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

Beam Energy Dependence of Moments of the Net-Charge Multiplicity Distributions in Au + Au Collisions at RHIC

We report the first measurements of the moments-mean (M), variance (σ2), skewness (S), and kurtosis (κ)-of the net-charge multiplicity distributions at midrapidity in Au + Au collisions at seven energies, ranging from √ˢᶰᶰ = 7.7 to 200 GeV, as a part of the Beam Energy Scan program at RHIC. The moments are related to the thermodynamic susceptibilities of net charge, and are sensitive to the location of the QCD critical point. We compare the products of the moments, σ2/M, Sσ, and κσ2, with the expectations from Poisson and negative binomial distributions (NBDs). The Sσ values deviate from the Poisson baseline and are close to the NBD baseline, while the κσ2 values tend to lie between the two. Within the present uncertainties, our data do not show nonmonotonic behavior as a function of collision energy. These measurements provide a valuable tool to extract the freeze-out parameters in heavy-ion collisions by comparing with theoretical models. © American Physical Societ

Dielectron Mass Spectra from Au plus Au Collisions at √ˢᴺᴺ = 200 GeV

We report the STAR measurements of dielectron (e+e-) production at midrapidity ( |yee| \u3c 1) in Au + Au collisions at √ˢᴺᴺ = 200 GeV. The measurements are evaluated in different invariant mass regions with a focus on 0.30-0.76 (ρ-like), 0.76-0.80 (ω-like), and 0.98-1.05 (ɸ-like) GeV/c2. The spectrum in the ω-like and ɸ-like regions can be well described by the hadronic cocktail simulation. In the ρ-like region, however, the vacuum ρ spectral function cannot describe the shape of the dielectron excess. In this range, an enhancement of 1.77 ±0.11(stat) ± 0.24(syst) ± 0.33(cocktail) is determined with respect to the hadronic cocktail simulation that excludes the ρ meson. The excess yield in the ρ-like region increases with the number of collision participants faster than the ω and ɸ yields. Theoretical models with broadened rho contributions through interactions with constituents in the hot QCD medium provide a consistent description of the dilepton mass spectra for the measurement presented here and the earlier data at the Super Proton Synchrotron energies

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

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

Fluctuations of Charge Separation Perpendicular to the Event Plane and Local Parity Violation in √ˢᴺᴺ = 200 GeV Au + Au Collisions at the BNL Relativistic Heavy Ion Collider

Previous experimental results based on data (~ 15 x 106 events) collected by the STAR detector at the BNL Relativistic Heavy Ion Collider suggest event-by-event charge-separation fluctuations perpendicular to the event plane in noncentral heavy-ion collisions. Here we present the correlator previously used split into its two component parts to reveal correlations parallel and perpendicular to the event plane. The results are from a high-statistics 200-GeV Au + Au collisions data set (57 x 106 events) collected by the STAR experiment. We explicitly count units of charge separation from which we find clear evidence for more charge-separation fluctuations perpendicular than parallel to the event plane. We also employ a modified correlator to study the possible P-even background in same- and opposite-charge correlations, and find that the P-even background may largely be explained by momentum conservation and collective motion

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

Measurement of Longitudinal Spin Asymmetries for Weak Boson Production in Polarized Proton-Proton Collisions at RHIC

We report measurements of single- and double-spin asymmetries for W± and Z/ɣ* boson production in longitudinally polarized p + p collisions at √s = 510 GeV by the STAR experiment at RHIC. The asymmetries for W± were measured as a function of the decay lepton pseudorapidity, which provides a theoretically clean probe of the proton\u27s polarized quark distributions at the scale of the W mass. The results are compared to theoretical predictions, constrained by polarized deep inelastic scattering measurements, and show a preference for a sizable, positive up antiquark polarization in the range 0.05 \u3c x \u3c 0.2

Measurement of Charge Multiplicity Asymmetry Correlations in High-Energy Nucleus-Nucleus Collisions at √ˢᶰᶰ = 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 √ˢᶰᶰ = 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 vobs2, where Δ = [ 1.3 ± 1.4(stat) +4.0 -1.0(syst)] x 10-5 + [3.2 ± 0.2(stat) +0.4-0.3 (syst)] x 10-3 vobs2 for 20- 40% Au + Au collisions. The implications for the proposed chiral magnetic effect are discussed

ΛΛ Correlation Function in Au + Au Collisions at √s(NN)=200 GeV

We present ΛΛ correlation measurements in heavy-ion collisions for Au + Au collisions at √sNN = 200 GeV using the STAR experiment at the Relativistic Heavy-Ion Collider. The Lednicky-Lyuboshitz analytical model has been used to fit the data to obtain a source size, a scattering length and an effective range. Implications of the measurement of the ΛΛ correlation function and interaction parameters for dihyperon searches are discussed