Thesis

At the Large Hadron Collider (LHC) at Geneva, collisions of nuclei are used to create a thermodynamic system described by fluid dynamics under extreme conditions of temperature and energy density. The temperature of the short-lived system is sufficiently
high in order to release the building blocks of matter at a subnucleonic scale, quarks and gluons. This state of matter is commonly called Quark Gluon Plasma (QGP). The space-time evolution of heavy-ion and light-ion collisions at the LHC is described by close-to-ideal hydrodynamics after a short pre-equilibrium lapse of time. However, key features of the
early stages of these collisions are largely unknown. These characteristics are crucial tounderstand the applicability limits of hydrodynamics as well as the thermalization process of a strongly interacting system.
In recent publications, it was pointed out that the dilepton production in the intermediate mass scale between 1.5 and 5 GeV/c² is highly sensitive to the ´thermalisation´ time scale towards the equilibrium QGP. In 2025, the LHC collided for the first time oxygen nuclei. In these collisions, the background conditions are less challenging than in lead-lead collisions
and the experimental sensitivity to the preequilibrium dileptons is enhanced.
The principal goal of this thesis is to perform the first preequilibrium dilepton production measurement at the LHC in Oxygen
-Oxygen collisions using the LHCb apparatus.
This measurement will be based on novel background rejection techniques exploiting the forward geometry of LHCb.
In parallel, the discrimination between the preequilibrium signal and other sources of dileptons, most notably the Drell
-Yan process, will be studied thanks to theoretical calculations and simulations. In a second part, the measurement feasibility will be studied with the current and the future LHCb detector. Depending on the result, a measurement in heavy
-ion collisions could be envisaged.