Nonmonotonic variation with collision energy (sqrts_{NN}) 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 critical point. We report the first evidence of a nonmonotonic variation in the kurtosis times variance of the net-proton number (proxy for net-baryon number) distribution as a function of sqrts_{NN} with 3.1 σ significance for head-on (central) gold-on-gold (Au+Au) collisions measured solenoidal tracker at Relativistic Heavy Ion Collider. Data in noncentral Au+Au collisions and models of heavy-ion collisions without a critical point show a monotonic variation as a function of sqrts_{NN}.
We present measurements of bulk properties of the matter produced in Au+Au collisions at $\sqrt{s}$$_ {NN}$= 7.7, 11.5, 19.6, 27, and 39 GeV using identified hadrons (π±, K±, p, and $\bar{p}$) from ...the STAR experiment in the Beam Energy Scan (BES) Program at the Relativistic Heavy Ion Collider (RHIC). Midrapidity (| y | < 0.1) results for multiplicity densities dN / dy, average transverse momenta $\langle$pT$\rangle$, and particle ratios are presented. The chemical and kinetic freeze-out dynamics at these energies are discussed and presented as a function of collision centrality and energy. These results constitute the systematic measurements of bulk properties of matter formed in heavy-ion collisions over a broad range of energy (or baryon chemical potential) at RHIC.
Fluctuations of conserved quantities such as baryon number, charge, and strangeness are sensitive to the correlation length of the hot and dense matter created in relativistic heavy-ion collisions ...and can be used to search for the QCD critical point. We report the first measurements of the moments of net-kaon multiplicity distributions in Au+Au collisions at sNN=7.7, 11.5, 14.5, 19.6, 27, 39, 62.4, and 200 GeV. The collision centrality and energy dependence of the mean (M), variance (σ2), skewness (S), and kurtosis (κ) for net-kaon multiplicity distributions as well as the ratio σ2/M and the products Sσ and κσ2 are presented. Comparisons are made with Poisson and negative binomial baseline calculations as well as with UrQMD, a transport model (UrQMD) that does not include effects from the QCD critical point. Within current uncertainties, the net-kaon cumulant ratios appear to be monotonic as a function of collision energy.
In this letter, measurements of the shared momentum fraction (zg) and the groomed jet radius (Rg), as defined in the SoftDrop algorithm, are reported in p+p collisions at s=200 GeV collected by the ...STAR experiment. These substructure observables are differentially measured for jets of varying resolution parameters from R=0.2−0.6 in the transverse momentum range 15<pT,jet<60 GeV/c. These studies show that, in the pT,jet range accessible at s=200 GeV and with increasing jet resolution parameter and jet transverse momentum, the zg distribution asymptotically converges to the DGLAP splitting kernel for a quark radiating a gluon. The groomed jet radius measurements reflect a momentum-dependent narrowing of the jet structure for jets of a given resolution parameter, i.e., the larger the pT,jet, the narrower the first splitting. For the first time, these fully corrected measurements are compared to Monte Carlo generators with leading order QCD matrix elements and leading log in the parton shower, and to state-of-the-art theoretical calculations at next-to-leading-log accuracy. We observe that PYTHIA 6 with parameters tuned to reproduce RHIC measurements is able to quantitatively describe data, whereas PYTHIA 8 and HERWIG 7, tuned to reproduce LHC data, are unable to provide a simultaneous description of both zg and Rg, resulting in opportunities for fine parameter tuning of these models for p+p collisions at RHIC energies. We also find that the theoretical calculations without non-perturbative corrections are able to qualitatively describe the trend in data for jets of large resolution parameters at high pT,jet, but fail at small jet resolution parameters and low jet transverse momenta.
Dihadron angular correlations in d+Au collisions at sNN=200 GeV are reported as a function of the measured zero-degree calorimeter neutral energy and the forward charged hadron multiplicity in the ...Au-beam direction. A finite correlated yield is observed at large relative pseudorapidity (Δη) on the near side (i.e. relative azimuth Δϕ∼0). This correlated yield as a function of Δη appears to scale with the dominant, primarily jet-related, away-side (Δϕ∼π) yield. The Fourier coefficients of the Δϕ correlation, Vn=〈cosnΔϕ〉, have a strong Δη dependence. In addition, it is found that V1 is approximately inversely proportional to the mid-rapidity event multiplicity, while V2 is independent of it with similar magnitude in the forward (d-going) and backward (Au-going) directions.
