Person:

Franklin, Melissa

A search for doubly charged scalar bosons decay- ing into W boson pairs is presented. It uses a data sample from proton–proton collisions corresponding to an integrated luminosity of 36.1 fb−1 collected by the ATLAS detector at the LHC at a centre-of-mass energy of 13 TeV in 2015 and 2016. This search is guided by a model that includes an exten- sion of the Higgs sector through a scalar triplet, leading to a rich phenomenology that includes doubly charged scalar bosons H ±± . Those bosons are produced in pairs in proton– proton collisions and decay predominantly into electroweak gauge bosons H ±± → W ± W ± . Experimental signatures with several leptons, missing transverse energy and jets are explored. No significant deviations from the Standard Model predictions are found. The parameter space of the benchmark model is excluded at 95% confidence level for H±± bosons with masses between 200 and 220 GeV.

The response of the ATLAS detector to large- radius jets is measured in situ using 36.2 fb−1 of √s = 13 TeV proton–proton collisions provided by the LHC and recorded by the ATLAS experiment during 2015 and 2016. The jet energy scale is measured in events where the jet recoils against a reference object, which can be either a calibrated photon, a reconstructed Z boson, or a system of well-measured small-radius jets. The jet energy resolution and a calibration of forward jets are derived using dijet bal- ance measurements. The jet mass response is measured with two methods: using mass peaks formed by W bosons and top quarks with large transverse momenta and by compar- ing the jet mass measured using the energy deposited in the calorimeter with that using the momenta of charged-particle tracks. The transverse momentum and mass responses in sim- ulations are found to be about 2–3% higher than in data. This difference is adjusted for with a correction factor. The results of the different methods are combined to yield a calibration over a large range of transverse momenta ( pT ). The precision of the relative jet energy scale is 1–2% for 200 GeV < pT < 2 TeV, while that of the mass scale is 2–10%. The ratio of the energy resolutions in data and sim- ulation is measured to a precision of 10–15% over the same pT range.

The results of a search for direct pair production of top squarks in events with two opposite-charge leptons (electrons or muons) are reported, using 36.1 fb−1 of integrated luminosity from proton--proton collisions at s√=13 TeV collected by the ATLAS detector at the Large Hadron Collider. To cover a range of mass differences between the top squark t̃ and lighter supersymmetric particles, four possible decay modes of the top squark are targeted with dedicated selections: the decay t̃→bχ̃±1 into a b-quark and the lightest chargino with χ̃±1→Wχ̃01, the decay t̃→tχ̃01 into an on-shell top quark and the lightest neutralino, the three-body decay t̃→bWχ̃01 and the four-body decay t̃→bℓνχ̃01. No significant excess of events is observed above the Standard Model background for any selection, and limits on top squarks are set as a function of the t̃ and χ̃01 masses. The results exclude at 95% confidence level t̃ masses up to about 720 GeV, extending the exclusion region of supersymmetric parameter space covered by previous searches.

Measurements of the azimuthal anisotropy in lead–lead collisions at sqrt sNN = 5.02 TeV are presented using a data sample corresponding to 0.49 nb−1 integrated luminosity collected by the ATLAS experiment at the LHC in 2015. The recorded minimum-bias sample is enhanced by triggers for “ultra-central” collisions, providing an opportunity to perform detailed study of flow harmonics in the regime where the initial state is dominated by fluctuations. The anisotropy of the charged-particle azimuthal angle distributions is characterized by the Fourier coefficients, v2 –v7 , which are mea- sured using the two-particle correlation, scalar-product and event-plane methods. The goal of the paper is to provide measurements of the differential as well as integrated flow harmonics vn over wide ranges of the transverse momentum, 0.5 < pT < 60 GeV, the pseudorapidity, eta < 2.5, and the collision centrality 0–80%. Results from different methods are compared and discussed in the context of previous and recent measurements in Pb+Pb collisions at sqrt sNN = 2.76 TeV and 5.02 TeV . In particular, the shape of the pT dependence of elliptic or triangular flow harmonics is observed to be very similar at different centralities after scaling the vn and pT values by constant factors over the centrality interval 0–60% andthe pT range0.5< pT <5GeV.

