Abstract
The production yield of Z bosons is measured in the electron and muon decay channels in Pb+Pb collisions at sNN=5.02 TeV with the ATLAS detector. Data from the 2015 LHC run corresponding to an integrated luminosity of 0.49 nb−1 are used for the analysis. The Z boson yield, normalised by the total number of minimum-bias events and the mean nuclear thickness function, is measured as a function of dilepton rapidity and event centrality. The measurements in Pb+Pb collisions are compared with similar measurements made in proton–proton collisions at the same centre-of-mass energy. The nuclear modification factor is found to be consistent with unity for all centrality intervals. The results are compared with theoretical predictions obtained at next-to-leading order using nucleon and nuclear parton distribution functions. The normalised Z boson yields in Pb+Pb collisions lie 1–3σ above the predictions. The nuclear modification factor measured as a function of rapidity agrees with unity and is consistent with a next-to-leading-order QCD calculation including the isospin effect.
Highlights
The measurement of electroweak (EW) boson production is a key part of the heavy-ion (HI) physics programme at the LargeHadron Collider (LHC)
Measurements performed by the ATLAS and CMS experiments with Z and W bosons decaying leptonically show that production rates of these non-strongly interacting particles are proportional to the amount of nuclear overlap, quantified by the mean nuclear thickness function, TAA [1,2,3,4,5,6]
The measurements performed in the two channels are combined using the Best Linear Unbiased Estimate (BLUE) method [52], accounting for the correlations of the systematic uncertainties across the channels and measurement bins
Summary
The measurement of electroweak (EW) boson production is a key part of the heavy-ion (HI) physics programme at the LargeHadron Collider (LHC). After being created at the initial stage of the collision in high-momentum exchange processes, Z and W bosons decay much faster than the timescale of the medium’s evolution. Their leptonic decay products are generally understood to not be affected by the strong interaction; they carry information about the initial stage of the collision and the partonic structure of the nuclei. Measurements performed by the ATLAS and CMS experiments with Z and W bosons decaying leptonically show that production rates of these non-strongly interacting particles are proportional to the amount of nuclear overlap, quantified by the mean nuclear thickness function, TAA [1,2,3,4,5,6]. The first includes the same background sources that are studied in pp collisions [15] and the second includes additional background sources specific to the Pb+Pb collision system
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