Abstract
Studying spin-momentum correlations in hadronic collisions offers a glimpse into a three-dimensional picture of proton structure. The transverse single-spin asymmetry for midrapidity isolated direct photons in p^{↑}+p collisions at sqrt[s]=200 GeV is measured with the PHENIX detector at the Relativistic Heavy Ion Collider (RHIC). Because direct photons in particular are produced from the hard scattering and do not interact via the strong force, this measurement is a clean probe of initial-state spin-momentum correlations inside the proton and is in particular sensitive to gluon interference effects within the proton. This is the first time direct photons have been used as a probe of spin-momentum correlations at RHIC. The uncertainties on the results are a 50-fold improvement with respect to those of the one prior measurement for the same observable, from the Fermilab E704 experiment. These results constrain gluon spin-momentum correlations in transversely polarized protons.
Highlights
The 200 transverse single-spin asymmetry for GeV is measured with the PHENIX midrapidity isolated direct detector at the Relativistic photons in Heavy Ion
Because direct photons in particular are produced from the hard scattering and do not interact via the strong force, this measurement is a clean probe of initial-state spin-momentum correlations inside the proton and is in particular sensitive to gluon interference effects within the proton
Direct photons are produced at leading order via the quantum chromodynamics (QCD) 2-to-2 hard scattering subprocesses quark-gluon Compton scattering (g þ q → γ þ q) and quark-antiquark annihilation
Summary
Because direct photons in particular are produced from the hard scattering and do not interact via the strong force, this measurement is a clean probe of initial-state spin-momentum correlations inside the proton and is in particular sensitive to gluon interference effects within the proton. Next-to-leading-order perturbative QCD calculations which only include effects from high energy parton scattering predict that these asymmetries should be small and fall off as mq=Q [7], where mq is the bare mass of the quark To explain these large TSSAs, they must be considered in the context of the dynamics present in proton-proton collisions that cannot be calculated perturbatively, namely, dynamics describing proton structure and/or the process of hadronization. Midrapidity inclusive hadron TSSA measurements are sensitive to gluon spin-momentum correlations in the proton and include potential effects from hadronization and final-state color interactions.
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