A search for new heavy particles manifested as resonances in two-jet final states is presented. The data were produced in 7 TeV proton-proton collisions by the Large Hadron Collider (LHC) and correspond to an integrated luminosity of 315 nb^-1 collected by the ATLAS detector. No resonances were observed. Upper limits were set on the product of cross section and signal acceptance for excited-quark (q*) production as a function of q* mass. These exclude at the 95% CL the q* mass interval 0.30 < mq* < 1.26 TeV, extending the reach of previous experiments.
The dijet mass distribution (NUMBER OF EVENTS).
95 PCT CL upper limit of the cross section x acceptance.
First measurements of the W -> lnu and Z/gamma* -> ll (l = e, mu) production cross sections in proton-proton collisions at sqrt(s) = 7 TeV are presented using data recorded by the ATLAS experiment at the LHC. The results are based on 2250 W -> lnu and 179 Z/gamma* -> ll candidate events selected from a data set corresponding to an integrated luminosity of approximately 320 nb-1. The measured total W and Z/gamma*-boson production cross sections times the respective leptonic branching ratios for the combined electron and muon channels are $\stotW$ * BR(W -> lnu) = 9.96 +- 0.23(stat) +- 0.50(syst) +- 1.10(lumi) nb and $\stotZg$ * BR(Z/gamma* -> ll) = 0.82 +- 0.06(stat) +- 0.05(syst) +- 0.09(lumi) nb (within the invariant mass window 66 < m_ll < 116 GeV). The W/Z cross-section ratio is measured to be 11.7 +- 0.9(stat) +- 0.4(syst). In addition, measurements of the W+ and W- production cross sections and of the lepton charge asymmetry are reported. Theoretical predictions based on NNLO QCD calculations are found to agree with the measurements.
Measured fiducial cross section times leptonic branching ratio for W+ production in the W+ -> e+ nu final state.
Measured fiducial cross section times leptonic branching ratio for W- production in the W- -> e- nubar final state.
Measured fiducial cross section times leptonic branching ratio for W+/- production in the combined W+ -> e+ nu and W- -> e- nubar final state.
A measurement of prompt photon-pair production in proton-proton collisions at $\sqrt{s}=13$ TeV is presented. The data were recorded by the ATLAS detector at the LHC with an integrated luminosity of 139 fb$^{-1}$. Events with two photons in the well-instrumented region of the detector are selected. The photons are required to be isolated and have a transverse momentum of $p_\mathrm{T,\gamma_{1(2)}} > 40(30)$ GeV for the leading (sub-leading) photon. The differential cross sections as functions of several observables for the diphoton system are measured and compared with theoretical predictions from state-of-the-art Monte Carlo and fixed-order calculations. The QCD predictions from next-to-next-to-leading-order calculations and multi-leg merged calculations are able to describe the measured integrated and differential cross sections within uncertainties, whereas lower-order calculations show significant deviations, demonstrating that higher-order perturbative QCD corrections are crucial for this process. The resummed predictions with parton showers additionally provide an excellent description of the low transverse-momentum regime of the diphoton system.
Differential cross section as a function of $p_{T,\gamma_{1}}$. The table contains the values measured in data and theory predictions from SHERPA, DIPHOX and NNLOJET.
Differential cross section as a function of $p_{T,\gamma_{2}}$. The table contains the values measured in data and theory predictions from SHERPA, DIPHOX and NNLOJET.
Integrated fiducial cross section measured in data and from different predictions.
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$\pm$2.4 fb.
The results of the fitted signal strength $\mu$ in the 1L/2LOS channel
The results of fitted inclusive ${t\bar{t}t\bar{t}}$ cross-section in the 1L/2LOS channel
Ranking of the nuisance parameters included in the fit according to their impact on the signal strength $\mu$. The impact of each nuisance parameter, $\Delta\mu$, is computed by comparing the nominal best-fit value of $\mu$ with the result of the fit when fixing the nuisance parameter to its best-fit value, $\hat{\theta}$, shifted by its pre-fit (post-fit) uncertainties $\pm \Delta\theta$ ($\pm \Delta\hat{\theta}$).
Measurements of both the inclusive and differential production cross sections of a top-quark-antiquark pair in association with a $Z$ boson ($t\bar{t}Z$) are presented. The measurements are performed by targeting final states with three or four isolated leptons (electrons or muons) and are based on $\sqrt{s} = 13$ TeV proton-proton collision data with an integrated luminosity of 139 fb$^{-1}$, recorded from 2015 to 2018 with the ATLAS detector at the CERN Large Hadron Collider. The inclusive cross section is measured to be $\sigma_{t\bar{t}Z} = 0.99 \pm 0.05$ (stat.) $\pm 0.08$ (syst.) pb, in agreement with the most precise theoretical predictions. The differential measurements are presented as a function of a number of kinematic variables which probe the kinematics of the $t\bar{t}Z$ system. Both absolute and normalised differential cross-section measurements are performed at particle and parton levels for specific fiducial volumes and are compared with theoretical predictions at different levels of precision, based on a $\chi^{2}/$ndf and $p$-value computation. Overall, good agreement is observed between the unfolded data and the predictions.
