Single transverse-spin asymmetries in hadronic pion production

Abstract

We analyze single transverse-spin asymmetries for hadronic pion production at large transverse momenta using QCD factorization. In the large x F region, leading contributions to the asymmetries are naturally produced by twist-3 parton correlation functions that couple quark fields and gluon field strengths. With a simple model for these matrix elements, leading-order asymmetries calculated from QCD are consistent with data on pion production from Fermilab, and can be used to predict single-spin asymmetries at BNL RHIC. We argue that our perturbative calculation for the asymmetries is relevant to pion transverse momenta as low as a few GeV. ͓S0556-2821͑98͒07423-2͔

Azimuthal and single spin asymmetries in hard scattering processes

“…More precisely, for the numerator of A N in the process A ↑ B → C + X (C unpolarized) they obtain, neglecting higher power corrections (we keep here the original notation of Ref. [191]),…”

“…Another step towards this interpretation was developed by Efremov and Teryaev [186,187,188] who pointed out that a nonvanishing SSA can be obtained in pQCD beyond the leading power expansion. However, the first consistent calculation in collinear pQCD of a sizable SSA in the forward region (large x F ) was given by Qiu and Sterman in their paper on hadronic direct photon production [189,190] and their subsequent application to pion production [191]. The asymmetries are evaluated in the framework of pQCD generalized factorization theorems with the introduction of new twist-three quark-gluon correlator functions convoluted with ordinary twist-two parton distribution functions and a short-distance hard scattering part.…”

A New Mechanism for Generating a Single Transverse Spin Asymmetry

“…Hence our STSA-generating mechanism is distinctively different from the Collins [27] and Sivers [15,16] effects, and is more akin to (though still different from) the higher-twist mechanisms of [17][18][19][20][21][22][23][24][25][26].…”

“…(32a) stems from the multiple interactions with the target (higher-twist effects), and its contribution is in fact zero in the linearized (leading-twist) approximation. In this sense the above mechanism for generating STSA is similar in spirit to the higher-twist mechanisms of [17][18][19][20][21][22][23][24][25][26], though a detailed comparison of the diagrams appears to indicate that the two approaches are, in fact, different.…”

“…Being a part of the PDF, this is an intrinsically non-perturbative process. The asymmetry could also be generated in the partonic scattering processes themselves; since the leading-order scattering process does not generate an asymmetry, any contribution would come from "higher-twist" terms in the interaction, as suggested in [17][18][19][20][21][22][23][24][25][26]. Such an asymmetry generated by the interaction may be perturbative, depending on the kinematics.…”