A Method to Polarize Stored Antiprotons to a High Degree

Abstract

Polarized antiprotons can be produced in a storage ring by spin-dependent interaction in a purely electron-polarized hydrogen gas target. The polarizing process is based on spin transfer from the polarized electrons of the target atoms to the orbiting antiprotons. After spin filtering for about two beam lifetimes at energies T ≈ 40−170 MeV using a dedicated large acceptance ring, the antiproton beam polarization would reach P = 0.2 − 0. . Unfortunately, both approaches do not allow efficient accumulation in a storage ring, which would greatly enhance the luminosity. Spin splitting using the Stern-Gerlach separation of the given magnetic substates in a stored antiproton beam was proposed in 1985 [3]. Although the theoretical understanding has much improved since then [4], spin splitting using a stored beam has yet to be observed experimentally.Interest in the polarization of antiprotons has recently been stimulated by a proposal to build a High Energy Storage Ring (HESR) for antiprotons at the new Facility for Antiproton and Ion Research (FAIR) at the Gesellschaft für Schwerionenforschung (GSI) in Darmstadt [5]. A Letter-of-Intent for spin-physics experiments has been submitted by the PAX collaboration [6] to employ a polarized antiproton beam incident on a polarized internal storage cell target [7]. A beam of polarized antiprotons would enable new experiments, such as the first direct 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 as compared to semi-inclusive deep-inelastic scattering, 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 [6].In 1992 an experiment at the Test Storage Ring (TSR) at MPI Heidelberg showed that an initially unpolarized stored 23 MeV proton beam can be polarized by spindependent interaction with a polarized hydrogen gas target [8,9,10]. In the presence of polarized protons of magnetic quantum number m = , which eventually caused the stored beam to acquire a polarization parallel to the proton spin of the hydrogen atoms during spin filtering. In an analysis by Meyer three different mechanisms were identified, that add up to the measured result [11]. One of these mechanisms is spin transfer from the polarized electrons of the hydrogen gas target to the circulating protons. Horowitz and Meyer derived the spin transfer cross section p + e → p + e (using c = = 1) [12],where α is the fine-structure constant, a is the anomalous magnetic moment of the proton, m e and m p are the rest mass of electron and proton, p is the momentum in the CM system, a 0 = 52900 fm is the Bohr radius and C 2 0 = 2πη/[exp(2πη) − 1] is the square of the Coulomb wave function at the origin. The Coulomb parameter η is given by η = −zα/v (for antiprotons, η is positive). z is the beam charge number and v the relative velocity of particle and projectile in ...

Spin observables and spin structure functions: Inequalities and dynamics

“…The estimate for the antiproton beam foreseen at the future GSI facility is a polarisation of the order of 0.2-0.4 [286]. However, another study does not confirm this optimistic result [287].…”

Antiproton scattering off3He and4He nuclei at low and intermediate energies

“…Coulomb effects are sizable at low energies, i.e., for T lab 25 MeV, as can be seen from the analysis of the FILTEX experiment [2] in which protons were scattered off polarized hydrogen at 23 MeV. Forp 3 He scattering, Coulomb effects could be even more important due to the twice-as-large electric charge of 3 He.…”

“…Its aim is to measure the proton transversity in the interaction of polarized antiprotons with protons. In order to produce an intense beam of polarized antiprotons, the collaboration intends to use antiproton elastic scattering off a polarized hydrogen target ( 1 H) in a storage ring [2]. The basic idea is connected to the result of the FILTEX experiment [3], where a sizable effect of polarization buildup was achieved in a storage ring by scattering of unpolarized protons off polarized hydrogen atoms at low beam energies of 23 MeV.…”

PAX - The Road towards Polarized Antiprotons

“…Spin filtering is known to work for protons since the FILTEX measurement, performed at the TSR of MPI Heidelberg in the 1990´s [5], which was later on proposed to be used for antiprotons in 2005 [18]. The PAX collaboration has taken on this proposal in connection with the FAIR/HESR project [19].…”