Search for new charged bosons and dark matter in final states with one lepton and missing transverse energy with the ATLAS detector at the LHC

The Standard Model (SM), the current theory of elementary particles and interactions, has been extremely successful in predicting and describing experimental results. The prediction of the electron’s anomalous magnetic moment served as an early triumph of quantum electrodynamics, and one success after another has followed, including the discovery of the weak interaction gauge bosons W± and Z⁰, and more recently the discovery of the Higgs boson at CERN’s Large Hadron Collider (LHC) in 2012. In spite of the success of the theory, though, there are phenomena which it does not explain, such as the dark matter and dark energy making up most of the universe. Extensions of the SM aiming to address its shortcomings typically predict observable deviations from the theory. Although theories predicting significant deviations from the SM in the energy regime so far explored can be immediately excluded, theories that predict deviations at higher, unexplored energies are still viable. Therefore, exploring physics at such energies is crucial in order to improve our understanding of nature at the most fundamental level. Currently, experimental data at the energy frontier are provided by the LHC experiments. In this thesis, we present a search for new physics in final states with one lepton and missing transverse energy using data from the ATLAS detector. No significant deviations from SM predictions are observed in the transverse mass distribution. The search is interpreted in terms of the production of hypothetical heavy, charged bosons, and also in terms of the production of dark matter particles in association with a leptonically decaying W boson. Limits on the cross sections and relevant mass scales of these processes are presented. In the case of new charged boson signal, the Sequential Standard Model (SSM) and the W* reference model are considered. The SSM is a reference model widely used to represent new gauge bosons, of which the charged ones are usually denoted W′, related to hypothetical symmetries of nature and the associated interactions. The W* boson is a common occurence in theories addressing the unreasonably large radiative corrections to the Higgs boson mass, and differs significantly from the new gauge bosons in its interactions with the SM fermions. We find that new charged bosons are excluded at 95% CL for masses up to 3.28 TeV in the case of the SSM W′ boson and 3.21 TeV in the case of the W* reference model. As a search for dark matter particles, the analysis is found to be competitive with ATLAS searches in other final states, in particular in the case of constructive interference in the so-called D5 effective field theory (EFT). Limits at 90% CL on the suppression scale in the effective coupling between dark matter particles and quarks are presented. For low dark matter particle masses, where collider searches are particularly sensitive, the limits extend to 1.4 TeV in the D9 EFT and 1.2 TeV in the D5 EFT in the case of constructive interference

Search for resonances decaying into photon pairs in 139 fb−1 of pp collisions at √s = 13 TeV with the ATLAS detector

Searches for new resonances in the diphoton final state, with spin 0 as predicted by theories with an extended Higgs sector and with spin 2 using a warped extra-dimension benchmark model, are presented using 139 fb−1 of √ s =13 TeV pp collision data collected by the ATLAS experiment at the LHC. No significant deviation from the Standard Model is observed and upper limits are placed on the production cross-section times branching ratio to two photons as a function of the resonance mass.publishedVersio

Search for heavy lepton resonances decaying to a ZZ boson and a lepton in pppp collisions at s=8\sqrt{s}=8 TeV with the ATLAS detector

A search for heavy leptons decaying to a $Z$ boson and an electron or a muon is presented. The search is based on $pp$ collision data taken at $\sqrt{s}=8$ TeV by the ATLAS experiment at the CERN Large Hadron Collider, corresponding to an integrated luminosity of 20.3 fb$^{-1}$, Three high-transverse-momentum electrons or muons are selected, with two of them required to be consistent with originating from a $Z$ boson decay. No significant excess above Standard Model background predictions is observed, and 95% confidence level limits on the production cross section of high-mass trilepton resonances are derived. The results are interpreted in the context of vector-like lepton and type-III seesaw models. For the vector-like lepton model, most heavy lepton mass values in the range 114-176 GeV are excluded. For the type-III seesaw model, most mass values in the range 100-468 GeV are excluded

Search for invisible decays of the Higgs boson produced in association with a hadronically decaying vector boson in pppp collisions at s=8\sqrt{s} = 8 TeV with the ATLAS detector

A search for Higgs boson decays to invisible particles is performed using 20.3 fb\).{-1}\) of $pp$ collision data at a centre-of-mass energy of 8 TeV recorded by the ATLAS detector at the Large Hadron Collider. The process considered is Higgs boson production in association with a vector boson $V$ = $W$ or $Z$ that decays hadronically, resulting in events with two or more jets and large missing transverse momentum. No excess of candidates is observed in the data over the background expectation. The results are used to constrain $VH$ production followed by $H$ decaying to invisible particles for the Higgs mass range $115<m_H<300$ GeV. The 95% confidence-level observed upper limit on $\sigma_{VH} \times \text{BR}(H\rightarrow \text{inv.})$ varies from 1.6 pb at 115 GeV to 0.13 pb at 300 GeV. Assuming Standard Model production and including the $gg\rightarrow H$ contribution as signal, the results also lead to an observed upper limit of 78% at 95% confidence level on the branching ratio of Higgs bosons decays to invisible particles at a mass of 125 GeV.publishedVersio

Search for heavy lepton resonances decaying to a ZZ boson and a lepton in pppp collisions at s=8\sqrt{s}=8 TeV with the ATLAS detector

