Performance of jet subtraction in pPb and PbPb collisions, without collectivity

We investigated the performance of basic jet subtraction, by the utilization of jet reconstruction generally used at the Large Hadron Collider (LHC). This was done by studying Z-bosons, in the dielectron decay channel, and the associated jet correlation in pp, pPb and PbPb collisions, using datasets generated by the Monte Carlo generator PYTHIA8. The PbPb collisions at $\sqrt{s_{NN}} = 2.76$ TeV and pPb collisions at $\sqrt{s_{NN}} = 5.012$ TeV were generated with the full simulated heavy-ion background by using the Angantyr model (PYTHIA), and compared to the results of pp collisions with a corresponding collision energy. The way the Angantyr model works is by essentially stacking individual nucleon-nucleon sub-collisions on top of each other and hadronize them together, allowing us to study only the microscopic interactions without any assumptions of a thermalized medium or collective interaction. The measurements of Z+leading-jet from these collisions are presented as a function of the transverse momentum balance between Z$^0$ and the jet, the azimuthal angular and the pseudorapidity separation between the Z$^0$ and the jet, and the jet profile. These measurements are presented as a function of collision centrality. The results of the investigation suggests that the jet subtraction performs considerably well correcting the transverse momenta of the jet for peripheral PbPb collisions and pPb events, but the shape of the jets remain strongly damped in the underlying event of central PbPb collisions.Popular Scientific Summary: Recreation of Big Bang When head-on collisions between massive lead ions are performed at the Large Hadron Collider, there are hundreds of collisions happening at once creating tens of thousands of particles, at energies of a few trillion electronvolts. This firework of particles forms for a brief moment a tiny "big bang" so hot that everything inside it "melts" into a so-called \emph{quark-gluon plasma}, or QGP for short. The quark-gluon plasma is a system so incredibly hot and dense that quarks and gluons, the building blocks of all ordinary matter, are no longer bound to their primary \emph{hadron}, but can freely interact with e.g. quarks from other hadrons. At this brief moment, we have recreated the conditions similar to those that exist in the very early moments of the universe. As this ultra hot fireball expands and cools down immediately afterwards, the individual quarks and gluons recombine into their natural bound states, such as protons and neutrons and other less familiar and unstable hadrons. These particles are then dispersed in all directions and detected with a gigantic detector system. The information carried by these particles is very important to our advancement of knowledge of \emph{quantum chromodynamics}, which is the theory describing the nature of quarks and gluons. Together with the electrons they form the building blocks of everything we see around us. In order to make any sense of the circus of particles registered by the detectors, we need a physical theory and models, that are able to explain the outcome and what has happened. These collisions are, however, normally described with statistical models, which deals with averaged properties, such as pressure, density and temperature. With this in mind, a group of people at the Lund University has developed a new event generator model, which will simulate heavy-ion collisions in great detail, down to the individual quarks and gluons, in order to separate effects of different origin. My role is to investigate the performance of basic methods of analysing heavy-ion collisions, simulated by the new event generator. These methods rely on an indirect observables that are sensitive to the effects of the QGP. The observable is a spray of particles, a so called jet, that passes through the plasma. At very rare occasions, the primary collision between a combination of two quarks/gluons from each lead ion, produces a very energetic electron-positron pair, that will pass the plasma unchanged. Together with this, we get a corresponding recoil of particles in the opposite direction of equal magnitude, a "hard jet", that will interact strongly with the surrounding quarks and gluons in the quark-gluon plasma. This type of hard scattering is extremely instructive, as it can provide information on the jet modification that occurred in the plasma, thus revealing properties of the quark-gluon plasma. This new event generator will serve as a very powerful tool and may help future studies, as it will contribute to the understanding of the quark-gluon plasma and the forces behind the most elementary particles. All in all, it will give us one more piece of the puzzle, that is to understand the origin of matter and the beginning of our universe