2 00 3 Universal threshold for single - spin asymmetries in fixed target experiments

The analysis of inclusive π-meson single spin asymmetry measurements was done. It was found that the single spin asymmetry starts to grow up at the same value of the π-meson energy in the center of mass system for fixed target experiments at the beam energy range from 13 to 200 GeV. Polarization experiments give us an unique opportunity to probe the nucleon internal structure. While spin averaged cross-sections can be calculated within acceptable accuracy, current theory of strong interactions can not describe large spin asymmetries and polarization. Unexpected large values of single spin asymmetry A N (SSA) in inclusive π-meson production are real challenge to the theory because pertur-bative Quantum Chromodynamics predicts small asymmetries decreasing with transverse momentum. Various models were developed to explain results from E704 (FNAL), PROZA-M and FODS (both Protvino) and several BNL experiments. Most of the models analyse experimental data in terms of x F and/or p T. To investigate the dependence of SSA on a secondary meson production angle, the measurements in the reaction π − + p ↑ → π 0 + X were carried out at the PROZA-M experiment (Protvino) at 40 GeV pion beam in the two different kinematic regions: at Feinman scaling variable x F ≈ 0 [1] and in the polarized target fragmentation region [2]. It was reported [2] that the asymmetry of inclusive π 0 production in the reaction π − + p ↑ → π 0 + X starts to grow up at the same centre of mass energy E cms 0 ≈ 1.7 GeV for the both kinematic regions. A similar behaviour was also found in the reaction p + p ↑ → π 0 + X at 70 GeV [3]. There is an indication that the asymmeties are zero till some threshold value and then start to rise up linearly and in principle may saturate at some level. In this case we can fit SSA by the function