Evidence of shadowing in inelastic nucleon-nucleon cross section

About

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 $\sigma_{\rm nn}^{\rm inel}$ which is nearly always taken from proton-proton measurements. At the LHC energies $\sigma_{\rm nn}^{\rm inel}$ 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 proton. Thus, $\sigma_{\rm nn}^{\rm inel}$ 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 $\sigma_{\rm nn}^{\rm inel}$ 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 $\sigma_{\rm nn}^{\rm inel}$ 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..

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