Measurement of Z bosons in p+Pb collisions at $\sqrt{s_{NN}}$=5.02 TeV with the ATLAS detector

The aim of this thesis is to study the geometry of heavy ion collisions and nuclear effects. This thesis describes the measurement of $Z$ bosons produced in p+Pb collisions at $\sqrt{s_{NN}}$= 5.02 TeV using the ATLAS detector at the LHC. Z bosons - produced by the Drell-Yan process - are measured in the muonic decay channel. From an integrated luminosity of 25.5 nb$^{−1}$, 1941 $Z$ bosons are observed with a background contamination, estimated from the number of same-sign dimuons, of 0.3%. The measurement is performed within different centrality groups. From the distribution of the yield within each centrality group, Z production is found not to scale with the number of binary collisions using the default Glauber Model. Instead it is shown, that a correct description of the centrality in p+Pb collisions might require the Glauber-Gribov Model, which includes fluctuations in the nucleon-nucleon cross-section. Apart from the centrality determination, the differential $Z → μμ$ cross-section is measured. The differential cross-section is found to be: $σ = 182^{+15.1}_{−13.4}\textrm{nb}$ No earlier measurements of Z production in p+A collisions exist, and direct comparison is therfore not performed. The comparison is instead done with the cross-section obtained in p+p collisions assuming binary scaling. It gives an enhancement of $Z$ production in p+Pb collisions of about 50% compared to $Z$ production in p+p collisions, which cannot be explained by well-know effects.