Neutral pion cross section and spin asymmetries at intermediate pseudorapidity in polarized proton collisions at √ s = 200 GeV

Adamczyk, L. and Adkins, J. K. and Agakishiev, G. and Aggarwal, M. M. and Ahammed, Z. and Alekseev, I. and Alford, J. and Anson, C. D. and Aparin, A. and Arkhipkin, D. and Aschenauer, E. C. and Averichev, G. S. and Balewski, J. and Banerjee, A. and Barber, B. and Barnovska, Z. and Beavis, D. R. and Bellwied, R. and Betancourt, M. J. and Bhasin, A. and Bhati, A. K. and Bhattarai, P. and Bichsel, H. and Bielcik, J. and Bielcikova, J. and Bland, L. C. and Bordyuzhin, I. G. and Borowski, W. and Bouchet, J. and Brandin, A. V. and Vértesi, Róbert (2014) Neutral pion cross section and spin asymmetries at intermediate pseudorapidity in polarized proton collisions at √ s = 200 GeV. PHYSICAL REVIEW D, 89 (1). ISSN 2470-0010

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Abstract

The differential cross section and spin asymmetries for neutral pions produced within the intermediate pseudorapidity range 0.8 < η < 2.0 in polarized proton-proton collisions at √ s = 200 GeV are presented. Neutral pions were detected using the endcap electromagnetic calorimeter in the STAR detector at RHIC. The cross section was measured over a transverse momentum range of 5 < pT < 16 GeV/c and is found to agree with a next-to-leading order perturbative QCD calculation. The longitudinal double-spin asymmetry, ALL, is measured in the same pseudorapidity range and spans a range of Bjorken-x down to x ≈ 0.01. The measured ALL is consistent with model predictions for varying degrees of gluon polarization. The parity-violating asymmetry, AL, is also measured and found to be consistent with zero. The transverse single-spin asymmetry, AN , is measured over a previously unexplored kinematic range in Feynman-x and pT . Such measurements may aid our understanding of the on-set and kinematic dependence of the large asymmetries observed at more forward pseudorapidity (η ≈ 3) and their underlying mechanisms. The AN results presented are consistent with a twist-3 model prediction of a small asymmetry over the present kinematic range.

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