Antiproton--Proton Scattering Experiments with Polarization

TL;DR

The paper argues for polarized antiproton-proton scattering experiments at FAIR, outlining a two-stage program that uses an Antiproton Polarizer Ring and a Cooler Synchrotron Ring to produce polarized antiprotons and collide them with polarized protons at the High Energy Storage Ring. The core aim is to access transversity via the double-spin asymmetry A_TT in Drell-Yan processes, complemented by single-spin asymmetries, time-like electromagnetic form factors, and hard/soft scattering studies, including generalized parton distributions. It provides a comprehensive technical design and feasibility assessment for APR, CSR, and HESR integration, detailing polarization buildup, spin preservation, and interaction-region concepts, along with luminosity and rate estimates. Collectively, the work establishes a concrete roadmap for a unique pbar-p spin physics program with potential to illuminate nucleon structure, test QCD spin phenomena, and probe spin-dependent hadron dynamics at intermediate energies.

Abstract

The document describes the physics case of the PAX experiment using polarized antiprotons, which has recently been proposed for the new Facility for Antiprotons and Ions Research (FAIR) at GSI--Darmstadt. Polarized antiprotons provide access to a wealth of single-- and double--spin observables, thereby opening a new window to physics uniquely accessible at the HESR. The polarized antiprotons would be most efficiently produced by spin--filtering in a dedicated Antiproton Polarizer Ring (APR) using an internal polarized hydrogen gas target. In the proposed collider scenario of the PAX experiment, polarized protons stored in a COSY--like Cooler Storage Ring (CSR) up to momenta of 3.5 GeV/c are bombarded head--on with 15 GeV/c polarized antiprotons stored in the HESR. This asymmetric double--polarized antiproton--proton collider is ideally suited to map, e.g., the transversity distribution in the proton. The proposed detector consists of a large--angle apparatus optimized for the detection of Drell--Yan electron pairs.

Figures (21)

• Figure 2: The SSA $A_{UT}^{\sin(\phi_h-\phi_S)q_\perp/M_N}$ in Drell-Yan lepton pair production, $p^{\uparrow} \bar{p} \to \mu^+ \mu^- X$, as function of the rapidity $y$ for typical PAX kinematics ($s = 45 \, \textrm{GeV}^2$, $M^2 = 2.5 \, \textrm{GeV}^2$). The different curves correspond to equally good fits to the HERMES dataHERMESSiv.
• Figure 3: Predicted single--spin asymmetry ${\cal A}_y={\cal P}_y$ for $\theta = 45^\circ$ in the time--like region for selected form factor fits: $F_2/F_1 \propto 1/Q$ fit brodsky, the $(\log^2 Q^2)/Q^2$ fit of Belitsky et al.belitsky02; an improved $(\log^2 Q^2)/Q^2$ fit BHHK; and a fit from Iachello et al., ijl.
• Figure 4: Predicted single--spin asymmetries (${\cal A}_y={\cal P}_y$) for $\theta = 45^\circ$ in the time--like region for two versions (O (old) & N (new)) of the analytic and unitary vector--meson dominance (VDM) models d
• Figure 5: All existing magnetic form factor data of the proton in the time--like region obtained with the hypothesis $|G_M|=|G_E|$ versus $s=q^2$, as compiled in Andreotti; the summary of the earlier data can be found in bib:psip.
• Figure 6: The energy dependence of $R_{1}=s^{10}{d\sigma_{pp}/ dt}|_{90^o}$ for the high energy $pp$ elastic scattering at $90^o$ c.m. angle compared to the model calculation RalstonPire from the interference of the Brodsky--Farrar and Landshoff mechanisms.