The production of $W^{\pm}Z$ events in proton--proton collisions at a centre-of-mass energy of 13 TeV is measured with the ATLAS detector at the LHC. The collected data correspond to an integrated luminosity of 3.2 fb$^{-1}$. The $W^{\pm}Z$ candidates are reconstructed using leptonic decays of the gauge bosons into electrons or muons. The measured inclusive cross section in the detector fiducial region for leptonic decay modes is $\sigma_{W^\pm Z \rightarrow \ell^{'} \nu \ell \ell}^{\textrm{fid.}} = 63.2 \pm 3.2$ (stat.) $\pm 2.6$ (sys.) $\pm 1.5$ (lumi.) fb. In comparison, the next-to-leading-order Standard Model prediction is $53.4^{+3.6}_{-2.8}$ fb. The extrapolation of the measurement from the fiducial to the total phase space yields $\sigma_{W^{\pm}Z}^{\textrm{tot.}} = 50.6 \pm 2.6$ (stat.) $\pm 2.0$ (sys.) $\pm 0.9$ (th.) $\pm 1.2$ (lumi.) pb, in agreement with a recent next-to-next-to-leading-order calculation of $48.2^{+1.1}_{-1.0}$ pb. The cross section as a function of jet multiplicity is also measured, together with the charge-dependent $W^+Z$ and $W^-Z$ cross sections and their ratio.
The measured fiducial cross section in the four channels and their combination. The first systematic uncertainty is the combined systematic uncertainty excluding luminosity uncertainty, the second is the luminosity uncertainty.
The measured fiducial cross section in the four channels and their combination. The first systematic uncertainty is the combined systematic uncertainty excluding luminosity uncertainty, the second is the luminosity uncertainty.
The measured fiducial cross section in the four channels and their combination. The first systematic uncertainty is the combined systematic uncertainty excluding luminosity uncertainty, the second is the luminosity uncertainty.
A search for electroweak production of supersymmetric particles in scenarios with compressed mass spectra in final states with two low-momentum leptons and missing transverse momentum is presented. This search uses proton-proton collision data recorded by the ATLAS detector at the Large Hadron Collider in 2015-2016, corresponding to 36.1 fb$^{-1}$ of integrated luminosity at $\sqrt{s}=13$ TeV. Events with same-flavor pairs of electrons or muons with opposite electric charge are selected. The data are found to be consistent with the Standard Model prediction. Results are interpreted using simplified models of R-parity-conserving supersymmetry in which there is a small mass difference between the masses of the produced supersymmetric particles and the lightest neutralino. Exclusion limits at 95% confidence level are set on next-to-lightest neutralino masses of up to 145 GeV for Higgsino production and 175 GeV for wino production, and slepton masses of up to 190 GeV for pair production of sleptons. In the compressed mass regime, the exclusion limits extend down to mass splittings of 2.5 GeV for Higgsino production, 2 GeV for wino production, and 1 GeV for slepton production. The results are also interpreted in the context of a radiatively-driven natural supersymmetry model with non-universal Higgs boson masses.
<b>Kinematics 1</b> Kinematic distributions after the background-only fit showing the data as well as the expected background in the inclusive electroweakino SRℓℓ-m<sub>ℓℓ</sub> [1, 60] (top) and slepton SRℓℓ-m<sub>T2</sub><sup>100</sup> [100, ∞] (bottom) signal regions. The arrow in the E<sub>T</sub><sup>miss</sup>/H<sub>T</sub><sup>lep</sup> variables indicates the minimum value of the requirement imposed in the final SR selection. The m<sub>ℓℓ</sub> and m<sub>T2</sub> distributions (right) have all the SR requirements applied. Background processes containing fewer than two prompt leptons are categorized as `Fake/nonprompt'. The category `Others' contains rare backgrounds from triboson, Higgs boson, and the remaining top-quark production processes listed in Table 1. The uncertainty bands plotted include all statistical and systematic uncertainties. The last bin includes overflow. The dashed lines represent benchmark signal samples corresponding to the Higgsino H̃ and slepton ℓ̃ simplified models. Orange arrows in the Data/SM panel indicate values that are beyond the y-axis range.
