This dissertation presents the observation of same-sign WW electroweak production in proton-proton collisions with a center-of-mass energy of 13 TeV at the Large Hadron Collider using the ATLAS detector. The W bosons are required to decay leptonically, giving a signature of two leptons (electrons or muons) with the same electric charge, two jets with a large invariant mass and rapidity separation, and missing transverse energy. The same-sign WW electroweak fiducial cross section is measured using 36.1 fb^{-1} of data recorded in 2015 and 2016. A total of 122 candidate events are observed with a fitted background of 69 +/- 7 events, corresponding to an observed signal significance of 6.5 standard deviations. The measured fiducial cross section is 2.89 +0.51 -0.48 (stat) +0.29 -0.28 (syst) fb and is in agreement with Standard Model predictions.

Supersymmetry, one of the of the most promising extensions to the Standard Model of Particle Physics, predicts a number of as-yet-undiscovered particles, evidence of which could be found in LHC collision events. This thesis presents a search for R-parity violating Supersymmetric signal events in ATLAS proton-proton collision data. The search uses $36.1~fb^{-1}$ of $\sqrt{s}=13~TeV$ collision data, taken by ATLAS in 2015 and 2016. As no excess of events above the background prediction was found, limits are set on the production cross sections of gluinos and neutralinos under two different decay scenarios. The resulting limits are a significant extension beyond the reach of the Run-1 search for the same process.

This dissertation presents a search for a resonant state decaying to a W and a Z boson

using events with three charged leptons (electrons or muons) and missing transverse energy.

The search is performed using proton-proton collisions at a center-of-mass energy of 8 TeV,

produced by the LHC and measured by the ATLAS detector in 2012. This data sample

corresponds to an integrated luminosity of 13.{fb^{−1}. Events are selected with reconstructed

W and Z bosons found at similar rapidity and large angular separation. The invariant

mass of three leptons and missing energy is calculated, assuming on-shell W production,

and compared to the Standard Model expectation. No significant excess is observed and the

results are interpreted as cross-section times branching ratio limits for two specific models.

In the context of an Extended Gauge Model, a W′ boson is excluded at a 95 % confidence

level for masses below 1180 GeV. In the context of a Low Scale Technicolor model, two-

dimensional limits are set for values of the technipion and technirho mass between 200 GeV

and 1000 GeV. These limits are generalizable to any model predicting a narrow resonance

that decays to a W and a Z boson.

This thesis describes the search for displaced photons in the exotic decays of the Standard Model Higgs boson using 139 fb−1 of proton-proton collision data recorded by the ATLAS detector. The signal model involves the decay of the Higgs boson into a pair of long-lived supersymmetric particles, each of which subsequently decay into a photon and a stable supersymmetric particle. The current upper limits on the decays of the Higgs boson to the undetected particles is around 21% making this search extremely relevant. This analysis utilizes the ATLAS electromagnetic calorimeter’s unique capabilities to measure the photon’s direction of flight and its time of arrival. No significant excess was observed compared to theestimated Standard Model background and this null result was used to set 95% confidence level limits on the branching ratio of the Higgs to the pair of long-lived particles for the very first time.

This dissertation presents the first measurement of the top quark pair differential cross-section as a function of the hadronic top candidate transverse momentum that uses boosted top quark identification techniques. This allows to extend the measurement into the TeV range, with the cross-section reported for top candidate transverse momentum between 300 GeV and 1.2 TeV. The measurement is performed using events reconstructed in the lepton+jets channel in 20.3 fb$^{-1}$ of $\sqrt{s}$ = 8 TeV proton-proton collision data collected by the ATLAS experiment. Previous measurements have reported the cross-section as a function of the top quark kinematics. To allow for a robust top candidate definition, independent of assumptions required to isolate the kinematics of individual partons, the cross-section presented here is reported at the stable particle level. The cross-section is reported within a fiducial region closely following the detector-level event selection to minimize uncertainties associated with extrapolations beyond the detector acceptance. To illustrate the evolution of the uncertainties associated with extrapolating the particle level results to the usual top parton definition, the results are also presented within the full phase-space at the parton level.

The transverse momentum and multiplicity of jets produced in top quark events are measured using 20.3 inverse fb of pp collision data at a center-of-mass energy of 8 tev. Jets are selected from top events requiring an opposite-charge $e\mu$ pair and two b-tagged jets in the final state.

The data are corrected to obtain the particle-level fiducial cross section for additional jets with rank 1-4, where rank=1 is the leading additional jet. These distributions are used to obtain the extra jet multiplicity as a function of minimum jet pt threshold. The results are compared with several next to leading order Monte Carlo generators. The resulting measurements can be used to tune Monte Carlo QCD modelling and may also reduce associated modelling uncertainties for LHC top quark physics measurements.

