A Search For Massive Resonances In Final States With Boosted Top Antitop Pairs Decaying Into A Lepton And Jets With The Atlas Detector At The Large Hadron Collider

A search for undiscovered particles decaying into top-antitop quark pairs produced in proton-proton collisions with the ATLAS experiment at the Large Hadron Collider utilizing 20.3 fb$^-1$ of data collected at $\sqrt{s}=8$ \tev\ center-of-mass energy during the 2012 data taking period is presented. The invariant mass spectrum of events containing multiple jets, exactly one lepton, and missing transverse energy and which are consistent with the decay of a top-antitop quark pair is studied and found to be consistent with that predicted by the Standard Model. Upper limits on the production cross section times branching ratio of several benchmark signal models are set at a 95\% confidence level.

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.

A search for the production of heavy resonances decaying into top quark-antiquark pairs is presented. The analysis is performed in the lepton+jets and fully hadronic channels using data collected in proton-proton collisions at $\sqrt{s}$ =13 TeV using the CMS detector at the LHC, corresponding to an integrated luminosity of 2.6 inverse femtobarns. The selection is optimized for massive resonances, where the top quarks have large Lorentz boosts. No evidence for resonant ttbar production is found in the data, and upper limits on the production cross section of heavy resonances are set. The exclusion limits for resonances with masses above 2 TeV are significantly improved compared to those of previous analyses at $\sqrt{s}$ = 8 TeV.

Many theories beyond the Standard Model predict new massive particles that decay preferentially to top-antitop quark pairs. This thesis investigates the dilepton final state where both W bosons from the top quarks decay into leptons. This final state features the highest purity for top quarks but is kinematically under-constrained due to the presence of the two neutrinos that leave the ATLAS detector undetected. Using kinematic constraints from the top-antitop quark decay chain, along with the missing energy, leptons, and jets, that are measured with the ATLAS detector, a set of analytic solutions for the four-vectors of the neutrinos can be obtained. With this information, the invariant mass can be reconstructed up to a four-fold ambiguity. Methods used to eliminate incorrect neutrino solutions are investigated and characterized. For a resonance particle with a mass much larger than the top quark mass, the top quarks will be highly boosted, resulting in collimated decay products of the top quark. Optimizing the lepton selection in such an environment is very important and is also discussed.

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.

ABSTRACT: We performed a search for"non-standard model tṫ resonances in the all jets final state channel." The main goal was to examine top-antitop invariant mass spectrum for the presence of narrow resonant states. The data analysis used 2.8fb−1 of CDF data; events were produced at the Tevatron collider in p[bar p] collisions with center-of-mass energy of 1.96 TeV. 2086 data events were analyzed and compared to Standard Model expectation. No evidence for new tṫ resonant production mechanisms was found. Upper limits were placed on the cross-section times branching ratio for resonance production at 805 GeV/c2. For signal modeling we considered leptophobic Z' boson in a topcolor-assisted technicolor model with the width of [capital gamma]= 1.2%MX0.

A search for heavy resonances that decay to tau lepton pairs is performed using proton-proton collisions at \sqrt{s}$ = 13 TeV. The data were collected with the CMS detector at the CERN LHC and correspond to an integrated luminosity of 2.2 inverse femtobarns. The observations are in agreement with standard model predictions. An upper limit at 95% confidence level on the product of the production cross section and branching fraction into tau lepton pairs is calculated as a function of the resonance mass. For the sequential standard model, the presence of Z' bosons decaying into tau lepton pairs is excluded for Z' masses below 2.1 TeV, extending previous limits for this final state. For the topcolor-assisted technicolor model, which predicts Z' bosons that preferentially couple to third-generation fermions, Z' masses below 1.7 TeV are excluded, representing the most stringent limit to date.

Results are reported of a general search for pair production of heavy resonances decaying to pairs of jets in events with at least four jets. The study is based on up to 19.4 inverse femtobarns of integrated luminosity from proton-proton collisions at a center-of-mass energy of 8 TeV, recorded with the CMS detector at the LHC. Limits are determined on the production of scalar top quarks (top squarks) in the framework of R-parity violating supersymmetry and on the production of color-octet vector bosons (colorons). First limits at the LHC are placed on top squark production for two scenarios. The first assumes decay to a bottom quark and a light-flavor quark and is excluded for masses between 200 and 385 GeV, and the second assumes decay to a pair of light-flavor quarks and is excluded for masses between 200 and 350 GeV at 95% confidence level. Previous limits on colorons decaying to light-flavor quarks are extended to exclude masses from 200 to 835 GeV.