References - Bogoliubov Laboratory of Theoretical Physics - JINR

STATUS OF THE PAX EXPERIMENT
Erhard Steffens 1 † ,
For the PAX Collaboration 2
(1) University of Erlangen-Nürnberg
(2) www.fz-juelich.de/ikp/pax/
† E-mail: steffens@physik.uni-erlangen.de
Abstract
The PAX experiment aims at the production of a spin-polarized antiproton beam
by means of Spin-Filtering, and the study of the spin-dependent pp cross sections.
The PAX collaboration is preparing for detailed investigations of the spin-filtering
process of protons at COSY (FZ Jülich) and of antiprotons at the AD (CERN).
The physics program with polarized antiprotons includes the study of the Drell-Yan
process as a direct measurement of transversity, a leading twist structure function of
the nucleon. In the talk, a recent measurement of the depolarization of an initially
polarized proton beam by co-moving electrons at low relative velocity is presented.
Implications for the role of electrons in the polarization buildup of a stored proton
beam are discussed.
1 The PAX experiment
There is consensus that the QCD physics potential of experiments with high energy polarized
antiprotons would be enormous, but up to now high–luminosity experiments were not
possible. The situation would change dramatically with the production of stored polarized
antiproton beams, and the subsequent realization of a double–polarized high–luminosity
antiproton–proton collider. The list of fundamental physics issues to be addressed with
such a collider includes the measurement of transversity, the quark transverse polarization
inside a transversely polarized proton, which constitutes the last missing leading twist
piece of the QCD description of the partonic structure of the nucleon. The transversity
can be directly accessed only via double–polarized antiproton–proton Drell–Yan production.
It should be noted, that without a measurement of the transversity, our knowledge
of the the spin structure of the proton will remain incomplete. Other items of interest
are the measurement of the phases of the timelike form factors of the proton, and
double–polarized hard antiproton–proton scattering.
The PAX collaboration (Polarized Antiproton eXperiments) has formulated its ambitious
physics program [1]. A Technical Proposal [2] has been submitted for the new
Facility for Antiproton and Ion Research (FAIR) to be built at GSI in Darmstadt, Germany.
The uniqueness and the strong scientific merits that would become available with
the advent of stored beams of polarized antiprotons have been well received [3], and there
is now an urgency to convincingly demonstrate experimentally that a high degree of antiproton
beam polarization can be reached. The suggested collider aims at luminosities in
excess of 10 31 cm −2 s −1 . An integral part of such a machine is a dedicated large–acceptance
Antiproton Polarizer Ring (APR) [4], for which a basic design has been developed [5].
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