Evidence of shadowing in inelastic nucleon-nucleon cross section

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.

Comments: 5 pages, 4 figures, to appear in the proceedings of the 10th International Conference on Hard and Electromagnetic Probes of High-Energy Nuclear Collisions, 1 - 5 June 2020, Austin, Texas, USA