J/ψ suppression has long been considered a sensitive signature of the formation of the Quark-Gluon Plasma (QGP) in relativistic heavy-ion collisions. In this letter, we present the first measurement ...of inclusive J/ψ production at mid-rapidity through the dimuon decay channel in Au+Au collisions at sNN=200 GeV with the STAR experiment. These measurements became possible after the installation of the Muon Telescope Detector was completed in 2014. The J/ψ yields are measured in a wide transverse momentum (pT) range of 0.15 GeV/c to 12 GeV/c from central to peripheral collisions. They extend the kinematic reach of previous measurements at RHIC with improved precision. In the 0-10% most central collisions, the J/ψ yield is suppressed by a factor of approximately 3 for pT>5 GeV/c relative to that in p+p collisions scaled by the number of binary nucleon-nucleon collisions. The J/ψ nuclear modification factor displays little dependence on pT in all centrality bins. Model calculations can qualitatively describe the data, providing further evidence for the color-screening effect experienced by J/ψ mesons in the QGP.
We report the direct virtual photon invariant yields in the transverse momentum ranges 1<pT<3 GeV/c and 5<pT<10 GeV/c at mid-rapidity derived from the dielectron invariant mass continuum region ...0.10<Mee<0.28 GeV/c2 for 0–80% minimum-bias Au+Au collisions at sNN=200 GeV. A clear excess in the invariant yield compared to the nuclear overlap function TAA scaled p+p reference is observed in the pT range 1<pT<3 GeV/c. For pT>6 GeV/c the production follows TAA scaling. Model calculations with contributions from thermal radiation and initial hard parton scattering are consistent within uncertainties with the direct virtual photon invariant yield.
Azimuthal correlations of charged hadrons with direct-photon (γdir) and neutral-pion (π0) trigger particles are analyzed in central Au+Au and minimum-bias p+p collisions at sNN=200 GeV in the STAR ...experiment. The charged-hadron per-trigger yields at mid-rapidity from central Au+Au collisions are compared with p+p collisions to quantify the suppression in Au+Au collisions. The suppression of the away-side associated-particle yields per γdir trigger is independent of the transverse momentum of the trigger particle (pTtrig), whereas the suppression is smaller at low transverse momentum of the associated charged hadrons (pTassoc). Within uncertainty, similar levels of suppression are observed for γdir and π0 triggers as a function of zT (≡pTassoc/pTtrig). The results are compared with energy-loss-inspired theoretical model predictions. Our studies support previous conclusions that the lost energy reappears predominantly at low transverse momentum, regardless of the trigger energy.
Here, we present measurements of three-particle correlations for various harmonics in Au+Au collisions at energies ranging from √sNN=7.7 to 200 GeV using the STAR detector. The quantity < ...cos(mΦ1+nΦ2–(m+n)Φ3) >, with Φ being the azimuthal angles of the particles is evaluated as a function of √sNN, collision centrality, transverse momentum, pT, pseudorapidity difference, Δη, and harmonics (m and n). These data provide detailed information on global event properties such as the three-dimensional structure of the initial overlap region, the expansion dynamics of the matter produced in the collisions, and the transport properties of the medium. A strong dependence on Δη is observed for most harmonic combinations, which is consistent with breaking of longitudinal boost invariance. An interesting energy dependence is observed when one of the harmonics m,n, or m+n is equal to two, for which the correlators are dominated by the two-particle correlations relative to the second-harmonic event plane. These measurements can be used to constrain models of heavy-ion collisions over a wide range of temperature and baryon chemical potential.
We report the first measurements of transverse single-spin asymmetries for inclusive jet and jet+π± production at midrapidity from transversely polarized proton-proton collisions at s=500 GeV. The ...data were collected in 2011 with the STAR detector sampled from 23 pb−1 integrated luminosity with an average beam polarization of 53%. Asymmetries are reported for jets with transverse momenta 6<pT<55 GeV/c and pseudorapidity |η|<1. Presented are measurements of the inclusive-jet azimuthal transverse single-spin asymmetry, sensitive to twist-3 initial-state quark-gluon correlators; the Collins asymmetry, sensitive to quark transversity coupled to the polarized Collins fragmentation function; and the first measurement of the “Collins-like” asymmetry, sensitive to linearly polarized gluons. Within the present statistical precision, inclusive-jet and Collins-like asymmetries are small, with the latter allowing the first experimental constraints on gluon linear polarization in a polarized proton. At higher values of jet transverse momenta, we observe the first nonzero Collins asymmetries in polarized-proton collisions, with a statistical significance of greater than 5σ. The results span a range of x similar to results from semi-inclusive deep-inelastic scattering but at much higher Q2. The Collins results enable tests of universality and factorization breaking in the transverse momentum-dependent formulation of perturbative quantum chromodynamics.
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