The production of particles with high transverse momentum from polarized proton collisions at high energies is sensitive to the quark and gluon spin structure of the proton. One challenge to theory has been to understand the sizable azimuthal asymmetry of particles produced in collisions of transversely polarized protons, known as analyzing power (AN) or transverse single spin asymmetry (SSA) for inclusive pion production in polarized p + p collisions over a broad range of collision energies and in semi-inclusive deep inelastic scattering (SIDIS) from transversely polarized proton targets.
Spin-correlated transverse momentum dependent (TMD)distribution functions (Sivers effect), in conjunction with initial- or final-state color-charge interactions, can explain large AN. These functions describe parton orbital motion within the proton, and so are important to explore to understand the structure of the proton.
The predictions from these pQCD based theoretical model calculations is that AN will increase as the longitudinal momentum (pL) of the pion increases, usually given by Feynman-x, xF = 2pL/s and at fixed xF , AN will decrease with increasing transverse momentum (pT ), for pT >1.2 GeV/c.
STAR has recently reported precision measurements of the Feynman-x dependence, and first measurements of the transverse momentum dependence, of transverse single spin asymmetries for the production of neutral pions from polarized proton collisions at center of mass energies of 200 GeV. The xF dependence of the results are in fair agreement with perturbative QCD model calculations that identify orbital motion of quarks and gluons within the proton as the origin of the spin effects. Results for the pT dependence at fixed xF are not consistent with pQCD-based calculations (Shown in the Figure).
Figure shows the variation of AN with pT for fixed xF compared to theory calculations as mentioned above. There is a clear tendency for AN to increase with pT , and no significant evidence over the measured range for AN to decrease with increasing pT, as expected by the calculations. This may reflect the presence of additional mechanisms for these spin effects. Future measurements capable of disentangling TMD fragmentation and distribution function contributions to neutral pion spin effects, and measurements of AN for real and virtual photon production sensitive to only Sivers contributions, are required to definitively establish if partonic orbital motion is the correct explanation of these effects. So stay tuned for more interesting results from STAR.
Further details can be found in the following STAR paper -