Large-distance effects on spin observables at RHIC∗

The impact of large-distance contributions on the behaviour of the slopes of the spin-non-flip and of the spin-flip amplitudes is analysed. It is shown that the long tail of the hadron potential in impact parameter space leads to a larger value of the slope for the spin-flip amplitude (without the kinematic factor √ |t|) than for the spin-non-flip amplitude. This effect is taken into account in the calculation of the analysing power in proton-nucleus reactions at high energies. The recent data from RHIC and HERA indicate that, even at high energy, the hadronic amplitude has a significant spin-flip contribution, Asf , which remains proportional to the spin-non-flip part, Anf , as energy is increased. Theoretically, when large-distance contributions are considered, one can obtain a more complicated spin structure for the pomeron coupling. The dependence of the hadron spin-flip amplitude on the momentum transfer at small angles is tightly connected with the basic structure of hadrons at large distances. We show that the slope of the “reduced” hadron spinflip amplitude (i.e. the hadron spin-flip amplitude without the kinematic factor √ |t|) can be larger than the slope of the hadron spin-non-flip amplitude as was observed long ago [1, 2]. This large slope has a small effect on the differential hadron cross section and on the real part of hadron non-flip amplitude [3]. presented by O.V.S. at the 16th International Spin Physics Symposium, spin2004, October 10-16 2004, Trieste, Italy Inst. de Physique, Univ. de Liège, Liège, Belgium BLTPh, JINR, Dubna, Russia. Dip. di Fisica Teorica Univ. di Torino and Sezione INFN di Torino, Italy.