Evidence of shadowing in inelastic nucleon-nucleon cross section

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 σ^inel which is nearly always taken from proton-proton measurements. At the LHC energies nn σ^inel depends on the QCD dynamics at small x and low interaction scales where the shadownn ing/saturation phenomena are expected to become relatively more important for larger nuclei than for the proton. Thus, σ_nn^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^± and Z measurements as a "standard candle" to extract σ_nn^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 σ^inel in Pb+Pb collisions is suppressed relative to the proton-proton measurements by tens of nn percents. We demonstrate that this is in line with expectations from nuclear PDFs.