The single inclusive prompt- production is the clearest signal for accessing the gluon polarization. It is dominated by the -gluon scattering. In order to measure , both the prompt- and the away-side jet must be detected in coincidence so that the kinematics of the incoming partons can be approximately calculated, and in particular one must select events with Bjorken of the up quark larger than 0.2-0.3 so that is sizable. The gluon spectrum at GeV, GeV/c and is shown in Figure 2.2 . It is peaked around 0.06, starts at 0.03 and doesn't extend much above 0.1. This range is lower than the one covered in COMPASS, and hence the two experiments will probe the gluon polarization over complementary intervals. The scale in COMPASS is set by and GeV ; for the high prompt- and jet events at RHIC the scale is set by the prompt- or jet and about a few 100 GeV . A potential uncertainty on the measurement comes from the role of NLO corrections to the prompt- production as well. These corrections were calculated for the unpolarized case and are not small.
Figure 2.2: The gluon momentum fraction covered in prompt- production.
There are some discrepancies on the reported accuracies for this measurement, in particular concerning the study of systematic effects. The reported accuracies range from  to and less  at GeV. The RHIC high luminosity option ( at GeV, 10 times higher than the original RHIC design luminosity) was assumed for these estimates. These measurement accuracies will lead to a sensitivity on ranging roughly from 0.15 to 0.05. Also a precise knowledge of is essential. In the present estimates it was assumed that at with infinite precision, while recent SMC results show still large uncertainty on this quantity for . Taking into account also this uncertainty the sensitivity on will decrease.