Fakultät für Physik und Astronomie
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The production of a \(Z\) boson and a photon in association with a high-mass dijet system is studied using 20.2 fb\(^{−1}\) of proton-proton collision data at a centre-of-mass energy of \(\sqrt{s}\) = 8 TeV recorded with the ATLAS detector in 2012 at the Large Hadron Collider. Final states with a photon and a Z boson decaying into a pair of either electrons, muons, or neutrinos are analysed. Electroweak and total \(pp\) → \(Zγjj\) cross-sections are extracted in two fiducial regions with different sensitivities to electroweak production processes. Quartic couplings of vector bosons are studied in regions of phase space with an enhanced contribution from pure electroweak production, sensitive to vector-boson scattering processes \(V V → Zγ\). No deviations from Standard Model predictions are observed and constraints are placed on anomalous couplings parameterized by higher-dimensional operators using effective field theory.
This article presents searches for the \({Zγ}\) decay of the Higgs boson and for narrow high-mass resonances decaying to \(Z\)γ, exploiting \(Z\) boson decays to pairs of electrons or muons. The data analysis uses 36.1 fb\(^{−1}\) of \({pp}\) collisions at \(\sqrt{s}=13\) recorded by the ATLAS detector at the CERN Large Hadron Collider. The data are found to be consistent with the expected Standard Model background. The observed (expected — assuming Standard Model \({pp} → H → {Z}γ\) production and decay) upper limit on the production cross section times the branching ratio for \({pp} → H → {Z}γ\) is 6.6. (5.2) times the Standard Model prediction at the 95% confidence level for a Higgs boson mass of 125.09 GeV. In addition, upper limits are set on the production cross section times the branching ratio as a function of the mass of a narrow resonance between 250 GeV and 2.4 TeV, assuming spin-0 resonances produced via gluon-gluon fusion, and spin-2 resonances produced via gluon-gluon or quark-antiquark initial states. For high-mass spin-0 resonances, the observed (expected) limits vary between 88 fb (61 fb) and 2.8 fb (2.7 fb) for the mass range from 250 GeV to 2.4 TeV at the 95% confidence level.
This article presents a search for flavour-changing neutral currents in the decay of a top quark into an up-type (\({q = c, u}\)) quark and a Higgs boson, where the Higgs boson decays into two photons. The proton-proton collision data set analysed amounts to 36.1 fb\(^{−1}\) at \(\sqrt{s} = 13\) TeV collected by the ATLAS experiment at the LHC. Top quark pair events are searched for, where one top quark decays into \(qH\) and the other decays into \(bW\). Both the hadronic and leptonic decay modes of the \(W\) boson are used. No significant excess is observed and an upper limit is set on the \({t → cH}\) branching ratio of 2.2 × 10\(^{−3}\) at the 95% confidence level, while the expected limit in the absence of signal is 1.6 × 10\(^{−3}\). The corresponding limit on the \(tcH\) coupling is 0.090 at the 95% confidence level. The observed upper limit on the \({t → uH}\) branching ratio is 2.4 × 10\(^{−3}\).
A search for supersymmetric partners of gluons and quarks is presented, involving signatures with jets and either two isolated leptons (electrons or muons) with the same electric charge, or at least three isolated leptons. A data sample of proton-proton collisions at root s = 13 TeV recorded with the ATLAS detector at the Large Hadron Collider between 2015 and 2018, corresponding to a total integrated luminosity of 139 fb(-1), is used for the search. No significant excess over the Standard Model expectation is observed. The results are interpreted in simplified supersymmetric models featuring both R-parity conservation and R-parity violation, raising the exclusion limits beyond those of previous ATLAS searches to 1600 GeV for gluino masses and 750 GeV for bottom and top squark masses in these scenarios.
A search is presented for the pair production of heavy vector-like \(T\) quarks, primarily targeting the \(T\) quark decays to a \(W\) boson and a \(b\)-quark. The search is based on 36.1 fb\(^{−1}\) of \(pp\) collisions at \(\sqrt{s}=13\) TeV recorded in 2015 and 2016 with the ATLAS detector at the CERN Large Hadron Collider. Data are analysed in the lepton-plus-jets final state, including at least one \(b\)-tagged jet and a large-radius jet identified as originating from the hadronic decay of a high-momentum \(W\) boson. No significant deviation from the Standard Model expectation is observed in the reconstructed \(T\) mass distribution. The observed 95% confidence level lower limit on the \(T\) mass are 1350 GeV assuming 100% branching ratio to \(Wb\). In the SU(2) singlet scenario, the lower mass limit is 1170 GeV. This search is also sensitive to a heavy vector-like \(B\) quark decaying to \(Wt\) and other final states. The results are thus reinterpreted to provide a 95% confidence level lower limit on the \(B\) quark mass at 1250 GeV assuming 100% branching ratio to \(Wt\); in the SU(2) singlet scenario, the limit is 1080 GeV. Mass limits on both \(T\) and \(B\) production are also set as a function of the decay branching ratios. The 100% branching ratio limits are found to be applicable to heavy vector-like \(Y\) and \(X\) production that decay to \(Wb\) and \(Wt\), respectively.