During Run 2 of the Large Hadron Collider, the ATLAS experiment recorded proton--proton collision events with a center of mass energy of 13 TeV, the highest energy ever achieved in a collider. Analysis of this dataset, which corresponds to an integrated luminosity of 139 fb$^{-1}$, has provided new opportunities for precision measurements of the Higgs boson. The interaction between the Higgs boson and the top quark, the heaviest known particle, is a fundamental probe of the Higgs mechanism, which describes how Standard Model particles obtain mass.

The coupling of the Higgs boson to the top quark can be directly probed through the production of a Higgs boson in association with a top-antitop quark pair ($t\bar{t}H$) at the LHC. A statistical combination of ATLAS searches in multiple Higgs boson decay channels using 79.8 fb$^{-1}$ of the Run 2 dataset yielded the first observation of $t\bar{t}H$ production at the level of $6.3\sigma$.

The Higgs to diphoton ($\gamma\gamma$) decay channel is among the most sensitive for $t\bar{t}H$ measurements due to the excellent diphoton mass resolution of the ATLAS detector and the clean signature of this decay. A powerful multivariate analysis was developed to select $t\bar{t}H(\gamma\gamma)$ events based on the momenta of jets, photons, and leptons in the final state. The $t\bar{t}H$ cross section times $H\rightarrow\gamma\gamma$ branching fraction was measured to be $\sigma_{t\bar{t}H}\times B_{\gamma\gamma} = 1.59 \; _{-0.39}^{+0.43}$ fb using the full Run 2 ATLAS dataset. Top quarks were reconstructed with high probability in selected $t\bar{t}H(\gamma\gamma)$ events.

Following the observation of the $t\bar{t}H$ process, a direct measurement of the CP properties of the Higgs-top interaction was carried out in the $H\rightarrow\gamma\gamma$ decay channel. A CP-sensitive multivariate categorization was developed using reconstructed top quark variables, yielding an observed $t\bar{t}H$ significance of 5.2$\sigma$ and an upper limit on the cross section of single-top-plus-Higgs ($tH$) production of 11.6 times the SM expectation. The observed data excludes a fully CP odd Higgs-top coupling at the level of 3.9$\sigma$, and the CP mixing angle is constrained to $|\alpha|>43^\circ$ at 95\% confidence level.

The production of a Vector Boson in association with a single charm quark is measured and presented here using 140 fb$^{-1}$ of proton--proton collision data collected with the ATLAS detector at the Large Hadron Collider.Measurements are performed using either a Z boson or W boson produced with a charm meson. The charm quark is tagged by the presence of a charmed hadron, reconstructed with a secondary-vertex fit. The $W$ boson is reconstructed from the decay to either an electron or a muon and the missing transverse momentum present in the event. The $Z$ boson is reconstructed from the decay to either a pair of electrons or muons. The reconstruction modes used for charm hadron reconstruction are $D^{+} \to K^- \pi^+ \pi^+$ and $D^{*+} \to D^{0} \pi^+ \to (K^- \pi^+) \pi^+$ and the charge conjugate decays. The \WplusDmeson integrated and normalized differential cross-sections as a function of the pseudorapidity of the lepton from the $W$ boson decay, and of the transverse momentum of the charmed hadron, are extracted from the data using a profile likelihood fit. The fiducial region is defined to include events passing $p_{\text{T}}(e, \mu) > \SI{30}{\GeV}$, $|\eta(e, \mu)| < 2.5$, $p_{\text{T}}(D^{(\ast)}) > \SI{8}{\GeV}$, and $|\eta(D^{(\ast)})| < 2.2$. The measured total fiducial cross-sections are: $\sigma^{\mathrm{OS-SS}}_{\mathrm{fid}}(W^{-}{+}D^{*+}) = 51.1\pm0.4\,\mathrm{(stat.)}\,^{+1.9}_{-1.8}\,\mathrm{(syst.)}\,\mathrm{pb}$, and $\sigma^{\mathrm{OS-SS}}_{\mathrm{fid}}(W^{+}{+}D^{*-}) = 50.0\pm0.4\,\mathrm{(stat.)}\,^{+1.9}_{-1.8}\,\mathrm{(syst.)}\,\mathrm{pb}$. Results are compared with the predictions of next-to-leading-order quantum chromodynamics calculations performed using state-of-the-art parton distribution functions. Additionally, the ratio of charm to anti-charm production cross-sections is studied to probe the $s$-$\bar{s}$ quark asymmetry. The ratio is found to be $R_c^\pm = 0.971\pm0.006\,\mathrm{(stat.)}\pm0.011\,\mathrm{(syst.)}$. The ratio and cross-section measurements are consistent with the predictions obtained with parton distribution function sets that have a symmetric $s$-$\bar{s}$ sea, indicating that any $s$-$\bar{s}$ asymmetry in the Bjorken-$x$ region relevant for this measurement is small. The \ZplusDmeson integrated and normalized differential cross-sections as a function of the transverse momentum of the charmed hadron, are extracted from the data using the same fit strategy. The fiducial region is defined to include events passing $p_{\text{T}}(e, \mu) > \SI{27}{\GeV}$, $|\eta(e, \mu)| < 2.5$, $p_{\text{T}}(D^{(\ast)}) > \SI{8}{\GeV}$, and $|\eta(D^{(\ast)})| < 2.2$. The measured total fiducial cross-sections in the \ZplusDmeson analysis are $\sigma_{\mathrm{fid}}(Z^{0}{+}D^{\pm}) = 21.9 \pm0.04 \,\mathrm{(stat.)}\,^{+1.5}_{-1.4}\,\mathrm{(syst.)}\,\mathrm{pb}$, and $\sigma_{\mathrm{fid}}(Z^{0}{+}D^{*\pm}) =31.1 \pm0.23 \,\mathrm{(stat.)}\,^{+1.3}_{-1.3}\,\mathrm{(syst.)}\,\mathrm{pb}$. These results are also compared with the predictions of next-to-leading-order quantum chromodynamics calculations performed using state-of-the-art parton distribution functions.

