On the Physics Potential of Polarized Nucleon-Nucleon Collisions at HERA

TL;DR

The paper evaluates the physics potential of polarized nucleon-nucleon collisions at HERA with an internal polarized target, aiming to illuminate nucleon spin structure and twist-3 dynamics. It argues that single-spin asymmetries across multiple processes can disentangle hard-scattering, fragmentation, and distribution contributions, while double-spin asymmetries in photon+jet and J/ψ+jet provide a path to map ΔG/G in a broad x_g range. The study presents realistic sensitivity estimates and compares potential measurements to RHIC, highlighting complementary capabilities and the added value of elastic-scattering spin tests. Overall, the HERA–N program promises significant, feasible insights into spin phenomena in high-energy hadronic collisions.

Abstract

The physics of polarized nucleon--nucleon collisions originating from an internal polarized target in the HERA proton beam is investigated. Based on 240 pb^-1 integrated luminosity at 40 GeV c.m. energy, statistical sensitivities are given over a wide (x_F, p_T) range for a variety of inclusive and exclusive final states. By measuring single spin asymmetries unique information can be obtained on higher twist contributions and their p_T-dependence. From double spin asymmetries in both photon and J/psi production it appears possible to measure the polarized gluon distribution in the range 0.1<x_gluon<0.4 with a good statistical accuracy.

Figures (7)

• Figure 1: Single spin asymmetry in inclusive pion production $p^{\uparrow}~+~p~\rightarrow~\pi^{0\pm}~+~X$ measured by the E704 Collaboration 704pi and shown for two subregions of $p_T$.
• Figure 2: Contours of the asymmetry sensitivity levels for $\pi^+$ production in the ($p_T,~x_F$) plane. Lines of constant laboratory angles of the pion are shown.
• Figure 3: Capability of HERA-$\vec{N}$ to discriminate predictions for different $p_T$.
• Figure 4: Inclusive photon production: Double spin asymmetry vs. a) $p_T$ and b) $\eta$ for the NLO 'valence set' of Ref. GorVog (full line), shown in conjunction with the HERA--$\vec{N}$ statistical sensitivity. The dotted line corresponds to set C of Ref. GehrStir and the dashed line is close to set A of Ref. GehrStir_LO
• Figure 5: Typical predictions for the polarized gluon distribution confronted with the projected statistical errors expected for HERA-$\vec{N}$ and RHIC experiments.