Measurement of four-jet differential cross sections in $\sqrt{s} = 8$~TeV proton–proton collisions using the ATLAS detector

The work presented in this thesis has been carried out in the frame of the ATLAS experiment at the Large Hadron Collider. It is centred around the study of jets, their properties, their measurement in the calorimeter, and their production cross section. One of the main aspects of this thesis is the measurement of the differential cross section for the production of events with many jets, using the dataset collected by the ATLAS detector in proton-proton collisions at $\sqrt{s} = 8$ TeV delivered by the LHC. The event selection requires that the four ${\textrm{anti-}}k_{t}$ $R=0.4$ jets in the event with the largest transverse momentum ($p_{\text{T}}$) within the rapidity range $|y|<2.8$ are well separated ($\Delta R_{\text{4j}}^{\min}>0.65$), all have $p_{\text{T}}>64$ GeV, and include at least one jet with $p_{\text{T}}>100$ GeV. The measurement considers a wide variety of variables, including distributions of jet momenta, invariant masses, minimum and maximum opening angles and other kinematic variables. At the same time, these variables were binned in regions of momenta and mass in order to probe more diverse kinematic regimes. The presented cross sections are corrected for detector effects and compared to some of the leading-order calculations of most widespread use in experimental physics, as well as next-to-leading-order theoretical predictions. Other aspects of the work regarded the measurement of the momentum resolution of jets, and the design and assembly of new scintillator detectors in the calorimeter. The first subject involves the study of jet properties related to its energy resolution. This is an important measurement within the collaboration, as many analysis have jets in their final state. The resolution is affected by the data taking conditions, and it can worsen considerably when the instant luminosity and the energy of the collisions are increased. Consequently, the improvement of the jet resolution is one of the main goals in the corrections applied in jet calibration, which in turn need to be tested and validated through in situ methods. The work related to scintillator detectors was carried out during the period in which data taking was halted, between 2013 and 2014, to allow for the repair and upgrade of the accelerator and the experiments. The irradiated scintillators used during Run 1 were removed for study, in order to make an improved design and assembly new detectors.