SPIN Physics at GSI

TL;DR

This paper discusses the potential of polarized antiprotons produced via spin filtering at GSI for exploring unique spin-dependent phenomena and measuring fundamental properties of protons, opening new avenues in hadronic physics research.

Contribution

It introduces the use of polarized antiprotons at GSI for novel measurements of spin observables, transversity, Sivers function sign, and proton form factors, advancing hadronic physics studies.

Findings

First measurement of proton transversity distribution.

Test of Sivers function sign prediction in Drell-Yan.

Measurement of proton electric and magnetic form factors.

Abstract

Polarized antiprotons produced by spin filtering with an internal polarized gas target provide access to a wealth of single-- and double--spin observables, thereby opening a window to physics uniquely accessible with the HESR at FAIR. This includes a first measurement of the transversity distribution of the valence quarks in the proton, a test of the predicted opposite sign of the Sivers--function, related to the quark distribution inside a transversely polarized nucleon, in Drell--Yan (DY) as compared to semi--inclusive DIS, and a first measurement of the moduli and the relative phase of the time--like electric and magnetic form factors G_{E,M} of the proton. In polarized and unpolarized proton--antiproton elastic scattering open questions like the contribution from the odd charge--symmetry Landshoff--mechanism at large |t| and spin--effects in the extraction of the forward scattering amplitude at low |t| can be addressed.