I am a part of the STAR collaboration at Brookhaven National Lab (BNL). STAR is the Solenoidal Tracker At RHIC, and RHIC is the Relativistic Heavy Ion Collider. RHIC is the only collider in the world capable of colliding high-energy polarized proton beams. The beams travel in opposite directions around RHIC’s 3.86 km two-lane racetracks. The STAR detector sits at the six o'clock position with respect to RHIC as indicated in the figure below.
Bird's-eye view of RHIC, and indicating the location of the STAR dectector.
The aim of my PhD is to study the Collins effect in STAR’s 2015 200 GeV p+Au dataset, where the proton is transversely polarized. The Collins effect [1, 2] describes the relationship between the quark transversity and the Collins fragmentation function. This effect manifests itself as a final-state transverse single-spin asymmetry (TSSA), where hadrons produced from the fragmenting transversely polarized quark are azimuthally distributed about the axis of the parent jet. See the diagram below for more details about the Collins effect. STAR has measured this effect in transversely polarized p+p collisions at center-of-mass energies of 200 and 500 GeV, and a non-zero Collins effect measurement has been observed for identified pions in jets [3, 4].
The above diagram shows a transversely polarized proton colliding with an unpolarized proton. The quarks inside of the transversely polarized proton have transverse polarization and are described by the transversity parton distribution function, h. The quarks inside the unpolarized proton are described by the unpolarized parton distribution function, f. When a hard scattering occurs between the polarized quark a and the unpolarized quark b, the outgoing quark c carries most of the initial-state polarization of quark a. Due to color confinement in QCD, quarks can not exist on their own. They must form colorless objects. Therefore, quark c undergoes fragmentation and hadronization that form final-state hadrons (such as protons, kaons, and pions). This process is described by the Collins fragmentation function, H. The relationship between the transversity parton distribution function and the Collins fragmentation function is called the Collins effect. Diagram made by Ting Lin and modified by me.
However, the universality of the Collins fragmentation function is subject to debate [2, 5, 6, 7, 8, 9], and studying the Collins effect in p+Au will provide a test for the universality of the Collins fragmentation function. For the Collins asymmetry to arise, an overall phase from the scattering amplitudes is needed to have a non-vanishing TSSA. The overall phase is expected to arise from the fragmentation and hadronization processes of the outgoing transversely polarized quark [2, 7]. Comparing p+p to p+Au Collins measurements can, experimentally, identify potential contributions to the overall phase from outside the fragmentation and hadronization processes of the outgoing transversely polarized quark [5, 6], because the Au nucleus offers a large color field for possible initial- or final-state color connections. If color entanglement due to color connections with the Au nucleus is observed in p+Au, then the factorization and universality of the Collins fragmentation function become questionable.