Abstract
The STAR collaboration reports a measurement of the transverse single-spin asymmetries, , for neutral pions produced in polarized proton collisions with protons (), with aluminum nuclei () and with gold nuclei () at a nucleon-nucleon center-of-mass energy of 200 GeV. Neutral pions are observed in the forward direction relative to the transversely polarized proton beam, in the pseudorapidity region . Results are presented for observed in the STAR forward meson spectrometer electromagnetic calorimeter in narrow Feynman x () and transverse momentum () bins, spanning the range and . For fixed , the asymmetries are found to rise with increasing transverse momentum. For larger , the asymmetry flattens or falls as increases. Parametrizing the ratio over the kinematic range, the ratio is found to depend only weakly on , with . No significant difference in is observed between the low- region, , where gluon saturation effects may play a role, and the high- region, . It is further observed that the value of is significantly larger for events with a large- isolated than for events with a nonisolated accompanied by additional jetlike fragments. The nuclear dependence is similar for isolated and nonisolated events.
6 More- Received 14 December 2020
- Accepted 16 February 2021
DOI:https://doi.org/10.1103/PhysRevD.103.072005
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