<b>Kinematics 1</b> Kinematic distributions after the background-only fit showing the data as well as the expected background in the inclusive electroweakino SRℓℓ-m<sub>ℓℓ</sub> [1, 60] (top) and slepton SRℓℓ-m<sub>T2</sub><sup>100</sup> [100, ∞] (bottom) signal regions. The arrow in the E<sub>T</sub><sup>miss</sup>/H<sub>T</sub><sup>lep</sup> variables indicates the minimum value of the requirement imposed in the final SR selection. The m<sub>ℓℓ</sub> and m<sub>T2</sub> distributions (right) have all the SR requirements applied. Background processes containing fewer than two prompt leptons are categorized as `Fake/nonprompt'. The category `Others' contains rare backgrounds from triboson, Higgs boson, and the remaining top-quark production processes listed in Table 1. The uncertainty bands plotted include all statistical and systematic uncertainties. The last bin includes overflow. The dashed lines represent benchmark signal samples corresponding to the Higgsino H̃ and slepton ℓ̃ simplified models. Orange arrows in the Data/SM panel indicate values that are beyond the y-axis range.
<b>Kinematics 2</b> Kinematic distributions after the background-only fit showing the data as well as the expected background in the inclusive electroweakino SRℓℓ-m<sub>ℓℓ</sub> [1, 60] (top) and slepton SRℓℓ-m<sub>T2</sub><sup>100</sup> [100, ∞] (bottom) signal regions. The arrow in the E<sub>T</sub><sup>miss</sup>/H<sub>T</sub><sup>lep</sup> variables indicates the minimum value of the requirement imposed in the final SR selection. The m<sub>ℓℓ</sub> and m<sub>T2</sub> distributions (right) have all the SR requirements applied. Background processes containing fewer than two prompt leptons are categorized as `Fake/nonprompt'. The category `Others' contains rare backgrounds from triboson, Higgs boson, and the remaining top-quark production processes listed in Table 1. The uncertainty bands plotted include all statistical and systematic uncertainties. The last bin includes overflow. The dashed lines represent benchmark signal samples corresponding to the Higgsino H̃ and slepton ℓ̃ simplified models. Orange arrows in the Data/SM panel indicate values that are beyond the y-axis range.
A search for new phenomena in events with two same-charge leptons or three leptons and jets identified as originating from $b$-quarks in a data sample of 36.1 fb$^{-1}$ of $pp$ collisions at $\sqrt{s}= 13$ TeV recorded by the ATLAS detector at the Large Hadron Collider is reported. No significant excess is found and limits are set on vector-like quark, four-top-quark, and same-sign top-quark pair production. The observed (expected) 95% CL mass limits for a vector-like $T$- and $B$-quark singlet are $m_T > 0.98$ $(0.99)$ TeV and $m_B > 1.00$ $(1.01)$ TeV respectively. Limits on the production of the vector-like $T_{5/3}$-quark are also derived considering both pair and single production; in the former case the lower limit on the mass of the $T_{5/3}$-quark is (expected to be) 1.19 (1.21) TeV. The Standard Model four-top-quark production cross-section upper limit is (expected to be) 69 (29) fb. Constraints are also set on exotic four-top-quark production models. Finally, limits are set on same-sign top-quark pair production. The upper limit on $uu \to tt$ production is (expected to be) 89 (59) fb for a mediator mass of 1 TeV, and a dark-matter interpretation is also derived, excluding a mediator of 3 TeV with a dark-sector coupling of 1.0 and a coupling to ordinary matter above 0.31.
Expected and observed limits on vector-like B-quark pair production as a function of mass, assuming the branching ratios expected in the singlet model.
Expected and observed limits on vector-like B-quark pair production as a function of mass, assuming the branching ratios expected in the singlet model.
Expected and observed limits on vector-like T-quark pair production as a function of mass, assuming the branching ratios expected in the singlet model.
The modification of the production of $J/\psi$, $\psi(\mathrm{2S})$, and $\mit{\Upsilon}(n\mathrm{S})$ ($n = 1, 2, 3$) in $p$+Pb collisions with respect to their production in $pp$ collisions has been studied. The $p$+Pb and $pp$ datasets used in this paper correspond to integrated luminosities of $28$ $\mathrm{nb}^{-1}$ and $25$ $\mathrm{pb}^{-1}$ respectively, collected in 2013 and 2015 by the ATLAS detector at the LHC, both at a centre-of-mass energy per nucleon pair of 5.02 TeV. The quarkonium states are reconstructed in the dimuon decay channel. The yields of $J/\psi$ and $\psi(\mathrm{2S})$ are separated into prompt and non-prompt sources. The measured quarkonium differential cross sections are presented as a function of rapidity and transverse momentum, as is the nuclear modification factor, $R_{p\mathrm{Pb}}$ for $J/\psi$ and $\mit{\Upsilon}(\mathrm{1S})$. No significant modification of the $J/\psi$ production is observed while $\mit{\Upsilon}(\mathrm{1S})$ production is found to be suppressed at low transverse momentum in $p$+Pb collisions relative to $pp$ collisions. The production of excited charmonium and bottomonium states is found to be suppressed relative to that of the ground states in central $p$+Pb collisions.
