Studying the proton spin structure with STAR

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.

In my postdoctoral project I have joined the effort on the STAR spin program with longitudinally polarized proton-proton collisions at the center-of-mass energy of 200 GeV. I concentrate on the inclusive jet analysis, which can further constrain the gluon contribution to the proton spin.

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