Gluon polarization measurements from longitudinally polarized proton-proton collisions at STAR

The gluon polarization contribution to the proton spin is an integral part to solve the longstanding proton spin puzzle. At the Relativistic Heavy Ion Collider (RHIC), the STAR experiment has measured jets produced in mid-pseudo-rapidity, |η| < 1.0, and full azimuth, ϕ, from longitudinally polarized pp collisions to study the gluon polarization in the proton. At center of mass energies s = 200 and 510 GeV, jet production is dominated by hard QCD scattering processes such as gluon-gluon (gg) and quark-gluon (qg), thus making the longitudinal double-spin asymmetry (ALL ) sensitive to the gluon polarization. Early STAR inclusive jet ALL results at s = 200 GeV provided the first evidence of the non-zero gluon polarization at momentum fraction x > 0.05. The higher center of mass energy s = 510 GeV allows to explore the gluon polarization as low as x ∼ 0.015. In this talk we will present the recent STAR inclusive jet and dijet ALL results at s = 510 GeV, and discuss the relevant new analysis techniques for the estimation of trigger bias and reconstruction uncertainty, the underlying event correction on the jet energy and its effect on jet ALL . Dijet results are shown for different topologies in regions of pseudo-rapidity, effectively scanning the x-dependence of the gluon polarization.

Thermal corrections to the gluon magnetic Debye mass

We compute the gluon polarization tensor in a thermo-magnetic environment in thestrong magnetic field limit at zero and high temperature. The magnetic field effects are introduced using Schwinger's proper time method. Thermal effects are computed in the HTL approximation. At zero temperature, we reproduce the well-known result whereby for a non-vanishing quark mass, the polarization tensor reduces to the parallel structure and its coefficient develops an imaginary part corresponding to the threshold for quark-antiquark pair production. This coefficient is infrared finite and simplifies considerably when the quark mass vanishes. Keeping always the field strength as the largest energy scale, in the high temperature regime we analyze two complementary hierarchies of scales: $q^2<< m_f^2<< T^2$ and $m_f^2<< q^2<< T^2$. In the latter, we show that the polarization tensor is infrared finite as $m_f$ goes to zero. In the former, we discuss the thermal corrections to the magnetic Debye mass.

Studies of gluon TMDs and their evolution using quarkonium-pair production at the LHC

$$J/\psi $$J/ψ- or $$\Upsilon $$Υ-pair production at the LHC are promising processes to study the gluon transverse momentum distributions (TMDs) which remain very poorly known. In this article, we improve on previous results by including the TMD evolution in the computation of the observables such as the pair-transverse-momentum spectrum and asymmetries arising from the linear polarization of gluons inside unpolarized protons. We show that the azimuthal asymmetries generated by the gluon polarization are reduced compared to the tree level case but are still of measurable size (in the 5–10% range). Such asymmetries should be measurable in the available data sets of $$J/\psi $$J/ψ pairs and in the future data sets of the high-luminosity LHC for $$\Upsilon $$Υ pairs.

Double Helicity Asymmetries of Forward Neutral Pions from s = 510 GeV pp Collisions at STAR

Longitudinally polarized [Formula: see text] scattering experiments provide access to gluon polarization via measurement of the double helicity asymmetry, [Formula: see text]. At the completion of the 2013 RHIC running period, a significant dataset of [Formula: see text]s corresponding to an integrated luminosity of 46 [Formula: see text] (2012) and 8 [Formula: see text] (2013) produced from polarized [Formula: see text] scattering at [Formula: see text] GeV with an average beam polarization of approximately [Formula: see text] was acquired. The [Formula: see text] kinematics were measured via isolation cones by the STAR Forward Meson Spectrometer, an electromagnetic calorimeter covering a forward pseudorapidity range of [Formula: see text]. The asymmetric [Formula: see text] subprocess becomes more dominant in this forward region than in the midrapidity region; furthermore, asymmetry measurements in the forward region are sensitive to low-[Formula: see text] gluons. Progress on [Formula: see text] determined from forward [Formula: see text] events, complementing previous midrapidity measurements, are presented.

Gluon Polarization in Longitudinally Polarized pp Collisions at STAR

The STAR Collaboration is performing a wide range of measurements to determine the gluon helicity distribution in the proton. Gluon-gluon and quark-gluon scattering dominate jet production in proton-proton collisions at RHIC, so the longitudinal double-spin asymmetry, [Formula: see text], for jet production places significant constraints on the gluon polarization in the proton. In recent years STAR has recorded large longitudinally polarized [Formula: see text] data sets at both [Formula: see text] GeV and [Formula: see text] GeV. The 2009 STAR inclusive jet [Formula: see text] measurements at [Formula: see text] GeV show the first experimental evidence of non-zero gluon polarization over the Bjorken-[Formula: see text] range, [Formula: see text]. Furthermore, data collected at [Formula: see text] GeV during the 2012 and 2013 RHIC runs allow access to the gluon polarization at lower [Formula: see text]. In this talk, I will present the final results of the 2009 inclusive jet [Formula: see text] measurement at [Formula: see text] GeV, the analysis status of the 2012 inclusive jet [Formula: see text] measurement at [Formula: see text] GeV, and the status of di-jet and other STAR measurements that are sensitive to gluon polarization.