronic contributions, are observed in the 40%–80% centrality of these collisions. The excess yields peak distinctly at low pT with a width (√⟨p2T⟩) between 40 and 60  MeV/c. The absolute cross section of the excess depends weakly on centrality, while those from a theoretical model calculation incorporating an in-medium broadened ρ spectral function and radiation from a quark gluon plasma or hadronic cocktail contributions increase dramatically with an increasing number of participant nucleons. Model calculations of photon-photon interactions generated by the initial projectile and target nuclei describe the observed excess yields but fail to reproduce the p2T distributions

Improved Measurement of the Longitudinal Spin Transfer to Λ and Λ¯ Hyperons in Polarized Proton-Proton Collisions at √\u3cem\u3es\u3c/em\u3e = 200 GeV

The longitudinal spin transfer DLL to Λ and Λ¯ hyperons produced in high-energy polarized proton-proton collisions is expected to be sensitive to the helicity distribution functions of strange quarks and antiquarks of the proton, and to longitudinally polarized fragmentation functions. We report an improved measurement of DLL from data obtained at a center-of-mass energy of √s = 200  GeV with the STAR detector at RHIC. The data have an approximately twelve times larger figure of merit than prior results and cover |η| \u3c 1.2 in pseudorapidity with transverse momenta pT up to 6  GeV/c. In the forward scattering hemisphere at largest pT, the longitudinal spin transfer is found to be DLL = −0.036 ± 0.048(stat) ± 0.013(sys) for Λ hyperons and DLL = 0.032 ± 0.043(stat) ± 0.013(sys) for Λ¯ antihyperons. The dependences on η and pT are presented and compared with model evaluations

\u3cem\u3eJ/ψ\u3c/em\u3e Production Cross Section and Its Dependence on Charged-Particle Multiplicity in \u3cem\u3ep\u3c/em\u3e +\u3cem\u3e p\u3c/em\u3e Collisions at √\u3cem\u3es\u3c/em\u3e=200 GeV

We present a measurement of inclusive J/ψ production at mid-rapidity (|y| \u3c 1) in p+p collisions at a center-of-mass energy of √s=200 GeV with the STAR experiment at the Relativistic Heavy Ion Collider (RHIC). The differential production cross section for J/ψ as a function of transverse momentum (pT) for 0 \u3c pT \u3c 14 GeV/c and the total cross section are reported and compared to calculations from the color evaporation model and the non-relativistic Quantum Chromodynamics model. The dependence of J/ψ relative yields in three pT intervals on charged-particle multiplicity at mid-rapidity is measured for the first time in p+p collisions at √s=200 GeV and compared with that measured at √s=7 TeV, PYTHIA8 and EPOS3 Monte Carlo generators, and the Percolation model prediction

Correlation Measurements Between Flow Harmonics in Au + Au Collisions at RHIC

Flow harmonics (vn) in the Fourier expansion of the azimuthal distribution of particles are widely used to quantify the anisotropy in particle emission in high-energy heavy-ion collisions. The symmetric cumulants, SC(m,n), are used to measure the correlations between different orders of flow harmonics. These correlations are used to constrain the initial conditions and the transport properties of the medium in theoretical models. In this Letter, we present the first measurements of the four-particle symmetric cumulants in Au+Au collisions at √sNN=39 and 200 GeV from data collected by the STAR experiment at RHIC. We observe that v2 and v3 are anti-correlated in all centrality intervals with similar correlation strengths from 39 GeV Au+Au to 2.76 TeV Pb+Pb (measured by the ALICE experiment). The v2–v4 correlation seems to be stronger at 39 GeV than at higher collision energies. The initial-stage anti-correlations between second and third order eccentricities are sufficient to describe the measured correlations between v2 and v3. The best description of v2–v4 correlations at √sNN=200 GeV is obtained with inclusion of the system\u27s nonlinear response to initial eccentricities accompanied by the viscous effect with η/s\u3e0.08. Theoretical calculations using different initial conditions, equations of state and viscous coefficients need to be further explored to extract η/s of the medium created at RHIC

Transverse Spin-Dependent Azimuthal Correlations of Charged Pion Pairs Measured in \u3cem\u3ep\u3c/em\u3e↑ + \u3cem\u3ep\u3c/em\u3e Collisions at √\u3cem\u3es\u3c/em\u3e = 500 GeV

The transversity distribution, which describes transversely polarized quarks in transversely polarized nucleons, is a fundamental component of the spin structure of the nucleon, and is only loosely constrained by global fits to existing semi-inclusive deep inelastic scattering (SIDIS) data. In transversely polarized p↑ + p collisions it can be accessed using transverse polarization dependent fragmentation functions which give rise to azimuthal correlations between the polarization of the struck parton and the final state scalar mesons. This letter reports on spin dependent di-hadron correlations measured by the STAR experiment. The new dataset corresponds to 25 pb−1 integrated luminosity of p↑ + p collisions at √s = 500 GeV, an increase of more than a factor of ten compared to our previous measurement at √s = 200 GeV. Non-zero asymmetries sensitive to transversity are observed at a Q2 of several hundred GeV and are found to be consistent with the former measurement and a model calculation. We expect that these data will enable an extraction of transversity with comparable precision to current SIDIS datasets but at much higher momentum transfers where subleading effects are suppressed

