Strange particle production in $t \bar{t}$ final states in $pp$ collisions at $\sqrt{s} = 7$ TeV with the ATLAS detector at the LHC

The LHC has opened up a new era in particle physics in as much it paved the way for the discovery of the Higgs boson, the missing piece in the so called standard model (SM) , pushing the lower limits for particles predicted in many beyond the standard model (BSM) scenarios to values well above the TeV scale, allowing precision measurements at unprecedented energies. This ambitious scientific program has been carried out with the construction of two big multipurpose detectors: ATLAS and CMS. The discovery of the Higgs boson is, no doubt, one of the most important milestones in the history of particle physics. Another field where one can reach unprecedented precision is in the study of the top quark. With the advent of unprecedented statistics for top quark pair production, one can think of directly measuring the Cabibbo-Kobayashi-Maskawa matrix elements where the top quark is involved, in particular $V_{ts}$. Ali et al. [Phys. Lett. B 693 (2010) 44-51] made the proposal to determine this matrix element by discerning the signal $t\bar{t} \rightarrow W b W s$ final state from the dominant background process $t\bar{t} \rightarrow W b W b$. This could be done by looking at neutral strange particle production which turns out to be leading in $s$-quark jets while non-leading in $b$-quark jets as $b$-quarks decay sequentially $b\rightarrow c \rightarrow s$. This proposal makes use of MVA techniques, with the possible existence of jet secondary vertices and soft leptons as additional variables with discriminant power. Therefore this proposal is dependent on the good agreement between data and Monte Carlo generators used in the training phase. However the proposal we refer to is qualitatively right, but quantitatively is somewhat overoptimistic as it is based on generator level data. The aim of the present thesis is twofold: on the one hand, as a continuation of the thesis presented in 2015 by Dr. Javier Llorente, who measured $b$- and light-jet shapes in $t\bar{t}$ events, we have investigated the improvements in the performance of the MVA discussed above when adding the jet shapes as a discriminant variable. This was presented at ICHEP 2014 in Valencia, [Nucl. Part. Phys. Proc. 273-275 (2016) 2761-2763]. This is the basis for Chapter 4 in this thesis. On the other hand, we made use of the ATLAS data at 7 TeV taken in 2011, to study neutral strange particle production in the dileptonic $t\bar{t}$ final states. This study has been published in Eur. Phys. J. C 79 (2019) 1017. It is found that neutral strange particle production embedded in jets, $b$-tagged or not, is well described by current MC models like the following generators: POWHEG+PYTHIA6, POWHEG+PYTHIA8, MC@NLO+HERWIG, POWHEG+HERWIG7, aMC@NLO+HERWIG7 or SHERPA. This lends support to the idea put forward by ABL that one could attempt to determine $V_{ts}$ with a TMVA. Furthermore, neutral strange particles produced outside jets, the dominant contribution, in underestimated by current MC generators by 30 %. Thus, these data have the potential to help tune models for MPI. This is summarised in Chapter 5. For the sake of completeness and without any pretension to originality, the first three chapters are devoted to present the most relevant aspects of the SM, Chapter 1, the ATLAS detector and the LHC, Chapter 2, and the objects (leptons, jets, missing energy) reconstruction, Chapter 3.