QCD tests in $pp$ collisions at $\sqrt{s} = 7$ TeV using the ATLAS detector at the LHC

The Large Hadron Collider facility at CERN, and in particular the ATLAS detector, have provided a testing ground for high energy physics at a level which couldn't be explored in previous experiments. It has not only increased the centre-of-mass energy to the up to now highest value of $\sqrt{s} = 7$ TeV but also has had an excellent luminosity performance during these years of data taking. This has been especially important for the discovery of the Higgs boson, as well as for allowing searches of new physics and studies of the properties of, up to now, poorly known final states as $t\bar{t}$ production. In this thesis, data taken during the year 2011 have been analyzed to probe the strong $SU(3)_{C}$ sector of the Standard Model. The parton shower and hadronisation models have been tested in this thesis by means of the measurement of jet shapes in $t\bar{t}$ final states. Relying on the top-quark decays $t\to bW$ and the subsequent $W$ boson hadronic decays $W\to q \bar{q'}$, samples with very high purity of $b$- and light-quark induced jets have been selected. This fact, together with the fine granularity of the ATLAS calorimeter system, has allowed us to compare $b$-jet shapes with those of light jets (ATLAS Collaboration, Eur. Phys. J. C \textbf{73} 2676 (2013)) and to determine the $b$-quark mass from the so-called angular screening effects, clearly visible in these data (J. Llorente and J. C., Nucl. Phys. B \textbf{889} 401 (2014)). Furthermore, $b$-jets are found to be broader than light jets and therefore $b$-jet shapes may be used for tagging purposes. The value of the strong coupling constant $\alpha_s$ at the $Z^{0}$-pole scale has also been measured in this thesis. This constant plays a key role in our understanding of nature at the subatomic scale, as it determines the strength of gluon radiation, as well as being responsible for the stability of nuclear matter. We have selected a sample of multijet events by demanding the scalar sum of transverse momenta of the two leading jets to be greater than 500 GeV while the transverse momenta of the subleading jets is required to be above 50 GeV. All jets are required to lie in the central part of the calorimeter, i.e. $|\eta| < 2.5$. We have performed a measurement of transverse energy-energy correlations, which are the natural extension of the energy-energy correlation function which was popular in $e^+ e^-$ colliders at PETRA-PEP and LEP-SLC energies. We have performed a comparison between the data (ATLAS Collaboration, arXiv:1508.01579 [hep-ex]) with NLO pQCD calculations (A. A., F. B., J. Llorente and W. W., Phys. Rev. D \textbf{86} 114017 (2012)) which allows us to determine $\alpha_s(m_Z) = 0.1173 \pm 0.0010 \mbox{ (exp.)} ^{+0.0065}_{-0.0026} \mbox{ (theo.)}$.