A search for heavy leptons decaying to a $Z$ boson and an electron or a muon is presented. The search is based on $pp$ collision data taken at $\sqrt{s}=8$ TeV by the ATLAS experiment at the CERN Large Hadron Collider, corresponding to an integrated luminosity of 20.3 fb$^{-1}$, Three high-transverse-momentum electrons or muons are selected, with two of them required to be consistent with originating from a $Z$ boson decay. No significant excess above Standard Model background predictions is observed, and 95% confidence level limits on the production cross section of high-mass trilepton resonances are derived. The results are interpreted in the context of vector-like lepton and type-III seesaw models. For the vector-like lepton model, most heavy lepton mass values in the range 114-176 GeV are excluded. For the type-III seesaw model, most mass values in the range 100-468 GeV are excluded.publishedVersio

Measurement of the muon reconstruction performance of the ATLAS detector using 2011 and 2012 LHC proton–proton collision data

This paper presents the performance of the ATLAS muon reconstruction during the LHC run with pp collisions at √s = 7–8 TeV in 2011–2012, focusing mainly on data collected in 2012. Measurements of the reconstruction efficiency and of the momentum scale and resolution, based on large reference samples of J/ψ → μμ, Z → μμ and ϒ → μμ decays, are presented and compared to Monte Carlo simulations. Corrections to the simulation, to be used in physics analysis, are provided. Over most of the covered phase space (muon |η| < 2.7 and 5 ≲pT ≲pT 100 GeV) the efficiency is above 99% and is measured with per-mille precision. The momentum resolution ranges from 1.7% at central rapidity and for transverse momentum pT ≃ 10 GeV, to 4% at large rapidity and pT ≃ 100 GeV. The momentum scale is known with an uncertainty of 0.05% to 0.2% depending on rapidity. A method for the recovery of final state radiation from the muons is also presented

Measurement of distributions sensitive to the underlying event in inclusive Z-boson production in pp collisions at √s = 7 TeV with the ATLAS detector

A measurement of charged-particle distributions sensitive to the properties of the underlying event is presented for an inclusive sample of events containing a TeX -boson, decaying to an electron or muon pair. The measurement is based on data collected using the ATLAS detector at the LHC in proton–proton collisions at a centre-of-mass energy of TeX TeV with an integrated luminosity of TeX fb TeX . Distributions of the charged particle multiplicity and of the charged particle transverse momentum are measured in regions of azimuthal angle defined with respect to the TeX -boson direction. The measured distributions are compared to similar distributions measured in jet events, and to the predictions of various Monte Carlo generators implementing different underlying event models

Search for dark matter in events with heavy quarks and missing transverse momentum in pppp collisions with the ATLAS detector

This article reports on a search for dark matter pair production in association with bottom or top quarks in $20.3 fb^{-1}$ of $pp$ collisions collected at $\sqrt{s} = 8$ TeV by the ATLAS detector at the LHC. Events with large missing transverse momentum are selected when produced in association with high-momentum jets of which one or more are identified as jets containing $b$-quarks. Final states with top quarks are selected by requiring a high jet multiplicity and in some cases a single lepton. The data are found to be consistent with the Standard Model expectations and limits are set on the mass scale of effective field theories that describe scalar and tensor interactions between dark matter and Standard Model particles. Limits on the dark-matter--nucleon cross-section for spin-independent and spin-dependent interactions are also provided. These limits are particularly strong for low-mass dark matter. Using a simplified model, constraints are set on the mass of dark matter and of a coloured mediator suitable to explain a possible signal of annihilating dark matter

Search for dark matter in events with heavy quarks and missing transverse momentum in pppp collisions with the ATLAS detector

This article reports on a search for dark matter pair production in association with bottom or top quarks in $20.3 fb^{-1}$ of $pp$ collisions collected at $\sqrt{s} = 8$ TeV by the ATLAS detector at the LHC. Events with large missing transverse momentum are selected when produced in association with high-momentum jets of which one or more are identified as jets containing $b$-quarks. Final states with top quarks are selected by requiring a high jet multiplicity and in some cases a single lepton. The data are found to be consistent with the Standard Model expectations and limits are set on the mass scale of effective field theories that describe scalar and tensor interactions between dark matter and Standard Model particles. Limits on the dark-matter--nucleon cross-section for spin-independent and spin-dependent interactions are also provided. These limits are particularly strong for low-mass dark matter. Using a simplified model, constraints are set on the mass of dark matter and of a coloured mediator suitable to explain a possible signal of annihilating dark matter.publishedVersio

Search for pair and single production of new heavy quarks that decay to a Z boson and a third-generation quark in pp collisions at √s=8 TeV with the ATLAS detector

A search is presented for the production of new heavy quarks that decay to a Z boson and a third-generation Standard Model quark. In the case of a new charge +2/3 quark (T ), the decay targeted is T → Zt, while the decay targeted for a new charge −1/3 quark (B) is B → Zb. The search is performed with a dataset corresponding to 20.3 fb−1 of pp collisions at TeX TeV recorded in 2012 with the ATLAS detector at the CERN Large Hadron Collider. Selected events contain a high transverse momentum Z boson candidate reconstructed from a pair of oppositely charged same-flavor leptons (electrons or muons), and are analyzed in two channels defined by the absence or presence of a third lepton. Hadronic jets, in particular those with properties consistent with the decay of a b-hadron, are also required to be present in selected events. Different requirements are made on the jet activity in the event in order to enhance the sensitivity to either heavy quark pair production mediated by the strong interaction, or single production mediated by the electroweak interaction. No significant excess of events above the Standard Model expectation is observed, and lower limits are derived on the mass of vector-like T and B quarks under various branching ratio hypotheses, as well as upper limits on the magnitude of electroweak coupling parameters.publishedVersio