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.
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 first measurements of e+e- pair production in the mass region 0.4 between 40 and 60 MeV/c. The absolute cross section of the excess depends weakly on centrality, while those from a ...theoretical model calculation incorporating an in-medium broadened ρ spectral function and radiation from a quark gluon plasma or hadronic cocktail contributions increase dramatically with an increasing number of participant nucleons. Model calculations of photon-photon interactions generated by the initial projectile and target nuclei describe the observed excess yields but fail to reproduce the p$\mathbb 2\atop{T}$ distributions.
The STAR Collaboration reports measurements of the longitudinal double-spin asymmetry, ALL, for neutral pions produced at forward directions in polarized proton-proton collisions, at a center-of-mass ...energy of 510 GeV. Results are given for transverse momenta in the range 2
We present the first measurements of the longitudinal double-spin asymmetry ALL for dijets with at least one jet reconstructed within the pseudorapidity range 0.8<η<1.8. The dijets were measured in ...polarized pp collisions at a center-of-mass energy s=200 GeV. Values for ALL are determined for several distinct event topologies, defined by the jet pseudorapidities, and span a range of parton momentum fraction x down to x~0.01. The measured asymmetries are found to be consistent with the predictions of global analyses that incorporate the results of previous RHIC measurements. They will provide new constraints on Δg(x) in this poorly constrained region when included in future global analyses.
Here, rapidity-odd directed flow measurements at midrapidity are presented for Λ, $\bar{Λ}$, K±, K$0\atop{s}$ and Φ at √sNN = 7.7, 11.5, 14.5, 19.6, 27, 39, 62.4 and 200 GeV in Au+Au collisions ...recorded by the STAR detector at the Relativistic Heavy Ion Collider. These measurements greatly expand the scope of data available to constrain models with differing prescriptions for the equation of state of quantum chromodynamics. Results show good sensitivity for testing a picture where flow is assumed to be imposed before hadron formation and the observed particles are assumed to form via coalescence of 3 constituent quarks. Lastly, the pattern of departure from a coalescence-inspired sum-rule can be a valuable new tool for probing the collision dynamics.
We report on the first measurement of charm-strange meson $D_s^{\pm}$ production at midrapidity in Au+Au collisions at $\sqrt{s_{_{\rm NN}}}$ = 200 GeV from the STAR experiment. The yield ratio ...between strange ($D_{s}^{\pm}$) and non-strange ($D^{0}$) open-charm mesons is presented and compared to model calculations. A significant enhancement, relative to a PYTHIA simulation of $p$+$p$ collisions, is observed in the $D_{s}^{\pm}/D^0$ yield ratio in Au+Au collisions over a large range of collision centralities. Model calculations incorporating abundant strange-quark production in the quark-gluon plasma (QGP) and coalescence hadronization qualitatively reproduce the data. The transverse-momentum integrated yield ratio of $D_{s}^{\pm}/D^0$ at midrapidity is consistent with a prediction from a statistical hadronization model with the parameters constrained by the yields of light and strange hadrons measured at the same collision energy. These results suggest that the coalescence of charm quarks with strange quarks in the QGP plays an important role in $D_{s}^{\pm}$ meson production in heavy-ion collisions.
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