Constraints on the gluon Sivers distribution via transverse single spin   asymmetries at midrapidity in p(transv. polarized) p -> pi^0 X processes at   BNL RHIC
  We consider the recent RHIC data on the transverse single spin asymmetry (SSA) A_N, measured in p(transv. polarized) p -> pi^0 X processes at mid-rapidity by the PHENIX collaboration. The measurement is consistent with a vanishing SSA. We analyze this experimental information within a hard scattering approach based on a generalized QCD factorization scheme, with unintegrated, transverse momentum dependent (TMD), parton distribution and fragmentation functions. It turns out that, in the kinematical region of the data, only the gluon Sivers effect could give a large contribution to A_N; its vanishing value is thus an indication about the possible size of the gluon Sivers function (GSF). Approximate upper limits on its magnitude are derived. Additional constraints obtained combining available parameterizations of the quark Sivers function and the Burkardt sum rule (BSR) for the Sivers distributions are also discussed. 
M. Anselmino (1), U. D'Alesio (2), S. Melis (2), F. Murgia (2) ((1)   University and INFN, Torino, Italy, (2) University and...