Measurement of the strong coupling constant with the LHCb detector

Strong interactions of quarks and gluons are described by the gauge field theory named Quantum ChromoDynamics (QCD), which is a component of the Standard Model of particle physics. The strong coupling constant αs indicates the intensity of the strong interaction and is a so called running coupling constant, as its value depends on the energy scale of the process under study. Among the other fundamental constants, αs is, at present, measured with the worst precision; it is crucial to improve it, both for theoretical and experimental purposes. The goal of this thesis work is precisely to obtain a preliminary estimation of αs and its main uncertainties studying data samples collected with the LHCb experiment at LHC, CERN. For the first time at LHCb, data samples taken in 2016 during LHC Run 2 at a center of mass energy of 13 TeV and integrated luminosity 1.6 fb^-1, and containing hadronic jets are analyzed for this aim. After introducing the current status of αs measurement, the LHCb experiment, the jet reconstruction techniques, and the performances of LHCb in reconstructing muons, the work is reported in two parts: the first one is the development of a fast simulation to establish the proper analysis strategy, which turns out to be the determination of the dijet cross section; the second one is the extraction of the dijet cross section from experimental data, and the comparison of it with the simulation in order to determine αs and its uncertainty. A detector calibration is performed to study the contributions to the systematic uncertainty, which come mainly from the simulation of the detector response in jets identification and are factored in with two energy correction parameters, related to the Jet Energy Scale (JES) and the Jet Energy Resolution (JER). The number of selected events, which is the product of the dijet cross section, the selection efficiency and the integrated luminosity of Run 2, is derived from experimental data. The effects of JES, JER and integrated luminosity are propagated inside the selection efficiency, and, in turn, they have an impact on the uncertainty of the αs measurement.