Measurement of W boson production in Pb+Pb collisions at 5.02 TeV with the ATLAS detector

In this thesis measurement of inclusive production of $W^\pm\to\ell^\pm\nu$ in the electron and muon channels in Pb+Pb collisions at $\sqrt{s_{\mathrm{NN}}} = 5.02$~TeV is presented. The fiducial production yields are measured in the phase space region defined by the charged lepton transverse momentum $p_{\mathrm{T}}^{\ell}>25$~GeV and pseudorapidity $|\eta_\ell|<2.5$, the transverse momentum of the neutrino $p_{\mathrm{T}}^\nu>25$~GeV and the transverse mass of the charged lepton--neutrino system $m_\mathrm{T}>40$~GeV. After background subtraction and efficiency correction, the normalised production yields, corrected for background and efficiency, are presented as a function of the absolute pseudorapidity of the charged lepton and the average number of nucleons participating in the collision, the later being a measure of the collision centrality. The normalised production yields for $W^\pm$ bosons are consistent between the two leptonic decay channels which are combined in this analysis. The combined normalised production yields are consistent with theoretical predictions based on the \textsc{CT14nlo} PDF set, while predictions obtained with the EPPS16 and nCTEQ15 nPDF sets underestimate the measured yields by 10--20\%. The measured yields for $W^\pm$ bosons are also used to obtain the lepton charge asymmetry, which is well described by the above mentioned theoretical predictions. The lepton charge asymmetry changes sign and becomes negative for $|\eta_\ell| > 2$, which an result of the isospin effect. It is expected that $W$ production should scale with the average value of nuclear thickness function $\langle T_{\mathrm{AA}} \rangle$ evaluated from the Glauber model. Normalised production yields for $W^\pm$ bosons are in agreement with the expected scaling with $\langle T_{\mathrm{AA}} \rangle$ for central events. In the range $N_{\mathrm{part}}<200$, a systematic excess of the normalised production yields of $W^\pm$ bosons is observed in the data in comparison to the theory predictions. The effect is largest in the most peripheral bin for $W^-$ bosons where the excess amounts to 1.7 standard deviations. A comparison of normalised production yields for geometric parameters obtained with two versions of the Glauber model v2.4 and v3.2 shows that the Glauber v3.2 results are somewhat closer to the predictions. However, the difference between the two results is smaller than the measurement uncertainties. Impact of the neutron-skin effect evaluated using the separate radial distributions for protons and neutrons provided by the Glauber model v3.2 was found to be at the level -1.4\% (1\%) for $W^+$ ($W^-$) bosons with respect to predictions calculated using a constant proton-to-neutron ratio. Given current measurement precision data are not sensitive enough to confirm the neutron-skin effect.