A search is conducted for new resonant and non-resonant high-mass phenomena in dielectron and dimuon final states. The search uses 36.1 fb\(^{−1}\) of proton-proton collision data, collected at \(\sqrt{s}=13\) TeV by the ATLAS experiment at the LHC in 2015 and 2016. No significant deviation from the Standard Model prediction is observed. Upper limits at 95% credibility level are set on the cross-section times branching ratio for resonances decaying into dileptons, which are converted to lower limits on the resonance mass, up to 4.1 TeV for the E\(_6\)-motivated \(Z^′_χ\). Lower limits on the \({qqℓℓ}\) contact interaction scale are set between 2.4 TeV and 40 TeV, depending on the model.
A search for direct top squark pair production resulting in events with either a same-flavour opposite-sign dilepton pair with invariant mass compatible with a \(Z\) boson or a pair of jets compatible with a Standard Model (SM) Higgs boson (\(h\)) is presented. Requirements on the missing transverse momentum, together with additional selections on leptons, jets, jets identified as originating from \(b\)-quarks are imposed to target the other decay products of the top squark pair. The analysis is performed using proton-proton collision data at \(\sqrt{s}\) = 13 TeV collected with the ATLAS detector at the LHC in 2015–2016, corresponding to an integrated luminosity of 36.1 fb\(^{-1}\). No excess is observed in the data with respect to the SM predictions. The results are interpreted in two sets of models. In the first set, direct production of pairs of lighter top squarks (\(\tilde{t}_1\)) with long decay chains involving \(Z\) or Higgs bosons is considered. The second set includes direct pair production of the heavier top squark pairs (\(\tilde{t}_2\)) decaying via \(\tilde{t}_2\) → \(Z\tilde{t}_1\) or \(\tilde{t}_2\) → \(h\tilde{t}_1\). The results exclude at 95% confidence level \(\tilde{t}_2\) and \(\tilde{t}_1\) masses up to about 800 GeV, extending the exclusion region of supersymmetric parameter space covered by previous LHC searches.
A search for pair production of a scalar partner of the top quark in events with four or more jets plus missing transverse momentum is presented. An analysis of 36.1 fb\(^{−1}\) of \(\sqrt{s}\) = 13 TeV proton-proton collisions collected using the ATLAS detector at the LHC yields no significant excess over the expected Standard Model background. To interpret the results a simplified supersymmetric model is used where the top squark is assumed to decay via \(\tilde{t}_1\) → \(t^{(∗)}\)\(\tilde{χ}^0_1\) and \(\tilde{t}_1\) → \(b\tilde{χ}^±_1\) → \({bW}^{(∗)}\tilde{χ}^0_1\), where \(\tilde{χ}^0_1\) (\(\tilde{χ}^±_1\)) denotes the lightest neutralino (chargino). Exclusion limits are placed in terms of the top-squark and neutralino masses. Assuming a branching ratio of 100% to \(t\tilde{χ}^0_1\), top-squark masses in the range 450–1000 GeV are excluded for \(\tilde{χ}^0_1\) masses below 160 GeV. In the case where \(m_{\tilde{t}_1}\) ∼ \(m_t\) + \(m_{\tilde{χ}^0_1}\), top-squark masses in the range 235–590 GeV are excluded.
To probe the \(W tb\) vertex structure, top-quark and \(W\)-boson polarisation observables are measured from \(t\)-channel single-top-quark events produced in proton-proton collisions at a centre-of-mass energy of 8 TeV. The dataset corresponds to an integrated luminosity of 20.2 fb\(^{−1}\), recorded with the ATLAS detector at the LHC. Selected events contain one isolated electron or muon, large missing transverse momentum and exactly two jets, with one of them identified as likely to contain a \(b\)-hadron. Stringent selection requirements are applied to discriminate \(t\)-channel single-top-quark events from background. The polarisation observables are extracted from asymmetries in angular distributions measured with respect to spin quantisation axes appropriately chosen for the top quark and the \(W\) boson. The asymmetry measurements are performed at parton level by correcting the observed angular distributions for detector effects and hadronisation after subtracting the background contributions. The measured top-quark and \(W\)-boson polarisation values are in agreement with the Standard Model predictions. Limits on the imaginary part of the anomalous coupling \(g_R\) are also set from model independent measurements.
Ratios of top-quark pair to \(Z\)-boson cross sections measured from proton-proton collisions at the LHC centre-of-mass energies of \(\sqrt{s}\) = 13 TeV, 8 TeV, and 7 TeV are presented by the ATLAS Collaboration. Single ratios, at a given \(\sqrt{s}\) for the two processes and at different \(\sqrt{s}\) for each process, as well as double ratios of the two processes at different \(\sqrt{s}\), are evaluated. The ratios are constructed using previously published ATLAS measurements of the \({t\overline{t}}\) and \(Z\)-boson production cross sections, corrected to a common phase space where required, and a new analysis of \(Z\) → ℓ\(^+\)ℓ\(^-\) where ℓ = \(e, µ\) at \(\sqrt{s}\) = 13 TeV performed with data collected in 2015 with an integrated luminosity of 3.2 fb\(^−1\). Correlations of systematic uncertainties are taken into account when evaluating the uncertainties in the ratios. The correlation model is also used to evaluate the combined cross section of the \(Z\) → \(e\)\(^+\)\(e\)\(^−\) and the \(Z\) → \(µ\)\(^+\)\(µ\)\(^−\) channels for each \(\sqrt{s}\) value. The results are compared to calculations performed at next-to-next-to-leading-order accuracy using recent sets of parton distribution functions. The data demonstrate significant power to constrain the gluon distribution function for the Bjorken-\(x\) values near 0.1 and the light-quark sea for \(x\) < 0.02.