Search for: All records

We report a new measurement of transverse single-spin asymmetries for dijet production in collisions of polarized protons at $\sqrt{s}=200\mathrm{GeV}$. Correlations between the proton spin and the transverse momenta of its partons, each perpendicular to the proton momentum direction, are probed at high $Q^2\approx 160{\mathrm{GeV}}^2$. Evidence for nonzero Sivers effects is measured for the first time in dijets from proton-proton collisions, but only when the jets are sorted by their net charge, which enhances the otherwise canceling opposite-sign $u$- or $d$-quark contributions to separate data samples. The resulting asymmetries are compared to recent theoretical calculations. Separately, the associated Sivers observable $⟨k_T⟩$, the average parton transverse momentum, is extracted using a simple kinematics approach which further enables a determination of the individual partonic contributions to the observed asymmetries. 

Abstract In a Quark-Gluon Plasma (QGP), the fundamental building blocks of matter, quarks and gluons, are under extreme conditions of temperature and density. A QGP could exist in the early stages of the Universe, and in various objects and events in the cosmos. The thermodynamic and hydrodynamic properties of the QGP are described by Quantum Chromodynamics (QCD) and can be studied in heavy-ion collisions. Despite being a key thermodynamic parameter, the QGP temperature is still poorly known. Thermal lepton pairs (e⁺e⁻andμ⁺μ⁻) are ideal penetrating probes of the true temperature of the emitting source, since their invariant-mass spectra suffer neither from strong final-state interactions nor from blue-shift effects due to rapid expansion. Here we measure the QGP temperature using thermale⁺e⁻production at the Relativistic Heavy Ion Collider (RHIC). The average temperature from the low-mass region (in-mediumρ⁰vector-meson dominant) is (2.01 ± 0.23) × 10¹²K, consistent with the chemical freeze-out temperature from statistical models and the phase transition temperature from Lattice QCD. The average temperature from the intermediate mass region (above theρ⁰mass, QGP dominant) is significantly higher at (3.25 ± 0.60) × 10¹²K. This work provides essential experimental thermodynamic measurements to map out the QCD phase diagram and understand the properties of matter under extreme conditions. 

The STAR experiment reports new, high-precision measurements of the transverse single-spin asymmetries for ${\pi }^{\pm }$within jets, namely the Collins asymmetries, from transversely polarized $p^{\uparrow }p$collisions at $\sqrt{s}=510\mathrm{GeV}$. The energy-scaled distribution of jet transverse momentum, $x_{\mathrm{T}}=2p_{\mathrm{T},\text{jet}}/\sqrt{s}$, shows a remarkable consistency for Collins asymmetries of ${\pi }^{\pm }$in jets between $\sqrt{s}=200\mathrm{GeV}$and 510 GeV. This indicates that the Collins asymmetries are nearly energy independent, with, at most, a very weak scale dependence in $p^{\uparrow }p$collisions. These results extend to high-momentum scales ( $Q^2\le 3400{\mathrm{GeV}}^2$) and enable unique tests of evolution and universality in the transverse-momentum-dependent formalism, thus providing important constraints for the Collins fragmentation functions.