The measured $t\bar{t}\text{Z}$ cross-section value and its uncertainty based on the fit results from the combined trilepton and tetralepton channels. The value corresponds to the phase-space region where the difermion mass from the Z boson decay lies in the range $70 < m_{f\bar{f}} < 110$ GeV.
The measured $t\bar{t}\text{Z}$ cross-section value and its uncertainty based on the fit results from the combined trilepton and tetralepton channels. The value corresponds to the phase-space region where the difermion mass from the Z boson decay lies in the range $70 < m_{f\bar{f}} < 110$ GeV.
List of relative uncertainties of the measured inclusive $t\bar{t}\text{Z}$ cross section from the combined fit. The uncertainties are symmetrised for presentation and grouped into the categories described in the text. The quadratic sum of the individual uncertainties is not equal to the total uncertainty due to correlations introduced by the fit.
Measurements of Higgs boson production cross-sections are carried out in the diphoton decay channel using 139 fb$^{-1}$ of $pp$ collision data at $\sqrt{s} = 13$ TeV collected by the ATLAS experiment at the LHC. The analysis is based on the definition of 101 distinct signal regions using machine-learning techniques. The inclusive Higgs boson signal strength in the diphoton channel is measured to be $1.04^{+0.10}_{-0.09}$. Cross-sections for gluon-gluon fusion, vector-boson fusion, associated production with a $W$ or $Z$ boson, and top associated production processes are reported. An upper limit of 10 times the Standard Model prediction is set for the associated production process of a Higgs boson with a single top quark, which has a unique sensitivity to the sign of the top quark Yukawa coupling. Higgs boson production is further characterized through measurements of Simplified Template Cross-Sections (STXS). In total, cross-sections of 28 STXS regions are measured. The measured STXS cross-sections are compatible with their Standard Model predictions, with a $p$-value of $93\%$. The measurements are also used to set constraints on Higgs boson coupling strengths, as well as on new interactions beyond the Standard Model in an effective field theory approach. No significant deviations from the Standard Model predictions are observed in these measurements, which provide significant sensitivity improvements compared to the previous ATLAS results.
Cross-sections times H->yy branching ratio for ggF +bbH, VBF, VH, ttH, and tH production, normalized to their SM predictions. The values are obtained from a simultaneous fit to all categories. The theory uncertainties in the predictions include uncertainties due to missing higher-order terms in the perturbative QCD calculations and choices of parton distribution functions and value of alpha_s, as well as the H->yy branching ratio uncertainty.
Correlation matrix for the measurement of production cross-sections of the Higgs boson times the H->yy branching ratio.
Best-fit values and uncertainties for STXS parameters in each of the 28 regions considered, normalized to their SM predictions. The values for the gg->H process also include the contributions from bbH production.
Cross-section measurements for a $Z$ boson produced in association with high-transverse-momentum jets ($p_{\mathrm{T}} \geq 100$ GeV) and decaying into a charged-lepton pair ($e^+e^-,\mu^+\mu^-$) are presented. The measurements are performed using proton-proton collisions at $\sqrt{s}=13$ TeV corresponding to an integrated luminosity of $139$ fb$^{-1}$ collected by the ATLAS experiment at the LHC. Measurements of angular correlations between the $Z$ boson and the closest jet are performed in events with at least one jet with $p_{\mathrm{T}} \geq 500$ GeV. Event topologies of particular interest are the collinear emission of a $Z$ boson in dijet events and a boosted $Z$ boson recoiling against a jet. Fiducial cross sections are compared with state-of-the-art theoretical predictions. The data are found to agree with next-to-next-to-leading-order predictions by NNLOjet and with the next-to-leading-order multi-leg generators MadGraph5_aMC@NLO and Sherpa.
Measured fiducial differential cross sections for the Z boson p$_{\mathrm{T}}$ in Z($\to \ell^{+} \ell^{-}$) + high p$_{\mathrm{T}}$ jets events. The statistical, systematic, and luminosity uncertainties are given.
Measured fiducial differential cross sections for the leading jet p$_{\mathrm{T}}$ in Z($\to \ell^{+} \ell^{-}$) + high p$_{\mathrm{T}}$ jets events. The statistical, systematic, and luminosity uncertainties are given.
Measured fiducial differential cross sections for the jet multiplicity in Z($\to \ell^{+} \ell^{-}$) + high p$_{\mathrm{T}}$ jets events. The statistical, systematic, and luminosity uncertainties are given.