Summary of results for prompt Psi(2S) to J/psi double ratio in p+Pb collisions at 5.02 TeV as a function of center-of-mass rapidity. Uncertainties are statistical and systematic, respectively.
Detailed measurements of $t$-channel single top-quark production are presented. They use 20.2 fb$^{-1}$ of data collected by the ATLAS experiment in proton-proton collisions at a centre-of-mass energy of 8 TeV at the LHC. Total, fiducial and differential cross-sections are measured for both top-quark and top-antiquark production. The fiducial cross-section is measured with a precision of 5.8 % (top quark) and 7.8 % (top antiquark), respectively. The total cross-sections are measured to be $\sigma_{\mathrm{tot}}(tq) = 56.7^{+4.3}_{-3.8}\;$pb for top-quark production and $\sigma_{\mathrm{tot}}(\bar{t}q) = 32.9^{+3.0}_{-2.7}\;$pb for top-antiquark production, in agreement with the Standard Model prediction. In addition, the ratio of top-quark to top-antiquark production cross-sections is determined to be $R_t=1.72 \pm 0.09$, with an improved relative precision of 4.9 % since several systematic uncertainties cancel in the ratio. The differential cross-sections as a function of the transverse momentum and rapidity of both the top quark and the top antiquark are measured at both the parton and particle levels. The transverse momentum and rapidity differential cross-sections of the accompanying jet from the $t$-channel scattering are measured at particle level. All measurements are compared to various Monte Carlo predictions as well as to fixed-order QCD calculations where available.
Predicted and observed event yields for the signal region (SR). The multijet background prediction is obtained from a binned maximum-likelihood fit to the $E_{\mathrm{T}}^{\mathrm{miss}}$ distribution. All the other predictions are derived using theoretical cross-sections, given for the backgrounds in Sect. 6 and for the signal in Sect. 1. The quoted uncertainties are in the predicted cross-sections or in the number of multijet events, in case of the multijet process.
Predicted and observed event yields for the signal region (SR). The multijet background prediction is obtained from a binned maximum-likelihood fit to the $E_{\mathrm{T}}^{\mathrm{miss}}$ distribution. All the other predictions are derived using theoretical cross-sections, given for the backgrounds in Sect. 6 and for the signal in Sect. 1. The quoted uncertainties are in the predicted cross-sections or in the number of multijet events, in case of the multijet process.
Definition of the fiducial phase space.
Measurements of the electroweak production of a $W$ boson in association with two jets at high dijet invariant mass are performed using $\sqrt{s} = 7$ and $8$ TeV proton-proton collision data produced by the Large Hadron Collider, corresponding respectively to 4.7 and 20.2 fb$^{-1}$ of integrated luminosity collected by the ATLAS detector. The measurements are sensitive to the production of a $W$ boson via a triple-gauge-boson vertex and include both the fiducial and differential cross sections of the electroweak process.
Integrated fiducial cross-sections for QCD+EW and EW-only $Wjj$ production in the inclusive region with $m_{jj} > 1.5$ TeV.
Integrated fiducial cross-sections for QCD+EW $Wjj$ production in the forward-lepton region.
Integrated fiducial cross-sections for QCD+EW and EW-only $Wjj$ production in the signal region with $m_{jj} > 1.0$ TeV.
A search for Higgs boson pair production in the $b\bar{b}b\bar{b}$ final state is carried out with up to 36.1 $\mathrm{fb}^{-1}$ of LHC proton--proton collision data collected at $\sqrt{s}$ = 13 TeV with the ATLAS detector in 2015 and 2016. Three benchmark signals are studied: a spin-2 graviton decaying into a Higgs boson pair, a scalar resonance decaying into a Higgs boson pair, and Standard Model non-resonant Higgs boson pair production. Two analyses are carried out, each implementing a particular technique for the event reconstruction that targets Higgs bosons reconstructed as pairs of jets or single boosted jets. The resonance mass range covered is 260--3000 GeV. The analyses are statistically combined and upper limits on the production cross section of Higgs boson pairs times branching ratio to $b\bar{b}b\bar{b}$ are set in each model. No significant excess is observed; the largest deviation of data over prediction is found at a mass of 280 GeV, corresponding to 2.3 standard deviations globally. The observed 95% confidence level upper limit on the non-resonant production is 13 times the Standard Model prediction.
The observed and expected 95% CL upper limits on the production cross section times branching ratio for the narrow-width scalar.
The observed and expected 95% CL upper limits on the production cross section times branching ratio for the bulk Randall-Sundrum model with $\frac{k}{\overline{M}_{\mathrm{Pl}}} = 1$.
