Studying WWγWW\gamma and WZγWZ\gamma production in proton-proton collisions at s=8\sqrt{s} = 8 TeV with the ATLAS experiment

Quartic gauge couplings are tested by this study of the production of $WW\gamma$ and $WZ\gamma$ events in 20.2 fb$^{-1}$ of proton--proton collisions at a centre-of-mass energy of $\sqrt{s} = 8$ TeV recorded with the ATLAS detector at the LHC. The final state of $WW\gamma$ events containing an electron, a muon and a photon is analysed as well as the final states of $WW\gamma$ and $WZ\gamma$ production containing an electron or a muon, two jets and a photon. For all final states two different fiducial regions are defined: one yielding the best sensitivity to the production cross-section of the process and one optimised for the detection of new physical phenomena. In the former region, the $WW\gamma$ production cross-section is computed and in both regions, upper limits on the $WW\gamma$ and $WZ\gamma$ production cross-section are derived. The results obtained in the second phase space are combined for the interpretation in the context of anomalous quartic gauge couplings using an effective field theory.Comment: Proceedings of the Fifth AnnualLHC

Analysis of WWγ production with the ATLAS experiment

In this thesis, triboson final states containing two W bosons and a photon are studied using proton-proton collisions. The data set was recorded with the ATLAS detector at a centre-of-mass energy of √s = 8 TeV and corresponds to an integrated luminosity of 20.3 fb⁻¹. The fiducial cross-section of the process W W γ → eνμνγ is measured for the first time in hadron collisions and corresponds to σ = (1.89 ± 0.93(stat.) ± 0.41(syst.) ± 0.05(lumi.)) fb. It is in good agreement with the Standard Model prediction at next-to-leading order in the strong coupling constant. As no deviation from the Standard Model expectation is observed, frequentist limits at 95 % confidence level are computed to exclude contributions from anomalous quartic gauge couplings. This analysis is sensitive to fourteen coupling parameters of mass dimension eight and the limits are derived for all parameters with and without unitarisation

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 invisible Higgs boson decays in vector boson fusion at √s = 13 TeV with the ATLAS detector

We report a search for Higgs bosons that are produced via vector boson fusion and subsequently decay into invisible particles. The experimental signature is an energetic jet pair with invariant mass of O(1) TeV and O(100) GeV missing transverse momentum. The analysis uses 36.1 fb−1 of pp collision data at √s=13 TeV recorded by the ATLAS detector at the LHC. In the signal region the 2252 observed events are consistent with the background estimation. Assuming a 125 GeV scalar particle with Standard Model cross sections, the upper limit on the branching fraction of the Higgs boson decay into invisible particles is 0.37 at 95% confidence level where 0.28 was expected. This limit is interpreted in Higgs portal models to set bounds on the wimp–nucleon scattering cross section. We also consider invisible decays of additional scalar bosons with masses up to 3 TeV for which the upper limits on the cross section times branching fraction are in the range of 0.3–1.7 pb.publishedVersio

Measurement of the tt¯ tt¯ production cross section in pp collisions at √s = 13 TeV with the ATLAS detector

A measurement of four-top-quark production using proton-proton collision data at a centre-of-mass energy of 13 TeV collected by the ATLAS detector at the Large Hadron Collider corresponding to an integrated luminosity of 139 fb−1 is presented. Events are selected if they contain a single lepton (electron or muon) or an opposite-sign lepton pair, in association with multiple jets. The events are categorised according to the number of jets and how likely these are to contain b-hadrons. A multivariate technique is then used to discriminate between signal and background events. The measured four-top-quark production cross section is found to be 26+17−15 fb, with a corresponding observed (expected) significance of 1.9 (1.0) standard deviations over the background-only hypothesis. The result is combined with the previous measurement performed by the ATLAS Collaboration in the multilepton final state. The combined four-top-quark production cross section is measured to be 24+7−6 fb, with a corresponding observed (expected) signal significance of 4.7 (2.6) standard deviations over the background-only predictions. It is consistent within 2.0 standard deviations with the Standard Model expectation of 12.0 ± 2.4 fb.publishedVersio

Search for new phenomena with top quark pairs in final states with one lepton, jets, and missing transverse momentum in pp collisions at √s = 13 TeV with the ATLAS detector

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.publishedVersio

Search for the Higgs boson decays H → ee and H → eμ in pp collisions at √s=13 TeV with the ATLAS detector

Searches for the Higgs boson decays H → ee and H → eμ are performed using data corresponding to an integrated luminosity of 139 fb−1 collected with the ATLAS detector in pp collisions at √s = 13 TeV at the LHC. No significant signals are observed, in agreement with the Standard Model expectation. For a Higgs boson mass of 125 GeV, the observed (expected) upper limit at the 95% confidence level on the branching fraction B(H → ee) is 3.6 × 10−4 (3.5 × 10−4) and on B(H → eμ) is 6.2 × 10−5 (5.9 × 10−5). These results represent improvements by factors of about five and six on the previous best limits on B(H → ee) and B(H → eμ) respectively.publishedVersio

Measurement of the CP-violating phase ϕs in B0s→J/ψϕ decays in ATLAS at 13 TeV

A measurement of the B0s→J/ψϕ decay parameters using 80.5fb−1 of integrated luminosity collected with the ATLAS detector from 13 TeV proton–proton collisions at the LHC is presented. The measured parameters include the CP-violating phase ϕs, the width difference ΔΓs between the B0s meson mass eigenstates and the average decay width Γs. The values measured for the physical parameters are combined with those from 19.2fb−1 of 7 and 8 TeV data, leading to the following: ϕs=−0.087±0.036 (stat.)±0.021 (syst.) rad ΔΓs=0.0657±0.0043 (stat.)±0.0037 (syst.) ps−1 Γs=0.6703±0.0014 (stat.)±0.0018 (syst.) ps−1 Results for ϕs and ΔΓs are also presented as 68% confidence level contours in the ϕs–ΔΓs plane. Furthermore the transversity amplitudes and corresponding strong phases are measured. ϕs and ΔΓs measurements are in agreement with the Standard Model predictions.publishedVersio

Search for charginos and neutralinos in final states with two boosted hadronically decaying bosons and missing transverse momentum in pp collisions at √s=13 TeV with the ATLAS detector

A search for charginos and neutralinos at the Large Hadron Collider using fully hadronic final states and missing transverse momentum is reported. Pair-produced charginos or neutralinos are explored, each decaying into a high-pT Standard Model weak boson. Fully hadronic final states are studied to exploit the advantage of the large branching ratio, and the efficient rejection of backgrounds by identifying the high-pT bosons using large-radius jets and jet substructure information. An integrated luminosity of 139  fb−1 of proton-proton collision data collected by the ATLAS detector at a center-of-mass energy of 13 TeV is used. No significant excess is found beyond the Standard Model expectation. Exclusion limits at the 95% confidence level are set on wino or higgsino production with various assumptions about the decay branching ratios and the type of lightest supersymmetric particle. A wino (higgsino) mass up to 1060 (900) GeV is excluded when the lightest supersymmetry particle mass is below 400 (240) GeV and the mass splitting is larger than 400 (450) GeV. The sensitivity to high-mass winos and higgsinos is significantly extended relative to previous LHC searches using other final states.publishedVersio