Abstract
Measurements of transverse-single-spin asymmetries (A(N)) in p + p collisions at root s = 62.4 and 200 GeV with the PHENIX detector at the Relativistic Heavy Ion Collider are presented. At midrapidity, A(N) is measured for neutral pion and eta mesons reconstructed from diphoton decay, and, at forward rapidities, neutral pions are measured using both diphotons and electromagnetic clusters. The neutral-pion measurement of A(N) at midrapidity is consistent with zero with uncertainties a factor of 20 smaller than previous publications, which will lead to improved constraints on the gluon Sivers function. At higher rapidities, where the valence quark distributions are probed, the data exhibit sizable asymmetries. In comparison with previous measurements in this kinematic region, the new data extend the kinematic coverage in root s and p(T), and it is found that the asymmetries depend only weakly on root s. The origin of the forward A(N) is presently not understood quantitatively. The extended reach to higher p(T) probes the transition between transverse momentum dependent effects at low p(T) and multiparton dynamics at high p(T).
Highlights
The proton is a fundamental and stable bound state of quantum chromodynamics
Collinear perturbative quantum chromodynamics at leading twist in the operator product expansion successfully describes the quark and gluon substructure of the proton observed in high energy scattering experiments [1]
The minimum bias trigger was defined as the coincidence of signals from two beam beam counters (BBCs) covering the full azimuthal angle and the pseudorapidity range 3.0 < jηj < 3.9 [36]
Summary
The proton is a fundamental and stable bound state of quantum chromodynamics. Collinear perturbative quantum chromodynamics (pQCD) at leading twist in the operator product expansion successfully describes the quark and gluon substructure of the proton observed in high energy scattering experiments [1]. Transverse-single-spin asymmetries or the analyzing power (AN) of hadrons h produced in the transversely polarized p↑ þ p → h þ X reaction were expected to be small [4], but experiments instead measured large asymmetries of up to AN ≈ 40% These asymmetries hapvffieffi been measured at increasing center-of-mass energies s over the past three decades, from 4.9 to 200 GeV [5,6,7,8,9,10]. The first of these, known as the Sivers effect, correlates the proton spin with the partonic transverse momentum kT [14] It has been measured in semi-inclusive deep inelastic scattering (SIDIS) experiments with sensitivity mainly to the quarks [15,16].
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