This paper presents measurements of the Formula omitted and Formula omitted cross-sections and the associated charge asymmetry as a function of the absolute pseudorapidity of the decay muon. The data ...were collected in proton-proton collisions at a centre-of-mass energy of 8 Formula omitted with the ATLAS experiment at the LHC and correspond to a total integrated luminosity of Formula omitted. The precision of the cross-section measurements varies between 0.8 and 1.5% as a function of the pseudorapidity, excluding the 1.9% uncertainty on the integrated luminosity. The charge asymmetry is measured with an uncertainty between 0.002 and 0.003. The results are compared with predictions based on next-to-next-to-leading-order calculations with various parton distribution functions and have the sensitivity to discriminate between them.
Searches for scalar leptoquarks pair-produced in proton-proton collisions at Formula omitted TeV at the Large Hadron Collider are performed by the ATLAS experiment. A data set corresponding to an ...integrated luminosity of 36.1 fb Formula omitted is used. Final states containing two electrons or two muons and two or more jets are studied, as are states with one electron or muon, missing transverse momentum and two or more jets. No statistically significant excess above the Standard Model expectation is observed. The observed and expected lower limits on the leptoquark mass at 95% confidence level extend up to 1.29 TeV and 1.23 TeV for first- and second-generation leptoquarks, respectively, as postulated in the minimal Buchmüller-Rückl-Wyler model, assuming a branching ratio into a charged lepton and a quark of 50%. In addition, measurements of particle-level fiducial and differential cross sections are presented for the Formula omitted, Formula omitted and Formula omitted processes in several regions related to the search control regions. Predictions from a range of generators are compared with the measurements, and good agreement is seen for many of the observables. However, the predictions for the Formula omitted measurements in observables sensitive to jet energies disagree with the data.
This paper presents measurements of charged-particle distributions sensitive to the properties of the underlying event in events containing a Formula omitted boson decaying into a muon pair. The data ...were obtained using the ATLAS detector at the LHC in proton-proton collisions at a centre-of-mass energy of 13 Formula omitted with an integrated luminosity of Formula omitted. Distributions of the charged-particle multiplicity and of the charged-particle transverse momentum are measured in regions of the azimuth defined relative to the Formula omitted boson direction. The measured distributions are compared with the predictions of various Monte Carlo generators which implement different underyling event models. The Monte Carlo model predictions qualitatively describe the data well, but with some significant discrepancies.
A study of the decays Bs0 → μ+μ− and B0 → μ+μ− has been performed using 26.3 fb−1 of 13 TeV LHC proton-proton collision data collected with the ATLAS detector in 2015 and 2016. Since the detector ...resolution in μ+μ− invariant mass is comparable to the Bs0-B0 mass difference, a single fit determines the signal yields for both decay modes. This results in a measurement of the branching fraction ℬBs0→μ+μ−=3.2−1.0+1.1×10−9\ \mathrm{\mathcal{B}}\left({B}_s^0\to {\mu}^{+}{\mu}^{-}\right)=\left({3.2}_{-1.0}^{+1.1}\right)\times {10}^{-9} \ and an upper limit ℬB0→μ+μ−<4.3×10−10\ \mathrm{\mathcal{B}}\left({B}^0\to {\mu}^{+}{\mu}^{-}\right)<4.3\times {10}^{-10} \ at 95% confidence level. The result is combined with the Run 1 ATLAS result, yielding ℬBs0→μ+μ−=2.8−0.7+0.8×10−9\ \mathrm{\mathcal{B}}\left({B}_s^0\to {\mu}^{+}{\mu}^{-}\right)=\left({2.8}_{-0.7}^{+0.8}\right)\times {10}^{-9} \ and ℬB0→μ+μ−<2.1×10−10\ \mathrm{\mathcal{B}}\left({B}^0\to {\mu}^{+}{\mu}^{-}\right)<2.1\times {10}^{-10} \ at 95% confidence level. The combined result is consistent with the Standard Model prediction within 2.4 standard deviations in the ℬB0→μ+μ−−ℬBs0→μ+μ−\ \mathrm{\mathcal{B}}\left({B}^0\to {\mu}^{+}{\mu}^{-}\right)-\mathrm{\mathcal{B}}\left({B}_s^0\to {\mu}^{+}{\mu}^{-}\right) \ plane.