This paper describes a strategy for a general search used by the ATLAS Collaboration to find potential indications of new physics. Events are classified according to their final state into many event classes. For each event class an automated search algorithm tests whether the data are compatible with the Monte Carlo simulated expectation in several distributions sensitive to the effects of new physics. The significance of a deviation is quantified using pseudo- experiments. A data selection with a significant deviation defines a signal region for a dedicated follow-up analysis with an improved background expectation. The analysis of the data-derived signal regions on a new dataset allows a sta- tistical interpretation without the large look-elsewhere effect. The sensitivity of the approach is discussed using Standard Model processes and benchmark signals of new physics. As an example, results are shown for 3.2 fb−1 of proton– proton collision data at a centre-of-mass energy of 13 TeV collected with the ATLAS detector at the LHC in 2015, in which more than 700 event classes and more than 105 regions have been analysed. No significant deviations are found and consequently no data-derived signal regions for a follow-up analysis have been defined.

This paper presents a direct measurement of the decay width of the top quark using tt ̄ events in the lepton+jets final state. The data sample was collected by the ATLAS detector at the LHC in proton–proton collisions at a centre-of-mass energy of 8 TeV and corresponds to an integrated luminosity of 20.2 fb−1. The decay width of the top quark is measured using a template fit to distributions of kinematic observables associated with the hadronically and semileptonically decaying top quarks. The result, Γt = 1.76 ± 0.33 (stat.) +0.79 (syst.) GeV −0.68 for a top-quark mass of 172.5 GeV, is consistent with the prediction of the Standard Model.

Multi-particle cumulants and corresponding Fourier harmonics are measured for azimuthal angle distributions of charged particles in pp collisions at s√ = 5.02 and 13 TeV and in p+Pb collisions at sNN‾‾‾‾√ = 5.02 TeV, and compared to the results obtained for low-multiplicity Pb+Pb collisions at sNN‾‾‾‾√ = 2.76 TeV. These measurements aim to assess the collective nature of particle production. The measurements of multi-particle cumulants confirm the evidence for collective phenomena in p+Pb and low-multiplicity Pb+Pb collisions. On the other hand, the pp results for four-particle cumulants do not demonstrate collective behaviour, indicating that they may be biased by contributions from non-flow correlations. A comparison of multi-particle cumulants and derived Fourier harmonics across different collision systems is presented as a function of the charged-particle multiplicity. For a given multiplicity, the measured Fourier harmonics are largest in Pb+Pb, smaller in p+Pb and smallest in pp collisions. The pp results show no dependence on the collision energy, nor on the multiplicity.

A search for doubly charged Higgs bosons with pairs of prompt, isolated, highly energetic leptons with the same electric charge is presented. The search uses a proton–proton collision data sample at a centre-of-mass energy of 13 TeV corresponding to 36.1 fb−1 of integrated luminosity recorded in 2015 and 2016 by the ATLAS detector at the LHC. This analysis focuses on the decays H±± → e±e±, H±± → e±μ± and H±± → μ±μ±, fitting the dilepton mass spectra in several exclusive signal regions. No significant evidence of a signal is observed and corresponding limits on the production cross-section and consequently a lower limit on m(H±±) are derived at 95% confidence level. With l±l± = e±e±/μ±μ±/e±μ±, the observed lower limit on the mass of a doubly charged Higgs boson only coupling to left-handed leptons varies from 770 GeV to 870 GeV (850 GeV expected) for B(H±± → l±l±) = 100% and both the expected and observed mass limits are above 450 GeV for B(H±± → l±l±) = 10% and any combination of partial branching ratios.