• LHC
• exotic
• dibosone
• 13 TeV
• semileptonic
• resonance
• diboson
• ATLAS
• Nuova Fisica
• risonanza
• esotico
• Run 2
• Beyond Standard Model
• HVT

Searches for resonances of massive gauge bosons are one of the major tools to unravel hints of physics signals beyond the Standard Model. Such resonances feature in a large number of Standard Model extensions, such as extended gauge models, warped dimensions and technicolour, and are generally predicted to occur at the TeV scale.
The aim of the analysis covered in this thesis is to search for a possible resonance decaying to a pair of W bosons or into a W and a Z boson by investigating the invariant mass spectrum of the semileptonic ($\ell\nu qq$) final state using proton-proton collisions at $\sqrt{s}$ = 13 TeV gathered at ATLAS in the 2015.

A previous search in the fully hadronic final state of the Diboson channel carried out by ATLAS on $\sqrt{s}$ = 8 TeV data highlighted a mild (3.4$\sigma$ significance) excess at values of the invariant mass close to 2 TeV, piquing the interest of the physical community. The $\ell\nu qq$ search on 2015 data is expected to slightly improve the sensitivity of the fully hadronic Run I analysis, and, covering the diboson final state with the second-highest branching fraction, is of primary importance as it can serve as a prompt and independent validation (or disproval) or the aforementioned excess.

While the main bulk of the analysis was carried out by the members of the analysis group in the months leading up to the 2015 End Of Year Event, this thesis reports on a number of original optimization and performance studies, conceived as a support to the main analysis, personally carried out in parallel to the work performed by the main analysis group. The structure and the contents of this thesis will now be briefly described.

In the first Chapter, I briefly introduce the reader to the experimental and historical context in which the analysis covered by this work of thesis is carried out. A summary of the previous results published by the ATLAS experiment, with a focus on the past searches in the Diboson channel, is given. I also provide a small summary of the main phenomenological features of the theoretical framework that I use as a benchmark model for my studies.

Chapter 2 is dedicated to the description of the experimental setup at ATLAS during the 2015 data taking, with a brief account of the purpose, experimental philosophy and critical design performance parameters being included for each of the main ATLAS sub-detectors.

Chapter 3 describes the experimental procedures employed in the reconstruction of the physical objects and variables that I use for the analysis. Among these procedures, a particular prominence is given to those involving the hadronic event, with a special focus on the relatively innovative techniques used to reconstruct and handle large-R jets, the defining feature of the $\ell\nu qq$ final state in the kinematic range targeted by the analysis.

The first of the original studies I carried out is reported on in Chapter 4, and aims at determining the reconstruction performance of the variables contributing to the calculation of the fitted observable as well as those featured prominently in the event selection. For each variable the ATLAS delivered performance is examined by the means of a comparison between the truth and the reconstructed levels of the Montecarlo simulation. This study allowed me to gauge the closeness of the reconstructed event to the original simulation of the physical process ("truth" level).

A second original study, described in Chapter 5, was held with the purpose of achieving a better understanding of the event selection employed in the analysis, as well as to optimize the chosen working points for the kinematic and topologic cuts applied on candidate events fulfilling the pre-selection requirements. A particular focus was once again lent to the boson tagging procedure, which was unfolded in its two defining stages and analysed in detail.

The results of the two aforementioned studies prompted a preliminary investigation on the possibility to implement in the event selection the information on the hadronic part of the event reconstructed with the ATLAS Inner Detector. Within this survey, reported in Chapter 6, a set of track based variables describing the signal large-R jet were tested to assess whether a further enhancement of a possible excess in the invariant mass spectrum could be achieved by applying requirements on them. As the track-driven tagging of the substructure of large-R jets is a novel and poorly optimized field, a number of guidelines for future developments on the subject are tentatively suggested.

The final chapter of the thesis is dedicated to the two conclusive stages of the $\ell\nu qq$ analysis: the comparison between the distribution of a select number of variables as reconstructed in data and in the simulated distributions and the final fit procedure. In the first section of the chapter, both the variables of primary interest to the analysis and the variables examined in Chapter 6 were checked for any possible disagreement with the various selections used in the analysis. The data/Montecarlo comparison was performed personally applying the standard analysis event selection on the provided datasets. In the second part of Chapter 7 I give an account of the procedure employed by the analysis team to perform the fit on the invariant mass distribution of the final state and the statistical tools used to extract the eventual results, which are then reported in the last section of the chapter and thesis.