The production of a W boson in association with a single charm quark is studied using140 fb−1 of √s = 13 TeV proton–proton collision data collected with the ATLAS detector at the Large Hadron Collider. The charm quark is tagged by the presence of a charmed hadron, reconstructed with a secondary-vertex fit. The W boson is reconstructed from the decay to either an electron or a muon and the missing transverse momentum present in the event. The W + D(∗) process is sensitive to the strange quark PDF, and can be combined with other measurements to improve uncertainties and central values of global PDF fits. Charmed mesons are reconstructed in the decay channels D+ → K−π+π+, D∗+ → D0π+ → (K−π+)π+, D+ → φπ → (KK)π, and Ds → φπ → (KK)π and the charge conjugate decays in the fiducial regions where pT(e, μ) > 30 GeV , |η(e, μ)| < 2.5, pT(D(∗)) > 8 GeV , and |η(D(∗))| < 2.2. The integrated and normalized differential cross-sections as a function of the pseudorapidity of the lepton from the W boson decay, and of the transverse momentum of the charmed hadron, are extracted for the D+ → Kππ and D∗ → D0π → Kππ decay modes. The total fiducial cross section is measured for the Ds → φπ → (KK)π and D+ → φπ → (KK)π decay modes. All observables are extracted from the data using a profile likelihood fit. The measured total fiducial cross-sections for the D+ → Kππ and D∗ → D0π → Kππ modes are σ(OS−SS, fid) (W −+D+) = 50.2 ± 0.2 (stat.) +2.4−2.3 (syst.) pb, σ (OS−SS, fid) (W ++D−) = 48.5 ± 0.2 (stat.) +2.3−2.2 (syst.) pb, σ(OS−SS, fid) (W −+D∗+) = 51.1 ± 0.4 (stat.) +1.9−1.8 (syst.) pb, and σ(OS−SS, fid) (W ++D∗−) = 50.0 ± 0.4 (stat.) +1.9−1.8 (syst.) pb. These differential and total cross section results are compared with the predictions of next-to-leading-order quantum chromodynamics calculations performed using state-of-the-art parton distribution functions. Additionally, the ratio of charm to anti-charm production cross-sections is studied to probe the s- ̄s quark asymmetry. The ratio is found to be Rc = 0.971 ± 0.006 (stat.) ± 0.011 (syst.). The ratio and cross-section measurements are consistent with the predictions obtained with parton distribution function sets that have a symmetric s- ̄s sea, indicating that any s- ̄s asymmetry in the Bjorken-x region relevant for this measurement is small. For the Ds → φπ → (KK)π and D+ → φπ → (KK)π analysis, the observables that are measured are the inclusive cross section in both charges for the W +Ds process, the ratio of the cross sections with respect to the charge, and the ratio of the Ds to the D+ process. The measured values are σ(OS−SS, fid) (W −+D+s ) =17.8±0.4 (stat.) +1.8−1.6 (syst.) pb, σ(OS−SS, fid) (W ++D−s ) = 15.8±0.4 (stat.) +1.5−1.4 (syst.) pb, Rc (Ds) =0.885 ± 0.029 (stat.)+0.059−0.057 (syst.), and R(D+/Ds) = 2.93 ± 0.06(stat.) ± 0.24 (sys.). These result are consistent with the differential W + D+ analysis within 1.5 standard deviations, and are at a lower precision due to lower data and MC simulation statistics and increased systematic uncertainties related to background modeling.