Evidence of shadowing in inelastic nucleon-nucleon cross section

TL;DR

This paper investigates the inelastic nucleon-nucleon cross section in heavy-ion collisions at LHC energies, revealing a significant suppression in Pb+Pb collisions compared to proton-proton data, consistent with shadowing effects in nuclear PDFs.

Contribution

It introduces a method using W and Z boson measurements as standard candles to extract the inelastic nucleon-nucleon cross section in heavy-ion collisions, highlighting shadowing effects.

Findings

The inelastic nucleon-nucleon cross section in Pb+Pb is suppressed by tens of percent.

The suppression aligns with predictions from nuclear parton distribution functions.

The method leverages ATLAS data and advanced QCD calculations.

Abstract

The Glauber modeling plays a key role in centrality-dependent measurements of heavy-ion collisions. A central input parameter in Glauber models is the inelastic nucleon-nucleon cross section $\sigma_{\text{nn}}^{\text{inel}}$ which is nearly always taken from proton-proton measurements. At the LHC energies $\sigma_{\text{nn}}^{\text{inel}}$ depends on the QCD dynamics at small $x$ and low interaction scales where the shadowing/saturation phenomena are expected to become relatively more important for larger nuclei than for the proton. Thus, $\sigma_{\text{nn}}^{\text{inel}}$ e.g. in Pb+Pb collisions may well be lower than what is seen in proton-proton collisions. In this talk, we demonstrate how to use the recent $W^\pm$ and $Z$ measurements as a "standard candle" to extract $\sigma_{\text{nn}}^{\text{inel}}$ in Pb+Pb collisions. Our analysis -- built on the ATLAS data, state-of-the-art NNLO QCD calculations and nuclear PDFs -- indicate that at the LHC energies $\sigma_{\text{nn}}^{\text{inel}}$ in Pb+Pb collisions is suppressed relative to the proton-proton measurements by tens of percents. We demonstrate that this is in line with expectations from nuclear PDFs.