Description of di-hadron saturation signals within a universal nuclear parton distribution function approach

Abstract

Di-hadron and di-jet correlation measurements in proton-nucleus ($p$+A) and electron--nucleus collisions are widely motivated as sensitive probes of novel, non-linear QCD saturation dynamics in hadrons, which are particularly accessible in the dense nuclear environment at low values of Bjorken-$x$ ($x_{\mathrm{A}})$. Current measurements at RHIC and the LHC observe a significant suppression in the per-trigger yield at forward rapidities compared to that in proton-proton collisions, nominally consistent with the "mono-jet" production expected in a saturation scenario. However, the width of the azimuthal correlation remains unmodified, in contradiction to the qualitative expectations from this physics picture. I investigate whether the construction of these observables leaves them sensitive to effects from simple nuclear shadowing as captured by, for example, universal nuclear parton distribution function (nPDF) analyses. I find that modern nPDF sets, informed by recent precision measurements sensitive to the shadowing of low-$x_{\mathrm{A}}$ gluon densities in LHC and other data, can describe all or the majority of the di-hadron/jet suppression effects in $p$+A data at both RHIC and the LHC, while giving a natural explanation for why the azimuthal correlation width is unmodified. Notably, this is achieved via a $(x_{\mathrm{A}},Q^2)$-differential suppression of overall cross-sections only, without requiring additional physics dynamics which alter the inter-event correlations.

Comments: 9 pages, 7 figures, final version as published