Probing the Pomeron spin structure with Coulomb-nuclear interference

Polarized pp elastic scattering at small angles in the Coulomb-nuclear interference (CNI) region offers
a unique opportunity to study the spin structure of the Pomeron. Electromagnetic effects in elastic
amplitude can be equivalently treated either as Coulomb corrections to the hadronic amplitude (Coulomb
phase), or as absorption corrections to the Coulomb scattering amplitude. We perform the first calculation
of the Coulomb phase for the spin-flip amplitude and found it significantly exceeding the widely used
non-flip Coulomb phase. The alternative description in terms of absorption corrections, though equivalent,
turned out to be a more adequate approach for the Coulomb corrected spin-flip amplitude. Inspired by
the recent high statistics measurements of single-spin asymmetry with the HJET polarimeter at the BNL,
we also performed a Regge analysis of data, aiming at disentangling the Pomeron contribution. However,
in spite of an exceptional accuracy of the data, they do not allow to single out the Pomeron term,
which strongly correlates with the major sub-leading Reggeons. A stable solution can be accessed only by
making additional ad hoc assumptions, e.g. assuming the Pomeron to be a simple Regge pole, or fixing
some unknown parameters. Otherwise, in addition to the STAR data at √s = 200 GeV new measurements,
say at 100 GeV or 500 GeV, could become decisive.