Saturation effects in SIDIS at very forward rapidities

E. Iancu; A. H. Mueller; D. N. Triantafyllopoulos; S. Y. Wei

doi:10.1007/JHEP07(2021)196

paper

Saturation effects in SIDIS at very forward rapidities

arXiv:2012.08562 · doi:10.1007/JHEP07(2021)196

Abstract

Using the dipole picture for electron-nucleus deep inelastic scattering at small Bjorken $x$ , we study the effects of gluon saturation in the nuclear target on the cross-section for SIDIS (single inclusive hadron, or jet, production). We argue that the sensitivity of this process to gluon saturation can be enhanced by tagging on a hadron (or jet) which carries a large fraction $z ≃ 1$ of the longitudinal momentum of the virtual photon. This opens the possibility to study gluon saturation in relatively hard processes, where the virtuality $Q^{2}$ is (much) larger than the target saturation momentum $Q_{s}^{2}$ , but such that $z (1 - z) Q^{2} ≲ Q_{s}^{2}$ . Working in the limit $z (1 - z) Q^{2} ≪ Q_{s}^{2}$ , we predict new phenomena which would signal saturation in the SIDIS cross-section. For sufficiently low transverse momenta $k_{⊥} ≪ Q_{s}$ of the produced particle, the dominant contribution comes from elastic scattering in the black disk limit, which exposes the unintegrated quark distribution in the virtual photon. For larger momenta $k_{⊥} ≳ Q_{s}$ , inelastic collisions take the leading role. They explore gluon saturation via multiple scattering, leading to a Gaussian distribution in $k_{⊥}$ centred around $Q_{s}$ . When $z (1 - z) Q^{2} ≪ Q^{2}$ , this results in a Cronin peak in the nuclear modification factor (the $R_{p A}$ ratio) at moderate values of $x$ . With decreasing $x$ , this peak is washed out by the high-energy evolution and replaced by nuclear suppression ( $R_{p A} < 1$ ) up to large momenta $k_{⊥} ≫ Q_{s}$ . Still for $z (1 - z) Q^{2} ≪ Q_{s}^{2}$ , we also compute SIDIS cross-sections integrated over $k_{⊥}$ . We find that both elastic and inelastic scattering are controlled by the black disk limit, so they yield similar contributions, of zeroth order in the QCD coupling.

arXiv comment53 pages, 13 figures, 6 appendices. New appendix on double-logarithmic corrections, 2 additional figures and minor corrections. Published in JHEP

Saturation effects in SIDIS at very forward rapidities

Abstract

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