Here, we report a systematic measurement of cumulants, Cn, for net-proton, proton, and antiproton multiplicity distributions, and correlation functions, κn, for proton and antiproton multiplicity ...distributions up to the fourth order in Au+Au collisions at √sNN = 7.7, 11.5, 14.5, 19.6, 27, 39, 54.4, 62.4, and 200 GeV. The Cn and κn are presented as a function of collision energy, centrality and kinematic acceptance in rapidity, y, and transverse momentum, pT. The data were taken during the first phase of the Beam Energy Scan (BES) program (2010–2017) at the BNL Relativistic Heavy Ion Collider (RHIC) facility. The measurements are carried out at midrapidity (|y| < 0.5) and transverse momentum 0.4 < pT < 2.0GeV/c, using the STAR detector at RHIC. We observe a nonmonotonic energy dependence (√sNN = 7.7–62.4 GeV) of the net-proton C4/C2 with the significance of 3.1σ for the 0–5% central Au+Au collisions. This is consistent with the expectations of critical fluctuations in a QCD-inspired model. Thermal and transport model calculations show a monotonic variation with √sNN. For the multiparticle correlation functions, we observe significant negative values for a two-particle correlation function, κ2, of protons and antiprotons, which are mainly due to the effects of baryon number conservation. Furthermore, it is found that the four-particle correlation function, κ4, of protons plays a role in determining the energy dependence of proton C4/C1 below 19.6 GeV, which cannot be understood by the effect of baryon number conservation.
Non-monotonic variation with collision energy (√sNN) of the moments of the net-baryon number distribution in heavy-ion collisions, related to the correlation length and the susceptibilities of the ...system, is suggested as a signature for the Quantum Chromodynamics (QCD) critical point. We report the first evidence of a nonmonotonic variation in kurtosis times variance of the net-proton number (proxy for net-baryon number) distribution as a function of √sNN with 3.1σ significance, for head-on (central) gold on-gold (Au+Au) collisions measured using the STAR detector at RHIC. Data in non-central Au+Au collisions and models of heavy-ion collisions without a critical point show a monotonic variation as a function of √sNN.
The first (v$even\atop{1}$), second (v2) and third (v3) harmonic coefficients of the azimuthal particle distribution at mid-rapidity, are extracted for charged hadrons and studied as a function of ...transverse momentum (pT) and mean charged particle multiplicity density $\langle$Nch$\rangle$ in U+U ($\sqrt{s_ {NN}}$ = 193 GeV), Au+Au, Cu+Au, Cu+Cu, d+Au and p+Au collisions at $\sqrt{s_ {NN}}$ = 200 GeV with the STAR Detector. For the same $\langle$Nch$\rangle$, the v$even\atop{1}$ and v3 coefficients are observed to be independent of collision system, while v2 exhibits such a scaling only when normalized by the initial-state eccentricity (ε2). The data also show that ln(v2/ε2) scales linearly with $\langle$Nch$\rangle$-1/3. Finally, these measurements provide insight into initial-geometry fluctuations and the role of viscous hydrodynamic attenuation on vn from small to large collision systems.
The first (v$ ^{fluc} _1 $), second (v2), and third (v3) harmonic coefficients of the azimuthal particle distribution at midrapidity are extracted for charged hadrons and studied as a function of ...transverse momentum (pT) and mean charged particle multiplicity density 〈Nch〉 in U + U (sNN= 193 GeV), Au + Au , Cu + Au , Cu + Cu , d + Au , and p + Au collisions at sNN = 200 GeV with the STAR detector. For the same 〈N ch〉, the v$ ^{fluc} _1 $ and v3 coefficients are observed to be independent of the collision system, while v2 exhibits such a scaling only when normalized by the initial-state eccentricity (ϵ2). The data also show that ln (v2/ϵ2) scales linearly with 〈N ch〉-1/3. These measurements provide insight into initial-geometry fluctuations and the role of viscous hydrodynamic attenuation on vn from small to large collision systems.
We present the first measurement of the proton- correlation function in heavy-ion collisions for central (0-40%) and peripheral (40-80%) Au+Au collisions at $\sqrt{s}$$_{NN}$ =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-correlation function are sensitive to the presence of a nucleon-bound state. These predictions are based on the proton- interaction extracted from (2+1)-avor lattice QCD calculations at the physical point. The measured ratio of proton-correlation function from peripheral (small system) to central (large system) collisions is less than unity for relative momentum smaller than 40 MeV/c. Comparison of our measured correlation ratio with the theoretical calculation slightly favors a proton-Omega bound system with a binding energy of ~ 27 MeV.
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 $\sqrt{s_{NN}} =200 GeV$ 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 proto-Ω bound system with a binding energy of ~ 27 MeV.
We present two-particle pt correlations as a function of event centrality for Au+Au collisions at sNN = 7.7 , 11.5, 14.5, 19.6, 27, 39, 62.4, and 200 GeV at the Relativistic Heavy Ion Collider using ...the STAR detector. These results are compared to previous measurements from CERES at the Super Proton Synchrotron and from ALICE at the Large Hadron Collider. The data are compared with UrQMD model calculations and with a model based on a Boltzmann-Langevin approach incorporating effects from thermalization. The relative dynamical correlations for Au+Au collisions at sNN = 200 GeV show a power-law dependence on the number of participant nucleons and agree with the results for Pb+Pb collisions at sNN = 2.76 TeV from ALICE. As the collision energy is lowered from sNN = 200 to 7.7 GeV, the centrality dependence of the relative dynamical correlations departs from the power-law behavior observed at the higher collision energies. In central collisions, the relative dynamical correlations increase with collision energy up to sNN = 200 GeV in contrast to previous measurements that showed little dependence on the collision energy.
The longitudinal spin transfer $\mathcal{D_{LL}}$ to Λ and Λ ¯ hyperons produced in high-energy polarized proton-proton collisions is projected 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 $\mathcal{D_{LL}}$ from data obtained at a center-of-mass energy of s=200GeV with the STAR detector at RHIC. The data have an approximately twelve times larger figure of merit than prior results and cover |η| < 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 $\mathcal{D_{LL}}$ = -0.036 ± 0.048 (stat ) ± 0.013 (sys) for Λ hyperons and $\mathcal{D_{LL}}$ = 0.032 ± 0.043 (stat) ± 0.013 (sys) for $\bar{Λ}$ antihyperons. The dependences on η and pT are presented and compared with model evaluations.
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