Jets and dijets produced in these collisions have their origins in hard scattering processes involving gluons for most of the kinematic range at RHIC. By choosing the configuration of the beam polarizations, one can thus gain sensitivity to gluon spin effects.
STAR recently published results for the double-spin asymmetry, ALL, in the differential jet and dijet production cross sections for parallel and anti-parallel configurations of the beam helicities. The data give insight in the gluon spin contribution to the proton spin.
Figure 2. Dijet ALL for event topologies with jet production angles of equal and opposite sign, probing different gluon fractional momenta.
Figure 1 shows ALL for inclusive jets with transverse momenta up to 34 GeV, corresponding to xT = 0.34. The green square markers show the new results [1], whereas the blue triangle markers show prior STAR data [2]. The curves show theory expectations from the DSSV14 [3] and NNPDFpol1.1 [4] collaborations. The corresponding dijet results are shown below for two different dijet topologies versus the dijet invariant mass, M. The prior STAR results provided first evidence for a positive polarization of the gluons in the polarized nucleon for gluon fractional momenta larger than 0.05 upon their inclusion in global analyses. The new results have an approximately twice larger figure of merit, with improved systematic uncertainties, and thus considerably strengthen this evidence.
Since STAR has concluded its data taking with longitudinally polarized protons, these results are anticipated to provide the most precise insights in gluon polarization well into the future, likely until the future Electron-Ion Collider comes online.
References
[1] A. Abdallah et al. (STAR Collaboration), Phys. Rev. D 103, L091103 (2021).
[2] L. Adamczyk et al. (STAR Collaboration), Phys. Rev. Lett. 115, 092002 (2015).
[3] D. de Florian, R. Sassot, M. Stratmann, and W. Vogelsang, Phys. Rev. Lett. 113, 012001 (2014).
[4] E. R. Nocera, R. D. Ball, S. Forte, G. Ridolfi, and J. Rojo (NNPDF Collaboration), Nucl. Phys. B887, 276 (2014).