The observed and expected 95% CL upper limits on the production cross section times branching ratio for the bulk Randall-Sundrum model with $\frac{k}{\overline{M}_{\mathrm{Pl}}} = 2$.
An observation of electroweak $W^{\pm}Z$ production in association with two jets in proton-proton collisions is presented. The data collected by the ATLAS detector at the Large Hadron Collider in 2015 and 2016 at a centre-of-mass energy of $\sqrt{s} =$ 13 TeV are used, corresponding to an integrated luminosity of 36.1 fb$^{-1}$. Events containing three identified leptons, either electrons or muons, and two jets are selected. The electroweak production of $W^{\pm}Z$ bosons in association with two jets is measured with an observed significance of 5.3 standard deviations. A fiducial cross-section for electroweak production including interference effects is measured to be $\sigma_{WZjj\mathrm{-EW}} = 0.57 \; ^{+ 0.14} _{- 0.13} \,(\mathrm{stat.}) \; ^{+ 0.07} _{- 0.06} \,(\mathrm{syst.}) \; \mathrm{fb}$. Total and differential fiducial cross-sections of the sum of $W^\pm Z jj$ electroweak and strong productions for several kinematic observables are also measured.
Fiducial cross section of the electroweak $W^{\pm}Z$ boson pair production in association with two jets. The first systematic uncertainty is experimental, the second is the theory modelling and interference systematics and the third one is the luminosity uncertainty.
Fiducial cross section of the $W^{\pm}Z$ boson pair production in association with two jets. The first systematic uncertainty is experimental, the second is the theory modelling and interference systematics and the third one is the luminosity uncertainty.
A measurement of the four-lepton invariant mass spectrum is made with the ATLAS detector, using an integrated luminosity of 36.1 fb$^{-1}$ of proton-proton collisions at $\sqrt{s}$ = 13 TeV delivered by the Large Hadron Collider. The differential cross-section is measured for events containing two same-flavour opposite-sign lepton pairs. It exhibits a rich structure, with different mass regions dominated in the Standard Model by single $Z$ boson production, Higgs boson production, and $Z$ boson pair production, and non-negligible interference effects at high invariant masses. The measurement is compared with state-of-the-art Standard Model calculations, which are found to be consistent with the data. These calculations are used to interpret the data in terms of $gg\rightarrow ZZ \rightarrow 4\ell$ and $Z \rightarrow 4\ell$ subprocesses, and to place constraints on a possible contribution from physics beyond the Standard Model.
Measured and expected differential cross-section $\text{d}\sigma / \text{d} m_{4l}$ as a function of $m_{4l}$
Measured and expected differential cross-section $\text{d}\sigma / \text{d} m_{4l}$ as a function of $m_{4l}$ in bin of 0$< p_{T}^{4l} <$20 GeV
Measured and expected differential cross-section $\text{d}\sigma / \text{d} m_{4l}$ as a function of $m_{4l}$ in bin of 20$< p_{T}^{4l} <$50 GeV
A search for resonant and non-resonant pair production of Higgs bosons in the $b\bar{b}\tau^+\tau^-$ final state is presented. The search uses 36.1 fb$^{-1}$ of $pp$ collision data with $\sqrt{s}= 13$ TeV recorded by the ATLAS experiment at the LHC in 2015 and 2016. The semileptonic and fully hadronic decays of the $\tau$-lepton pair are considered. No significant excess above the expected background is observed in the data. The cross-section times branching ratio for non-resonant Higgs boson pair production is constrained to be less than 30.9 fb, 12.7 times the Standard Model expectation, at 95% confidence level. The data are also analyzed to probe resonant Higgs boson pair production, constraining a model with an extended Higgs sector based on two doublets and a Randall-Sundrum bulk graviton model. Upper limits are placed on the resonant Higgs boson pair production cross-section times branching ratio, excluding resonances $X$ in the mass range $305~{\rm GeV} < m_X < 402~{\rm GeV}$ in the simplified hMSSM minimal supersymmetric model for $\tan\beta=2$ and excluding bulk Randall-Sundrum gravitons $G_{\mathrm{KK}}$ in the mass range $325~{\rm GeV} < m_{G_{\mathrm{KK}}} < 885~{\rm GeV}$ for $k/\overline{M}_{\mathrm{Pl}} = 1$.
Upper limits on the production cross-section times the HH to bbtautau branching ratio for non-resonant HH at 95% CLS and their interpretation as multiples of the SM prediction
Upper limits on the production cross-section times the HH to bbtautau branching ratio divided by the SM prediction for non-resonant HH at 95% CL
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