The Proton-Ω Correlation Function in Au + Au Collisions at √s\u3csub\u3eNN\u3c/sub\u3e = 200 GeV

We present the first measurement of the proton–Ω correlation function in heavy-ion collisions for the central (0–40%) and peripheral (40–80%) Au + Au collisions at √sNN = 200 GeV by the STAR experiment at the Relativistic Heavy-Ion Collider (RHIC). Predictions for the ratio of peripheral collisions to central collisions for the proton–Ω correlation function are sensitive to the presence of a nucleon–Ω bound state. These predictions are based on the proton–Ω interaction extracted from (2+1)-flavor lattice QCD calculations at the physical point. The measured ratio of the proton–Ω correlation function between the peripheral (small system) and central (large system) collisions is less than unity for relative momentum smaller than 40 MeV/c. Comparison of our measured correlation ratio with theoretical calculation slightly favors a proton–Ω bound system with a binding energy of ∼ 27 MeV

Beam Energy Dependence of Rapidity-Even Dipolar Flow in Au + Au Collisions

New measurements of directed flow for charged hadrons, characterized by the Fourier coefficient v1, are presented for transverse momenta pT, and centrality intervals in Au+Au collisions recorded by the STAR experiment for the center-of-mass energy range √sNN=7.7–200 GeV. The measurements underscore the importance of momentum conservation, and the characteristic dependencies on √sNN, centrality and pT are consistent with the expectations of geometric fluctuations generated in the initial stages of the collision, acting in concert with a hydrodynamic-like expansion. The centrality and pT dependencies of v1even, as well as an observed similarity between its excitation function and that for v3, could serve as constraints for initial-state models. The v1even excitation function could also provide an important supplement to the flow measurements employed for precision extraction of the temperature dependence of the specific shear viscosity

Measurement of the Cross Section and Longitudinal Double-Spin Asymmetry for Dijet Production in Polarized \u3cem\u3epp\u3c/em\u3e Collisions at √\u3cem\u3es\u3c/em\u3e = 200 GeV

We report the first measurement of the longitudinal double-spin asymmetry ALL for midrapidity dijet production in polarized pp collisions at a center-of-mass energy of √s = 200  GeV. The dijet cross section was measured and is shown to be consistent with next-to-leading order (NLO) perturbative QCD predictions. ALL results are presented for two distinct topologies, defined by the jet pseudorapidities, and are compared to predictions from several recent NLO global analyses. The measured asymmetries, the first such correlation measurements, support those analyses that find positive gluon polarization at the level of roughly 0.2 over the region of Bjorken-x \u3e 0.05

Longitudinal Double-Spin Asymmetries for Dijet Production at Intermediate Pseudorapidity in Polarized \u3cem\u3epp\u3c/em\u3e Collisions at √\u3cem\u3es\u3c/em\u3e = 200 GeV

We present the first measurements of the longitudinal double-spin asymmetry ALL for dijets with at least one jet reconstructed within the pseudorapidity range 0.8 \u3c η \u3c 1.8. The dijets were measured in polarized pp collisions at a center-of-mass energy √s = 200  GeV. Values for ALL are determined for several distinct event topologies, defined by the jet pseudorapidities, and span a range of parton momentum fraction x down to x ∼ 0.01. The measured asymmetries are found to be consistent with the predictions of global analyses that incorporate the results of previous RHIC measurements. They will provide new constraints on Δg(x) in this poorly constrained region when included in future global analyses

Azimuthal Anisotropy in Cu+Au Collisions at √\u3cem\u3es\u3csub\u3eNN\u3c/sub\u3e\u3c/em\u3e = 200 GeV

The azimuthal anisotropic flow of identified and unidentified charged particles has been systematically studied in Cu+Au collisions at √sNN = 200 GeV for harmonics n = 1–4 in the pseudorapidity range |η| \u3c 1. The directed flow in Cu+Au collisions is compared with the rapidity-odd and, for the first time, the rapidity-even components of charged particle directed flow in Au+Au collisions at √sNN = 20 GeV. The slope of the directed flow pseudorapidity dependence in Cu+Au collisions is found to be similar to that in Au+Au collisions, with the intercept shifted toward positive pseudorapidity values, i.e., the Cu-going direction. The mean transverse momentum projected onto the spectator plane ⟨px⟩ in Cu+Au collision also exhibits approximately linear dependence on pseudorapidity with the intercept at about η ≈ −0.4 (shifted from zero in the Au-going direction), closer to the rapidity of the Cu+Au system center of mass. The observed dependencies find a natural explanation in a picture of the directed flow originating partly due the “tilted source” and partly due to the asymmetry in the initial density distribution. A charge dependence of ⟨px⟩ was also observed in Cu+Au collisions, consistent with an effect of the initial electric field created by charge difference of the spectator protons in two colliding nuclei. The rapidity-even component of directed flow in Au+Au collisions is close to that in Pb+Pb collisions at √sNN = 2.76 TeV, indicating a similar magnitude of dipolelike fluctuations in the initial-state density distribution. Higher harmonic flow in Cu+Au collisions exhibits similar trends to those observed in Au+Au and Pb+Pb collisions and is qualitatively reproduced by a viscous hydrodynamic model and a multiphase transport model. For all harmonics with n ≥ 2 we observe an approximate scaling of vn with the number of constituent quarks; this scaling works as well in Cu+Au collisions as it does in Au+Au collisions