How the spin of a nucleon arises from the spins and orbital angular momenta of quarks and gluons? This fundamental question in hadron physics remains still not completely answered. Experiments with polarized proton-proton collisions can shed light on the proton spin puzzle.
The main goal of the spin program of the STAR experiment at RHIC (Relativistic Heavy Ion Collider) is the investigation on how spin phenomena in quantum chromodynamics arise at the quark and gluon level. One of the key components of this program is the determination of the contribution of gluons and quarks to the spin of the proton.
At the center-of-mass energies available at RHIC, direct access to gluons is possible through several gluon-dominated hard scattering processes such as, e.g, inclusive high-$p_T$ jet production. Using longitudinally polarized beams, one can probe the helicity distributions of gluons in a broad range of momentum fraction $x$, by measuring the so-called longitudinal spin asymmetries. The helicity-dependent parton density encodes to what extent partons with a given momentum fraction $x$ tend to have their spins aligned with the spin direction of a longitudinally polarized nucleon. It is essential for the understanding of the internal structure of hadronic matter, but also it directly addresses the question of the origin of the nucleon spin.
I joined the STAR collaboration in 2018, and since then I’ve been focusing my efforts on studying the spin structure of the proton with jets. In our paper on longitudinal double-spin asymmetries for inclusive jets and dijets, Phys. Rev. D 103, L091103, we present the new results from the most recent and precise 200 GeV STAR longitudinal data in the central region of pseudorapidity. The data conclude the 200 GeV longitudinally polarized program at RHIC and will provide leading constraints on the gluon helicity distribution for gluon fractional momenta x > 0.05 in perturbative-QCD analyses of world data well into the future.
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