New mechanism for generating a single transverse spin asymmetry

Abstract

We propose a new mechanism for generating a single transverse spin asymmetry (STSA) in polarized proton-proton and proton-nucleus collisions in the high-energy scattering approximation. In this framework the STSA originates from the q → q G splitting in the projectile (proton) light-cone wave function followed by a perturbative (C-odd) odderon interaction, together with a C-even interaction, between the projectile and the target. We show that some aspects of the obtained expression for the STSA of the produced quarks are in qualitative agreement with experiment: STSA decreases with decreasing projectile xF and is a non-monotonic function of the transverse momentum kT . In our framework the STSA peaks at kT near the saturation scale Qs. Our mechanism predicts that the quark STSA in proton-nucleus collisions should be much smaller than in proton-proton collisions. We also observe that in our formalism the STSA for prompt photons is zero.

Sivers function in the quasiclassical approximation

“…The way to understand this approximation is as follows. As shown in [17], the preferred direction generated by odderontype rescattering couples to transverse gradients of the nuclear profile function, ∇T (b). The length scale over which these gradients become important is on the order of the nuclear radius; these gradients are therefore O A −1/3 ∼ O (α 2 s ) suppressed (in addition to an extra power of α s entering the lowest-order odderon amplitude corresponding to the triple-gluon exchange [52,53,54,55,56,57,58,59,17]) and are beyond the precision of the quasi-classical formula (51).…”

“…To that end a significant progress has been achieved by Brodsky, Hwang and Schmidt (BHS) in [10] (see also [16,5,6,7,9,11,17]), where, in a quark-di-quark proton model calculation, it has been shown that the STSA in SIDIS can be generated through an interference of the finalstate parton rescattering diagram with the Born-level amplitude. In essence, it was shown in [10,11] that multiple partonic rescatterings are key to generating the asymmetry.…”

“…Multiple rescatterings can be resummed in the GM/MV model as an expansion in powers of the parameter α 2 s A 1/3 [34]: the presence of a resummation parameter allows for a controlled approximation to the problem at hand. In the past there was a number of efforts to include spin effects in the saturation/CGC framework [35,36,37,38,39,17,40,41].…”

Single spin asymmetry in forwardpAcollisions

“…For example, the small-x/Regge behavior of the polarized parton distribution functions ∆q(x) and ∆g(x) is relevant to the nucleon spin decomposition problem [1][2][3][4]. Also, various single spin asymmetries (SSAs) in pp and pA collisions have been computed by including the gluon saturation effects [5][6][7][8][9][10][11][12][13]. On the experimental side, RHIC has recently reported its first measurement of SSA on a nuclear target [14] that might call for a saturation-based explanation.…”

Flat transverse momentum dependence of the single-spin asymmetry in inclusive neutral pion production

“…The most recent progress in this field is described in Refs. [1,2]. The decreasing dependence has not been directly observed experimentally, but the experimental data were not very conclusive due to large statistical errors.…”