Polarized Hyperons from pA Scattering in the Gluon Saturation Regime

Abstract

We study the production of transversely polarized Lambda hyperons in high-energy collisions of protons with large nuclei. The large gluon density of the target at saturation provides an intrinsic semi-hard scale which should naturally allow for a weak-coupling QCD description of the process in terms of a convolution of the quark distribution of the proton with the elementary quark-nucleus scattering cross section (resummed to all twists) and a fragmentation function. In this case of transversely polarized Lambda production we employ a so-called polarizing fragmentation function, which is an odd function of the transverse momentum of the Lambda relative to the fragmenting quark. Due to this k_t-odd nature, the resulting Lambda polarization is essentially proportional to the derivative of the quark-nucleus cross section with respect to transverse momentum, which peaks near the saturation momentum scale. Such processes might therefore provide generic signatures for high parton density effects and for the approach to the ``black-body'' (unitarity) limit of hadronic scattering.

Figures (2)

• Figure 1: Kinematics of the $p \, A \to \Lambda \, X$ process. The direction of positive $\Lambda$ polarization is indicated for each quadrant in the $\Lambda$ production plane.
• Figure 2: Transverse momentum distribution of the transverse $\Lambda$ polarization. Left: at fixed longitudinal momentum fraction $\xi=0.5$ and varying target saturation scale, $Q_s=2,3$ GeV, respectively. Right: For $Q_s=2$ GeV and various $\xi$.