Centrality dependence of electroweak boson production in PbPb collisions at the LHC

TL;DR

This paper investigates the centrality dependence of electroweak boson production in PbPb collisions at the LHC, proposing that a centrality bias explains the data instead of shadowing effects, aligning observations with nuclear parton distribution functions.

Contribution

It offers an alternative explanation for the observed data by introducing a centrality bias, challenging previous interpretations based on shadowing effects in the Glauber model.

Findings

Data aligns with nuclear PDFs when considering centrality bias.

A larger Pb nucleus skin thickness could influence collision cross sections.

Centrality bias may resolve discrepancies without invoking shadowing effects.

Abstract

Recent data on the nuclear modification of W and Z boson production measured by the ATLAS collaboration in PbPb collisions at $\sqrt{s_{\rm nn}}=5.02$ TeV show an enhancement in peripheral collisions, seemingly contradicting predictions of the Glauber model. The data were previously explained by arguing that the nucleon-nucleon cross section may be shadowed in nucleus-nucleus collisions, and hence suppressed compared to the proton-proton cross section at the same collision energy. This interpretation has quite significant consequences for the understanding of heavy-ion data, in particular in the context of the Glauber model. Instead, we provide an alternative explanation of the data by assuming that there is a mild bias present in the centrality determination of the measurement; on the size of the related systematic uncertainty. Using this assumption, we show that the data is in agreement with theoretical calculations using nuclear parton distribution functions. Finally, we speculate that the centrality dependence of the W$^-$/W$^{+}$ ratio may point to the relevance of a larger skin thickness of the Pb nucleus, which, if present, would result in a few percent larger PbPb cross section than currently accounted for in the Glauber model and may hence be the root of the centrality bias.