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Schlagwörter:
STAR
Zusammenfassung:
The STAR Collaboration reports a measurement of the transverse single-spin asymmetries, A_{N}, for neutral pions produced in polarized proton collisions with protons (pp), with aluminum nuclei (p\rm{Al}) and with gold nuclei (p\rm{Au}) 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 pseudo-rapidity region 2.7<\eta<3.8. Results are presented for \pi^0s observed in the STAR FMS electromagnetic calorimeter in narrow Feynman x (x_F) and transverse momentum (p_T) bins, spanning the range 0.17<x_F<0.81 and 1.7<p_{T}<6.0 GeV/c. For fixed x_F<0.47, the asymmetries are found to rise with increasing transverse momentum. For larger x_F, the asymmetry flattens or falls as {p_T} increases. Parametrizing the ratio r(A) \equiv A_N(pA)/A_N(pp)=A^P over the kinematic range, the ratio r(A) is found to depend only weakly on A, with {\langle}P{\rangle} = -0.027 \pm 0.005. No significant difference in P is observed between the low-p_T region, p_T<2.5 GeV/c, where gluon saturation effects may play a role, and the high-p_T region, p_T>2.5 GeV/c. It is further observed that the value of A_N is significantly larger for events with a large-p_T isolated \pi^0 than for events with a non-isolated pi^0 accompanied by additional jet-like fragments. The nuclear dependence r(A) is similar for isolated and non-isolated pi^0 events.
