• ATLAS
• diboson
• Standard Model

The Large Hadron Collider (LHC) is a particle accelerator designed as a high energy, high intensity proton-proton collider. The study of the diboson production at LHC provides an important test of the electroweak interaction structure at high energies. The goal of this thesis is to measure the diboson $pp\rightarrow WW+WZ$ production cross section in the semileptonic decay channel using the data collected by the ATLAS detector at LHC at a center-of-mass energy $\sqrt{s} = 8$ TeV.\\
Beside providing an important test of the Standard Model structure the diboson measurement also stands as a mean to detect new physics signals that could be unravelled through the measurement of an anomalous Triple Gauge Coupling. The study of the $WW$ and $WZ$ processes is also important for the searches of new physics or Higgs physics where the $WW/WZ$ diboson production may stand as irreducible backgrounds.\\
In the semileptonic decay channel a W decays into a lepton $\ell = e, \mu$ and a neutrino and the other boson (W or Z) decays into hadrons.
This final state is typically reconstructed by one lepton, large missing transverse energy, and two separate jets produced from the hadronically decayed boson. At sufficiently high transverse momentum, the decay products of the hadronically decaying boson become too close to each other and it is difficult to resolve them as two separate jets. This thesis focuses on events where the hadronically decaying boson has high transverse momentum and is reconstructed as a single jet with a large cone. The properties of the substructure of such a jet are used to enhance the discrimination between signal jets, having a two prong structure, and background jets. This is the first time the diboson production cross-section is measured in the boosted regime using experimental techniques based on the large cone jets.\\
The final measurement of the $WW/WZ$ cross section is the result of a joint effort among the components of the ``$WW/WZ\rightarrow\ell\nu jj$ Analysis Team''. In the following paragraph I depict the structure of the thesis highlighting my personal contribution to the analysis.
During the preparation of this thesis work I benefited of the suggestions of my supervisor and of the Analysis Team members, with whom I have been constantly in contact through frequent meetings.\\
The structure of the thesis is as follows: in Chapter 1 I give an overview of the theoretical framework and I summarise the recent results of massive diboson cross section measurements obtained using the ATLAS and CMS detectors. In Chapter 2 I illustrate the ATLAS experimental apparatus. In Chapter 3 I give a description of the reconstruction of the particle objects. Chapter 4 is focused on the description of the event selection of the candidate $WW/WZ\rightarrow\ell\nu_\ell qq'$ events. Candidate events are required to have exactly one lepton, large missing transverse energy and a single jet. The first tasks during my Master's thesis work has been to implement the event selection and to validate it. I have also studied the jet reconstruction techniques and the substructure variables, arriving to the implementation in my code and in a new event selection. Once obtained the full selection, I have measured the $WW+WZ$ signal yield by performing a binned maximum-likelihood fit to the jet mass distribution of the data using templates based on Monte Carlo simulations. In Chapter 5 I summarise how I obtained the final Monte Carlo templates and how they are used in the fit to extract the production cross section.\\
The main background processes in the boosted regime are W+jet, and $t\bar{t}$ and single top production. I have developed two selections as close as possible to the signal selection to define two control regions, each one enriched of one of the two main background processes. The definition of these control regions is described in Chapter 6. These regions allow to understand the level of agreement between data and simulated events. I dedicated a particular study to optimize the selection cuts to reduce the top background and I describe it in Chapter 7. The sources of systematic uncertainties that affect the measurement of the $WW/WZ\rightarrow \ell\nu_\ell J$ production cross section are discussed in Chapter 8. I have personally prepared the templates of most of the systematic uncertainties that are relevant for this analysis. In Chapter 9 the preliminary results on the cross-section measurement and on the limit on the anomalous triple gauge couplings are described. I personally performed the fit procedure to extract the signal cross-section.