Algorithms used for the reconstruction and identification of electrons in the central region of the ATLAS detector at the Large Hadron Collider (LHC) are presented in this paper; these algorithms are ...used in ATLAS physics analyses that involve electrons in the final state and which are based on the 2015 and 2016 proton-proton collision data produced by the LHC at Formula omitted = 13 Formula omitted. The performance of the electron reconstruction, identification, isolation, and charge identification algorithms is evaluated in data and in simulated samples using electrons from Formula omitted and Formula omitted decays. Typical examples of combinations of electron reconstruction, identification, and isolation operating points used in ATLAS physics analyses are shown.
This paper presents measurements of \W^{\pm }Z\ production cross sections in pp collisions at a centre-of-mass energy of 13 \\text {TeV}\. The data were collected in 2015 and 2016 by the ATLAS ...experiment at the Large Hadron Collider, and correspond to an integrated luminosity of \36.1~\hbox {fb}^{-1}\. The \W^{\pm }Z\ candidate events are reconstructed using leptonic decay modes of the gauge bosons into electrons and muons. The measured inclusive cross section in the detector fiducial region for a single leptonic decay mode is \\sigma _{W^\pm Z \rightarrow \ell ^{'} \nu \ell \ell }^{\text {fid.}} = 63.7 \, \pm ~1.0~\text {(stat.)} \, \pm ~2.3~\text {(syst.)} \, \pm ~1.4~\text {(lumi.)}\ fb, reproduced by the next-to-next-to-leading-order Standard Model prediction of \61.5^{+1.4}_{-1.3}\ fb. Cross sections for \W^+Z\ and \W^-Z\ production and their ratio are presented as well as differential cross sections for several kinematic observables. An analysis of angular distributions of leptons from decays of W and Z bosons is performed for the first time in pair-produced events in hadronic collisions, and integrated helicity fractions in the detector fiducial region are measured for the W and Z bosons separately. Of particular interest, the longitudinal helicity fraction of pair-produced vector bosons is also measured.
This paper describes a search for pairs of neutral, long-lived particles decaying in the ATLAS calorimeter. Long-lived particles occur in many extensions to the Standard Model and may elude searches ...for new promptly decaying particles. The analysis considers neutral, long-lived scalars with masses between 5 and 400 GeV, produced from decays of heavy bosons with masses between \125\ and \1000\ GeV, where the long-lived scalars decay into Standard Model fermions. The analysis uses either \10.8~{\hbox {fb}}^{-1}\ or \33.0~{\hbox {fb}}^{-1}\ of data (depending on the trigger) recorded in 2016 at the LHC with the ATLAS detector in proton–proton collisions at a centre-of-mass energy of 13 TeV. No significant excess is observed, and limits are reported on the production cross section times branching ratio as a function of the proper decay length of the long-lived particles.
Inclusive and differential cross-sections for the production of a top-quark pair in association with a photon are measured with proton-proton collision data corresponding to an integrated luminosity ...of 36.1 Formula omitted, collected by the ATLAS detector at the LHC in 2015 and 2016 at a centre-of-mass energy of 13 Formula omitted. The measurements are performed in single-lepton and dilepton final states in a fiducial volume. Events with exactly one photon, one or two leptons, a channel-dependent minimum number of jets, and at least one b-jet are selected. Neural network algorithms are used to separate the signal from the backgrounds. The fiducial cross-sections are measured to be Formula omitted and Formula omitted for the single-lepton and dilepton channels, respectively. The differential cross-sections are measured as a function of photon transverse momentum, photon absolute pseudorapidity, and angular distance between the photon and its closest lepton in both channels, as well as azimuthal opening angle and absolute pseudorapidity difference between the two leptons in the dilepton channel. All measurements are in agreement with the theoretical predictions.
It has been found that the theoretical predictions for W and Z boson cross sections, and for the W boson charge asymmetry, which are labelled as NNPDF3.1 1 have in fact been calculated using the ...NNPDF3.0 PDF set 2 instead.
The performance of identification algorithms ("taggers") for hadronically decaying top quarks and W bosons in pp collisions at Formula omitted = 13 TeV recorded by the ATLAS experiment at the Large ...Hadron Collider is presented. A set of techniques based on jet shape observables are studied to determine a set of optimal cut-based taggers for use in physics analyses. The studies are extended to assess the utility of combinations of substructure observables as a multivariate tagger using boosted decision trees or deep neural networks in comparison with taggers based on two-variable combinations. In addition, for highly boosted top-quark tagging, a deep neural network based on jet constituent inputs as well as a re-optimisation of the shower deconstruction technique is presented. The performance of these taggers is studied in data collected during 2015 and 2016 corresponding to 36.1 fb Formula omitted for the Formula omitted and Formula omitted and 36.7 fb Formula omitted for the dijet event topologies.
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