Positivity constraints on initial spin observables in inclusive reactions

Abstract

For any inclusive reaction of the type $A_1(spin1/2)+A_2(spin1/2)\to B+X$, we derive new positivity constraints on spin observables and study their implications for theoretical models in view, in particular, of accounting for future data from the polarized $pp$ collider at BNL-RHIC. We find that the single transverse spin asymmetry $A_N$ for several processes, in the central region for example jet production, direct photon production, lepton-pair production, must be such that $|A_N|\text{lappeq}1/2$, rather than the usual bound $|A_N|\le 1$.

Comments: 6 pages, version to be published in Phys.Rev.Lett