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.
The fundamental building blocks of the proton-quarks and gluons-have been known for decades. However, we still have an incomplete theoretical and experimental understanding of how these particles and ...their dynamics give rise to the quantum bound state of the proton and its physical properties, such as its spin
. The two up quarks and the single down quark that comprise the proton in the simplest picture account only for a few per cent of the proton mass, the bulk of which is in the form of quark kinetic and potential energy and gluon energy from the strong force
. An essential feature of this force, as described by quantum chromodynamics, is its ability to create matter-antimatter quark pairs inside the proton that exist only for a very short time. Their fleeting existence makes the antimatter quarks within protons difficult to study, but their existence is discernible in reactions in which a matter-antimatter quark pair annihilates. In this picture of quark-antiquark creation by the strong force, the probability distributions as a function of momentum for the presence of up and down antimatter quarks should be nearly identical, given that their masses are very similar and small compared to the mass of the proton
. Here we provide evidence from muon pair production measurements that these distributions are considerably different, with more abundant down antimatter quarks than up antimatter quarks over a wide range of momenta. These results are expected to revive interest in several proposed mechanisms for the origin of this antimatter asymmetry in the proton that had been disfavoured by previous results
, and point to future measurements that can distinguish between these mechanisms.
Quark interactions with topological gluon configurations can induce local chirality imbalance and parity violation in quantum chromodynamics, which can lead to the chiral magnetic effect (CME)—an ...electric charge separation along the strong magnetic field in relativistic heavy-ion collisions. The CME-sensitive azimuthal correlator observable ( Δγ) is contaminated by background arising, in part, from resonance decays coupled with elliptic anisotropy (v2) . We report here differential measurements of the correlator as a function of the pair invariant mass (minv) in 20–50% centrality Au + Au collisions at $\sqrt{s_{NN}}$ = 200 GeV by the STAR experiment at the BNL Relativistic Heavy Ion Collider. Strong resonance background contributions to Δγ are observed. At large minv where this background is significantly reduced, the Δγ value is found to be significantly smaller. An event-shape-engineering technique is deployed to determine the v2 background shape as a function of minv . We extract a v2-independent and minv -averaged signal Δγsig = (0.03 ± 0.06 ± 0.08 ) × 10 -4, or (2 ± 4 ± 5)% of the inclusive Δγ(minv > 0.4 GeV/c2) = ( 1.58 ± 0.02 ± 0.02 ) × 10-4, within pion pT = 0.2 –0.8 GeV/c and averaged over pseudorapidity ranges of -1 < η < - 0.05 and 0.05 < η < 1. This represents an upper limit of 0.23 × 10 -4 , or 15% of the inclusive result, at 95% confidence level for the minv -integrated CME contribution.
Here we present high-precision measurements of elliptic, triangular, and quadrangular flow v2, v3, and v4, respectively, at midrapidity for identified hadrons π, p, K, φ, Ks, Λ as a function of ...centrality and transverse momentum in Au+Au collisions at the center-of-mass energy √sNN = 200 GeV. We observe similar vn trends between light and strange mesons which indicates that the heavier strange quarks flow as strongly as the lighter up and down quarks. The number-of-constituent-quark scaling for v2, v3, and v4 is found to hold within statistical uncertainty for 0–10%, 10–40%, and 40–80% collision centrality intervals. The results are compared to several viscous hydrodynamic calculations with varying initial conditions, and could serve as an additional constraint to the development of hydrodynamic models.
Evidence for a flavor asymmetry between the $\bar{u}$ and $\bar{d}$ quark distributions in the proton has been found in deep-inelastic scattering and DrellYan experiments. The pronounced dependence ...of this flavor asymmetry on x (fraction of nucleon momentum carried by partons) observed in the Fermilab E866 Drell-Yan experiment suggested a drop of the $\bar{d}$(x)/$\bar{u}$(x) ratio in the x > 0.15 region. We report results from the SeaQuest Fermilab E906 experiment with improved statistical precision for $\bar{d}$(x)/$\bar{u}$(x) in the large x region up to x = 0.45 using the 120 GeV proton beam. Two different methods for extracting the Drell-Yan cross section ratios, σpd/2σpp, from the SeaQuest data give consistent results. The $\bar{d}$(x)/$\bar{u}$(x) ratios and the $\bar{d}$(x)- $\bar{u}$(x) differences are deduced from these cross section ratios for 0.13 < x < 0.45. The SeaQuest and E866/NuSea $\bar{d}$(x)/$\bar{u}$(x) ratios are in good agreement for the x≲0.25 region. The new SeaQuest data, however, show that $\bar{d}$(x) continues to be greater than $\bar{u}$(x) up to the highest x value (x = 0.45). The new results on $\bar{d}$(x)/$\bar{u}$(x) and $\bar{d}$(x)-$\bar{u}$(x) are compared with various parton distribution functions and theoretical calculations.
Notwithstanding decades of progress since Yukawa first developed a description of the force between nucleons in terms of meson exchange1, a full understanding of the strong interaction remains a ...considerable challenge in modern science. One remaining difficulty arises from the non-perturbative nature of the strong force, which leads to the phenomenon of quark confinement at distances on the order of the size of the proton. Here we show that, in relativistic heavy-ion collisions, in which quarks and gluons are set free over an extended volume, two species of produced vector (spin-1) mesons, namely ф and K·0, emerge with a surprising pattern of global spin alignment. In particular, the global spin alignment for ф is unexpectedly large, whereas that for K·0 is consistent with zero. The observed spin-alignment pattern and magnitude for ф cannot be explained by conventional mechanisms, whereas a model with a connection to strong force fields2-6, that is, an effective proxy description within the standard model and quantum chromodynamics, accommodates the current data. This connection, if fully established, will open a potential new avenue for studying the behaviour of strong force fields.
We report the first multi-differential measurements of strange hadrons of K-, φ and Ξ - yields as well as the ratios of φ/K-and φ/Ξ - in Au+Au collisions at √ sNN = 3 GeV with the STAR experiment ...fixed target configuration at RHIC. The φ mesons and Ξ - hyperons are measured through hadronic decay channels, φ → K+K- and Ξ - → Λπ -. Collision centrality and rapidity dependence of the transverse momentum 3 spectra for these strange hadrons are presented. The 4π yields and ratios are compared to thermal model and hadronic transport model predictions. At this collision energy, thermal model with grand canonical ensemble (GCE) under-predicts the φ/K- and φ/Ξ - ratios while the result of canonical ensemble (CE) calculations reproduce φ/K-, with the correlation length rc ~ 2.7 fm, and φ/Ξ -, rc ~ 4.2 fm, for the 0-10% central collisions. Hadronic transport models including high mass resonance decays could also describe the ratios. While thermal calculations with GCE work well for strangeness production in high energy collisions, the change to CE at 3 GeV implies a rather different medium property at high baryon density.
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