Quark helicity distributions in the nucleon for up, down, and strange quarks from semi--inclusive deep--inelastic scattering

TL;DR

This study uses semi-inclusive deep-inelastic scattering with hadron tagging to extract flavor-separated quark helicity distributions in the nucleon. Employing a leading-order QCD framework and a Monte Carlo-tuned purity method, it delivers the first detailed, flavor-specific quark polarization results, finding Δu>0, Δd<0, and Δubar≈Δdbar≈Δs≈0 within uncertainties. An isoscalar approach provides an independent check on the strange quark polarization, while unfolding techniques account for radiative and detector smearing effects, yielding robust Born-level asymmetries and correlated uncertainties. Overall, the moments of the extracted helicity densities align with inclusive fits, and the data favor flavor symmetry in the light sea within experimental errors, offering important constraints for spin-structure models.

Abstract

Polarized deep--inelastic scattering data on longitudinally polarized hydrogen and deuterium targets have been used to determine double spin asymmetries of cross sections. Inclusive and semi--inclusive asymmetries for the production of positive and negative pions from hydrogen were obtained in a re--analysis of previously published data. Inclusive and semi--inclusive asymmetries for the production of negative and positive pions and kaons were measured on a polarized deuterium target. The separate helicity densities for the up and down quarks and the anti--up, anti--down, and strange sea quarks were computed from these asymmetries in a ``leading order'' QCD analysis. The polarization of the up--quark is positive and that of the down--quark is negative. All extracted sea quark polarizations are consistent with zero, and the light quark sea helicity densities are flavor symmetric within the experimental uncertainties. First and second moments of the extracted quark helicity densities in the measured range are consistent with fits of inclusive data.

Figures (24)

• Figure 1: Diagram of the deep-inelastic scattering process. The incoming lepton emits a virtual photon which is absorbed by one of the quarks in the nucleon. In the case depicted, the struck quark fragments into a pion in the final state. In semi--inclusive processes, the scattered lepton and part of the hadronic final state are detected in coincidence.
• Figure 2: Schematic diagram of the HERA accelerator layout until 2000 with the location of the four experiments. Also shown are the locations of the spin--rotators and the two polarimeters.
• Figure 3: Diagram of the HERMES polarized target. Shown are the atomic beam source (ABS), the target gas analyzer (TGA) and the Breit--Rabi polarimeter (BRP). SFT, MFT, and WFT label the strong, medium, and weak field transitions in the ABS and the BRP.
• Figure 4: Side view of the HERMES spectrometer. The positron beam enters from the left. The spectrometer is split into two halves, one above the beam and one below, by a flux exclusion plate to protect the beams from the magnetic field. See the text for further details on the detectors.
• Figure 5: Typical PID detector responses. The distributions are based on a small set of the data collected in 2000, except for the threshold Čerenkov response, which was computed from a data set of similar size collected in 1997. The truncated mean is shown in the case of the TRD. The relative size of the lepton (dashed line) and hadron distributions (solid line) was scaled to the flux ratio in the respective data--taking periods to give a better idea of the level of contamination possible from each detector. The flux ratio of electrons to hadrons is typically $\sim 10\,\%$ for these data.