This thesis presents two studies in a search for new heavy resonances with 139 fb-1 of proton-proton collisions delivered by the Large Hadron Collider at √s = 13 TeV as measured by the ATLAS detector. In many Standard Model extensions, the top quark plays an important role in the dynamics of electroweak symmetry breaking, and so the studies presented here focus on heavy resonance searches in the top-antitop decay channel and their combination with searches for heavy resonances decaying into other final states. The first study detailed in this thesis is a search for new heavy resonances in a semileptonically decaying top-antitop final state where no significant excesses of data were observed with respect to the background prediction. That being so, exclusion limits at the 95% confidence level were placed on possible resonant masses for a Z′ in a leptophobic topcolor-assisted technicolor model and for a bulk Randall-Sundrum graviton at 4.2 TeV and 1.0 TeV, respectively. This top-antitop search was combined with searches in orthogonal final states performed by the ATLAS Collaboration from which limit contours were placed at the 95% confidence level in the space of couplings to Standard Model particles for the Heavy Vector Triplet model.

This concise primer reviews the latest developments in the field of jets. Jets are collinear sprays of hadrons produced in very high-energy collisions, e.g. at the LHC or at a future hadron collider. They are essential to and ubiquitous in experimental analyses, making their study crucial. At present LHC energies and beyond, massive particles around the electroweak scale are frequently produced with transverse momenta that are much larger than their mass, i.e., boosted. The decay products of such boosted massive objects tend to occupy only a relatively small and confined area of the detector and are observed as a single jet. Jets hence arise from many different sources and it is important to be able to distinguish the rare events with boosted resonances from the large backgrounds originating from Quantum Chromodynamics (QCD). This requires familiarity with the internal properties of jets, such as their different radiation patterns, a field broadly known as jet substructure. This set of notes begins by providing a phenomenological motivation, explaining why the study of jets and their substructure is of particular importance for the current and future program of the LHC, followed by a brief but insightful introduction to QCD and to hadron-collider phenomenology. The next section introduces jets as complex objects constructed from a sequential recombination algorithm. In this context some experimental aspects are also reviewed. Since jet substructure calculations are multi-scale problems that call for all-order treatments (resummations), the bases of such calculations are discussed for simple jet quantities. With these QCD and jet physics ingredients in hand, readers can then dig into jet substructure itself. Accordingly, these notes first highlight the main concepts behind substructure techniques and introduce a list of the main jet substructure tools that have been used over the past decade. Analytic calculations are then provided for several families of tools, the goal being to identify their key characteristics. In closing, the book provides an overview of LHC searches and measurements where jet substructure techniques are used, reviews the main take-home messages, and outlines future perspectives.

A search for new particles that decay into top quark pairs is reported. The search is performed with the ATLAS experiment at the LHC using an integrated luminosity of 20.3 fb-1 of proton-proton collision data collected at a centre-of-mass energy of \(\sqrt{s}=8 \) TeV. The lepton-plus-jets final state is used, where the top pair decays to \({W}^{+}b{W}^{-}\overline{b} \), with one W boson decaying leptonically and the other hadronically. The invariant mass spectrum of top quark pairs is examined for local excesses or deficits that are inconsistent with the Standard Model predictions. No evidence for a top quark pair resonance is found, and 95% confidence-level limits on the production rate are determined for massive states in benchmark models. The upper limits on the cross-section times branching ratio of a narrow Z' boson decaying to top pairs range from 4.2 pb to 0.03 pb for resonance masses from 0.4 TeV to 3.0 TeV. A narrow leptophobic topcolour Z' boson with mass below 1.8 TeV is excluded. Upper limits are set on the cross-section times branching ratio for a broad colour-octet resonance with [Gamma]/m = 15% decaying to \(t\overline{t} \). These range from 4.8 pb to 0.03 pb for masses from 0.4 TeV to 3.0 TeV. A Kaluza-Klein excitation of the gluon in a Randall-Sundrum model is excluded for masses below 2.2 TeV.

Questo documento riassume lo stato attuale degli ricerche studi, teorici e sperimentali, sulla produzione di coppie di bosoni di Higgs, e sui vincoli, sia diretti che indiretti, al valore del termine di auto-interazione del bosone di Higgs, con l’intento di servire da referenza per i prossimi anni. Il documento discute lo stato degli studi teorici, includendo le più recenti stime della sezione di produzione di coppie di bosoni di Higgs, sviluppi sulle teorie di campo efficaci, e studi su specifici scenari di nuova fisica che possono contribuire alla produzione di due bosoni di Higgs. Sono presentati i più recenti risultati sperimentali sulle ricerche di coppie di bosoni di Higgs e sui limiti diretti e indiretti al termine di auto-interazione, ottenuti al Large Hadron Collider di Ginevra, con una panoramica delle tecniche sperimentali. Infine, sono discusse le capacità dei collisionatori futuri di determinare il termine di auto-interazione del bosone di Higgs. Questo lavoro è iniziato come raccolta di contributi della conferenza “Di-Higgs ai Colliders”, che ha avuto luogo a Fermilab dal 4 al 9 settembre 2018, ma gli argomenti discussi vanno al di là di quelli presentati alla conferenza, includendo ulteriori sviluppi.

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We present a search for pair production of new physics resonances decaying into a top quark and a light parton in final states with two leptons, interpreting the results in the context of an R-parity violating supersymmetric model. We use 19.5 fb[-]1 of data collected by the CMS experiment at the LHC from proton-proton collisions at s = 8 TeV in 2012. The experimental signature consists of two leptons (e or [MICRO SIGN]), two jets identified as originating from the decay of a b quark, and two jets identified as coming from light flavor quarks or gluons. We reconstruct and analyze potential resonant decays. The dominant standard model background is top quark pair production with additional jets from initialor final-state radiation. We perform an extended unbinned maximum likelihood fit to the transverse momenta of the two leading light jets and the reconstructed resonance mass. The observation is consistent with the standard model expectation, and we set upper limits on the signal cross section for R-parity violating bottom squarks with masses between 250 and 600 GeV. We exclude R-parity violating bottom squark pair production at the 95% confidence level between 250 GeV and 326 GeV.

In the chiral B scenario with a 100% branching ratio of the decay B->Wt, the observed (expected) 95% CL lower limit on the B mass is 814 GeV (756 GeV).

Abstract: A search for new phenomena with top quark pairs in final states with one isolated electron or muon, multiple jets, and large missing transverse momentum is performed. Signal regions are designed to search for two-, three-, and four-body decays of the directly pair-produced supersymmetric partner of the top quark (stop). Additional signal regions are designed specifically to search for spin-0 mediators that are produced in association with a pair of top quarks and decay into a pair of dark-matter particles. The search is performed using the Large Hadron Collider proton-proton collision dataset at a centre-of-mass energy of s√ = 13 TeV recorded by the ATLAS detector from 2015 to 2018, corresponding to an integrated luminosity of 139 fb−1. No significant excess above the Standard Model background is observed, and limits at 95% confidence level are set in the stop-neutralino mass plane and as a function of the mediator mass or the dark-matter particle mass. Stops are excluded up to 1200 GeV (710 GeV) in the two-body (three-body) decay scenario. In the four-body scenario stops up to 640 GeV are excluded for a stop-neutralino mass difference of 60 GeV. Scalar and pseudoscalar dark-matter mediators are excluded up to 200 GeV when the coupling strengths of the mediator to Standard Model and dark-matter particles are both equal to one and when the mass of the dark-matter particle is 1 GeV