Comparison of transverse single-spin asymmetries for forward π0 production in polarized pp, pAl and pAu collisions at nucleon pair c.m. energy sNN=200GeV

J. Adam et al. (STAR Collaboration)
Phys. Rev. D 103, 072005 – Published 16 April 2021

Abstract

The STAR collaboration reports a measurement of the transverse single-spin asymmetries, AN, for neutral pions produced in polarized proton collisions with protons (pp), with aluminum nuclei (pAl) and with gold nuclei (pAu) 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 2.7<η<3.8. Results are presented for π0s observed in the STAR forward meson spectrometer electromagnetic calorimeter in narrow Feynman x (xF) and transverse momentum (pT) bins, spanning the range 0.17<xF<0.81 and 1.7<pT<6.0GeV/c. For fixed xF<0.47, the asymmetries are found to rise with increasing transverse momentum. For larger xF, the asymmetry flattens or falls as pT increases. Parametrizing the ratio r(A)AN(pA)/AN(pp)=AP over the kinematic range, the ratio r(A) is found to depend only weakly on A, with P=0.027±0.005. No significant difference in P is observed between the low-pT region, pT<2.5GeV/c, where gluon saturation effects may play a role, and the high-pT region, pT>2.5GeV/c. It is further observed that the value of AN is significantly larger for events with a large-pT isolated π0 than for events with a nonisolated π0 accompanied by additional jetlike fragments. The nuclear dependence r(A) is similar for isolated and nonisolated π0 events.

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  • Received 14 December 2020
  • Accepted 16 February 2021

DOI:https://doi.org/10.1103/PhysRevD.103.072005

© 2021 American Physical Society

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Nuclear PhysicsParticles & Fields

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Vol. 103, Iss. 7 — 1 April 2021

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