Abstract The relation between elastic amplitudes for polarized

The relation between elastic amplitudes for polarized proton-proton collisions
and spin-dependent scattering observables is studied near the forward direction
with a view to providing information on particular amplitudes at low values of
momentum transfer. Such information is particularly important, for example, in
considerations relating to the use of the transverse single-spin asymmetry in
estimating the beam polarization achieved by the increasing number of
accelerators aspiring to offer spin-polarized hadronic particles at higher energy.
Elastic scattering, a process at the confluence of "soft" physics and QCD, and
where the role of spin could be more significant and complex than previously
thought, is becoming topical again at high energies, as it might give a link
between the perturbative and nonperturbative regimes of QCD. This interest is
further enhanced by the progress in the acceleration and storage of polarized
proton beams at the new RHIC collider, and by the related problem of measuring
the polarization of such beams. The spin-dependence of high energy $pp$
forward elastic scattering is associated to a number of critical features of
small-angle differential cross sections, spin asymmetries and total cross-sections
in definite initial spin states. Electromagnetic and hadronic forces are of similar
magnitude at small scattering angles, and they interfere in the spin-averaged
forward differential cross-section as well as in a number of spin-dependent
asymmetries. Previous investigations on possible contributions of an hadronic
spin-flip amplitude to the polarization asymmetry in the Coulomb-nuclear
interference (CNI) region, are extended to include complete consideration of all
helicity amplitudes near the forward direction, and comparison with experimental
data is performed whenever possible.