  We discuss different models for the spin structure of the nonperturbative
pomeron: scalar, vector, and rank-2 symmetric tensor. The ratio of
single-helicity-flip to helicity-conserving amplitudes in polarised high-energy
proton-proton elastic scattering, known as the complex r_5 parameter, is
calculated for these models. We compare our results to experimental data from
the STAR experiment. We show that the spin-0 (scalar) pomeron model is clearly
excluded by the data, while the vector pomeron is inconsistent with the rules
of quantum field theory. The tensor pomeron is found to be perfectly consistent
with the STAR data.
