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

TL;DR

This paper shows that nuclear shadowing effects, modeled through universal nuclear parton distribution functions, can explain di-hadron and di-jet suppression in proton-nucleus collisions without invoking non-linear saturation dynamics, aligning with experimental observations.

Contribution

It demonstrates that modern nPDF analyses can account for observed suppression effects and the unchanged azimuthal correlation width in $p$+A collisions, challenging the necessity of saturation-based explanations.

Findings

nPDFs describe di-hadron/jet suppression at RHIC and LHC

Unmodified azimuthal correlation width explained by shadowing

Suppression explained without additional non-linear physics

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.