Neutron spin structure with polarized deuterons and spectator proton tagging at EIC

W Cosyn,D W Higinbotham,C Weiss,V Guzey,S Kuhn,K Park,P Nadel-Turonski,M Strikman,C Hyde,M Sargsian

doi:10.1088/1742-6596/543/1/012007

Abstract

The neutron's deep-inelastic structure functions provide essential information for the flavor separation of the nucleon parton densities, the nucleon spin decomposition, and precision studies of QCD phenomena in the flavor-singlet and nonsinglet sectors. Traditional inclusive measurements on nuclear targets are limited by dilution from scattering on protons, Fermi motion and binding effects, final-state interactions, and nuclear shadowing at x ≪ 0.1. An Electron-Ion Collider (EIC) would enable next-generation measurements of neutron structure with polarized deuteron beams and detection of forward-moving spectator protons over a wide range of recoil momenta (0 < pR < several 100MeV in the nucleus rest frame). The free neutron structure functions could be obtained by extrapolating the measured recoil momentum distributions to the on-shell point. The method eliminates nuclear modifications and can be applied to polarized scattering, as well as to semi-inclusive and exclusive final states. We review the prospects for neutron structure measurements with spectator tagging at EIC, the status of R&D efforts, and the accelerator and detector requirements.

Highlights

The program of exploring short-range nucleon structure and strong interaction dynamics with high–energy lepton scattering relies on measurements on the neutron as much as those on the proton target
Neutron and proton data are needed to separate the isoscalar and isovector combinations of the deep-inelastic scattering (DIS) structure functions, which are subject to different short–distance dynamics (QCD evolution, higher–twist effects, small–x behavior) and give access to different combinations of the parton densities
In this note we summarize the potential of DIS on the deuteron with spectator proton tagging for precision measurements of the neutron structure functions at Electron–Ion Collider (EIC)

Summary

Introduction

The program of exploring short-range nucleon structure and strong interaction dynamics with high–energy lepton scattering relies on measurements on the neutron as much as those on the proton target. Neutron and proton data are needed to separate the isoscalar and isovector combinations of the deep-inelastic scattering (DIS) structure functions, which are subject to different short–distance dynamics (QCD evolution, higher–twist effects, small–x behavior) and give access to different combinations of the parton densities (gluons and singlet quarks vs non-singlet quarks). It is clear that better experimental control of the nuclear environment is needed to improve the precision of neutron structure extraction This is relevant if a combination of neutron and proton data are to be used to study subtle QCD effects, such as the separation of leading and higher twist, non-singlet QCD evolution, and non-singlet small–x behavior. Spectator proton tagging with unpolarized deuterium was explored in a pioneering fixed-target experiment at JLab with 6 GeV beam energy (CLAS BoNuS detector, covers recoil momenta pR 70 MeV) [13] and will be studied further at 11 GeV.

Neutron structure with spectator tagging

Neutron spin structure

Semi-inclusive and exclusive measurements

Accelerator and detector requirements