Abstract:
Single-spin asymmetries (SSAs) in the scattering of leptons off transversely polarized nucleons
are studied across several final states, with a particular focus on single-inclusive processes. Fully
analytic expressions for the spin-dependent cross sections are derived within collinear twist-3 factorization.
The calculations are performed at next-to-leading order (NLO) accuracy in perturbative
Quantum Chromodynamics (pQCD) for single-inclusive hadron production ( N↑ → hX), jet
production ( N↑ → jetX) and photon production ( N↑ → γX). Moreover, the γSIDIS process
N↑ → γX is also considered, i.e. photon production in semi-inclusive deep inelastic scattering.
Within the employed collinear twist-3 factorization approach, results are expressed in terms
of twist-3 multiparton correlation functions, which describe either parton distributions inside a
transversely polarized nucleon or fragmentation into an observed hadron. The present analysis
focuses exclusively on the former, as only these distribution-type twist-3 functions contribute to
the transverse SSA in jet and photon production. For hadron production, twist-3 fragmentation
effects would also enter, but are not included here.
For the single-inclusive processes, the results derived in this work establish the validity of
collinear twist-3 factorization at one loop. The calculation reflects the typical complexity associated
with higher-twist observables, including the appearance of derivative terms of the twist-3 functions.
Each process exhibits unique features that render it particularly informative for probing twist-3
multiparton distribution functions. For hadron and jet production, the analytic results confirm that
the entire support in the longitudinal momentum fractions x, x of the qgq distribution functions
Fq and Gq is accessed. This is also true for single-inclusive photon production, which introduces
additional, less-studied twist-3 distribution functions. The distinctive aspect of the γSIDIS process
is its ability to scan the x, x support of Fq and Gq point-by-point.
To enable numerical predictions, a realistic model for these functions is constructed building on
the Sivers transverse-momentum–dependent parton distribution function (TMD) f
⊥,q
1T . Based on
this model, an exploratory phenomenological analysis of the transverse SSA is conducted for the
kinematics of a future Electron-Ion Collider (EIC), and a comparison is made to HERMES data for
single-inclusive pion production (ep↑ → π±X). The NLO corrections are found to be significant
for both hadron and jet production, and all four processes are shown to be highly sensitive to
the functional form of the twist-3 distribution functions Fq and Gq. Furthermore, regions in EIC
phase space where the γSIDIS asymmetry AγSIDIS
UT becomes particularly sizeable are identified.
