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.
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.
The longitudinal and transverse spin transfers to Λ ($\overline{Λ}$) hyperons in polarized proton-proton collisions are expected to be sensitive to the helicity and transversity distributions, ...respectively, of (anti)strange quarks in the proton, and to the corresponding polarized fragmentation functions. We report improved measurements of the longitudinal spin transfer coefficient, DLL, and the transverse spin transfer coefficient, DTT, to Λ and $\overline{Λ}$ in polarized proton-proton collisions at √s = 200 GeV by the STAR experiment at RHIC. The dataset includes longitudinally polarized proton-proton collisions with an integrated luminosity of 52 pb–1, and transversely polarized proton-proton collisions with a similar integrated luminosity. Both datasets have about twice the statistics of previous results and cover a kinematic range of |ηΛ($\overline{Λ}$)| < 1.2 and transverse momentum pT,Λ($\overline{Λ}$) up to 8GeV/c. We also report the first measurements of the hyperon spin transfer coefficients DLL and DTT as a function of the fractional jet momentum z carried by the hyperon, which can provide more direct constraints on the polarized fragmentation functions
Particle production sensitive to nonfactorizable and nonperturbative processes that contribute to the underlying event associated with a high transverse momentum ( pT) jet in proton + proton ...collisions at √s = 200 GeV is studied with the STAR detector. Each event is divided into three regions based on the azimuthal angle with respect to the highest- p T jet direction: in the leading jet direction ("Toward"), opposite to the leading jet ("Away"), and perpendicular to the leading jet ("Transverse"). In the Transverse region, the average charged particle density is found to be between 0.4 and 0.6 and the mean transverse momentum, ⟨pT⟩, between 0.5 and 0.7 GeV/c for particles with pT > 0.2 GeV/c at mid-pseudorapidity (|η| < 1) and jet pT > 15 GeV / c . Both average particle density and ⟨pT⟩depend weakly on the leading jet pT. Closer inspection of the Transverse region hints that contributions to the underlying event from initial- and final-state radiation are significantly smaller in these collisions than at the higher energies, up to 13 TeV, recorded at the LHC. Underlying event measurements associated with a high-pT jet will contribute to our understanding of QCD processes at hard and soft scales at RHIC energies, as well as provide constraints to modeling of underlying event dynamics.
We present measurements of the differential cross sections of inclusive J/ψ meson production as a function of transverse momentum (pTJ/ψ) using the μ+μ− and e+e− decay channels in proton+proton ...collisions at center-of-mass energies of 510 and 500 GeV, respectively, recorded by the STAR detector at the Relativistic Heavy Ion Collider. The measurement from the μ+μ− channel is for 0<pTJ/ψ<9 GeV/c and rapidity range |yJ/ψ|<0.4, and that from the e+e− channel is for 4<pTJ/ψ<20 GeV/c and |yJ/ψ|<1.0. The ψ(2S) to J/ψ ratio is also measured for 4<pTmeson<12 GeV/c through the e+e− decay channel. Model calculations, which incorporate different approaches toward the J/ψ production mechanism, are compared with experimental results and show reasonable agreement within uncertainties.
The STAR collaboration reports a measurement of the transverse single-spin asymmetries, AN , for neutral pions produced in polarized proton collisions with protons ( pp ), with aluminum nuclei ( pAl ...) and with gold nuclei ( pAu ) at a nucleon-nucleon center-of-mass energy of 200 GeV. Neutral pions are observed in the forward direction relative to the transversely polarized proton beam, in the pseudorapidity region 2.7<η<3.8 . Results are presented for π0 s observed in the STAR forward meson spectrometer electromagnetic calorimeter in narrow Feynman x ( xF ) and transverse momentum ( pT ) bins, spanning the range 0.17<xF<0.81 and 1.7<pT<6.0 GeV/c . For fixed xF<0.47 , the asymmetries are found to rise with increasing transverse momentum. For larger xF , the asymmetry flattens or falls as pT increases. Parametrizing the ratio r ( A ) ≡AN ( pA ) / AN ( pp ) =AP over the kinematic range, the ratio r ( A ) is found to depend only weakly on A , with ( P ) =-0.027±0.005 . No significant difference in P is observed between the low- pT region, pT<2.5 GeV/c , where gluon saturation effects may play a role, and the high- pT region, pT>2.5 GeV/c . It is further observed that the value of AN is significantly larger for events with a large- pT isolated π0 than for events with a nonisolated π0 accompanied by additional jetlike fragments. The nuclear dependence r ( A ) is similar for isolated and nonisolated π0 events.
Neutrinoless double-beta (0vββ) decay is a hypothesized lepton-number-violating process that offers the only known means of asserting the possible Majorana nature of neutrino mass. The Cryogenic ...Underground Observatory for Rare Events (CUORE) is an upcoming experiment designed to search for 0vββ decay of sup.130Te using an array of 988 TeO.sub.2 crystal bolometers operated at 10 mK. The detector will contain 206 kg of sup.130Te and have an average energy resolution of 5 keV; the projected 0vββ decay half-life sensitivity after five years of livetime is 1.6 x 10.sup.26 γ at 1σ (9.5 x 10.sup.25 γ at the 90% confidence level), which corresponds to an upper limit on the effective Majorana mass in the range 40-100 meV (50-130 meV). In this paper, we review the experimental techniques used in CUORE as well as its current status and anticipated physics reach.
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| < 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.
We report on new measurements of inclusive J / ψ polarization at midrapidity in p + p collisions at √s = 200 GeV by the STAR experiment at the Relativistic Heavy Ion Collider. The polarization ...parameters, λθ, λϕ, and λθϕ, are measured as a function of transverse momentum ( pT ) in both the helicity and Collins-Soper (CS) reference frames within pT < 10 GeV / c. Except for λθ in the CS frame at the highest measured pT, all three polarization parameters are consistent with 0 in both reference frames without any strong pT dependence. Several model calculations are compared with data, and the one using the Color Glass Condensate effective field theory coupled with nonrelativistic QCD gives the best overall description of the experimental results, even though other models cannot be ruled out due to experimental uncertainties.
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