CERN Accelerating science

In Nature, free quarks and gluons cannot be observed due to the confinement properties of quantum chromodynamics (QCD). However, in extreme conditions, a state of matter called the Quark-Gluon Plasma (QGP) is formed in which quarks and gluons are deconfined. The QGP can be created and studied by colliding ultrarelativistic heavy nuclei. At the Large Hadron Collider (LHC), collisions between lead (Pb) nuclei take place and are studied at the various experiments. In this thesis we present measurements on Z bosons and \jpsi, reconstructed in the muon spectrometer of the ALICE detector. Observations from PbPb collisions are subject to effects unrelated to the QGP, and it is important to disentangle these from QGP effects. In particular, parton distributions are different in heavy nuclei than in unbound protons and neutrons. In order to describe the production of probes with large transverse momentum, special objects called nuclear parton distribution functions (nPDFs) are necessary. They are a crucial ingredient of calculations of cross sections in heavy ion collisions, but cannot be obtained from first principles calculations in QCD. Instead, they are instead obtained from fits to data which include, among others, measurements of electroweak bosons. The latter can therefore be used to constrain or verify the accuracy of the nPDFs. In this thesis, the production of Z bosons in PbPb collisions at a center-of-mass energy per nucleon pair of 5.02 TeV with the ALICE detector is reported. The results strongly show a preference for models that take nuclear modifications to PDFs into account. This indicates that parton distributions in nuclei are not a simple superposition of protons and neutrons. In the near future, proton-oxygen and oxygen-oxygen collisions will take place at the LHC. We detail projections of Z boson production in these collisions, obtained by extrapolating existing cross sections from the ALICE and LHCb experiments. With the expected integrated luminosities for the two collision systems, measurements are found to be unfeasible. In proton-proton collisions, heavy particles such as the \jpsi are produced from a single partonic scattering (SPS). It is possible to create a pair of \jpsi from such a process, as opposed to a single \jpsi. However, \jpsi pairs can also be created through another mechanism. Because the densities of partons increase rapidly at high energies, it becomes possible for two parton scatterings to occur during a single pp collision. This is referred to as double parton scattering (DPS). \jpsi pairs can therefore be used as a tool to study relative contributions of SPS and DPS. In this thesis, a measurement of the production cross section of \jpsi pairs in proton-proton collisions at sqrt(s) = 13 TeV is presented. The measurement serves as a first of its kind in ALICE. It shows good agreement the cross section obtained by LHCb collaboration, although experimental uncertainties are large. Future LHC data taking periods will provide a larger data sample, which will allow for a measurement with better experimental precision. This could enable a measurement in which SPS and DPS contributions can be separated.