A measurement of single top-quark production in the s-channel is performed in proton$-$proton collisions at a centre-of-mass energy of 13 TeV with the ATLAS detector at the CERN Large Hadron Collider. The dataset corresponds to an integrated luminosity of 139 fb$^{-1}$. The analysis is performed on events with an electron or muon, missing transverse momentum and exactly two $b$-tagged jets in the final state. A discriminant based on matrix element calculations is used to separate single-top-quark s-channel events from the main background contributions, which are top-quark pair production and $W$-boson production in association with jets. The observed (expected) signal significance over the background-only hypothesis is 3.3 (3.9) standard deviations, and the measured cross-section is $\sigma=8.2^{+3.5}_{-2.9}$ pb, consistent with the Standard Model prediction of $\sigma^{\mathrm{SM}}=10.32^{+0.40}_{-0.36}$ pb.
Result of the s-channel single-top cross-section measurement, in pb. The statistical and systematic uncertainties are given, as well as the total uncertainty. The normalisation factors for the $t\bar{t}$ and $W$+jets backgrounds are also shown, with their total uncertainties.
Distribution of ${E}_{T}^{miss}$ after the fit of the multijet backgrounds, in the electron channel, in the signal region, without applying the cut on ${E}_{T}^{miss}$. Simulated events are normalised to the expected number of events given the integrated luminosity, after applying the normalisation factors obtained in the multijet fit. The last bin includes the overflow. The uncertainty band indicates the simulation's statistical uncertainty, the normalisation uncertainties for different processes ($40$ % for $W$+jets production, $30$ % for multijet background and $6$ % for top-quark processes) and the multijet background shape uncertainty in each bin, summed in quadrature. The lower panel of the figure shows the ratio of the data to the prediction.
Distribution of ${E}_{T}^{miss}$ after the fit of the multijet backgrounds, in the electron channel, in the $W$+jets VR, without applying the cut on ${E}_{T}^{miss}$. Simulated events are normalised to the expected number of events given the integrated luminosity, after applying the normalisation factors obtained in the multijet fit. The last bin includes the overflow. The uncertainty band indicates the simulation's statistical uncertainty, the normalisation uncertainties for different processes ($40$ % for $W$+jets production, $30$ % for multijet background and $6$ % for top-quark processes) and the multijet background shape uncertainty in each bin, summed in quadrature. The lower panel of the figure shows the ratio of the data to the prediction.
Measurements of differential cross sections are presented for inclusive isolated-photon production in $pp$ collisions at a centre-of-mass energy of 13 TeV provided by the LHC and using 139 fb$^{-1}$ of data recorded by the ATLAS experiment. The cross sections are measured as functions of the photon transverse energy in different regions of photon pseudorapidity. The photons are required to be isolated by means of a fixed-cone method with two different cone radii. The dependence of the inclusive-photon production on the photon isolation is investigated by measuring the fiducial cross sections as functions of the isolation-cone radius and the ratios of the differential cross sections with different radii in different regions of photon pseudorapidity. The results presented in this paper constitute an improvement with respect to those published by ATLAS earlier: the measurements are provided for different isolation radii and with a more granular segmentation in photon pseudorapidity that can be exploited in improving the determination of the proton parton distribution functions. These improvements provide a more in-depth test of the theoretical predictions. Next-to-leading-order QCD predictions from JETPHOX and SHERPA and next-to-next-to-leading-order QCD predictions from NNLOJET are compared to the measurements, using several parameterisations of the proton parton distribution functions. The measured cross sections are well described by the fixed-order QCD predictions within the experimental and theoretical uncertainties in most of the investigated phase-space region.
Measured cross sections for inclusive isolated-photon production as a function of $E_{\rm T}^{\gamma}$ for $|\eta^{\gamma}|<0.6$ and photon isolation cone radius $R=0.4$.
Measured cross sections for inclusive isolated-photon production as a function of $E_{\rm T}^{\gamma}$ for $0.6<|\eta^{\gamma}|<0.8$ and photon isolation cone radius $R=0.4$.
Measured cross sections for inclusive isolated-photon production as a function of $E_{\rm T}^{\gamma}$ for $0.8<|\eta^{\gamma}|<1.37$ and photon isolation cone radius $R=0.4$.
This paper presents studies of Bose-Einstein correlations (BEC) in proton-proton collisions at a centre-of-mass energy of 13 TeV, using data from the ATLAS detector at the CERN Large Hadron Collider. Data were collected in a special low-luminosity configuration with a minimum-bias trigger and a high-multiplicity track trigger, accumulating integrated luminosities of 151 $\mu$b$^{-1}$ and 8.4 nb$^{-1}$ respectively. The BEC are measured for pairs of like-sign charged particles, each with $|\eta|$ < 2.5, for two kinematic ranges: the first with particle $p_T$ > 100 MeV and the second with particle $p_T$ > 500 MeV. The BEC parameters, characterizing the source radius and particle correlation strength, are investigated as functions of charged-particle multiplicity (up to 300) and average transverse momentum of the pair (up to 1.5 GeV). The double-differential dependence on charged-particle multiplicity and average transverse momentum of the pair is also studied. The BEC radius is found to be independent of the charged-particle multiplicity for high charged-particle multiplicity (above 100), confirming a previous observation at lower energy. This saturation occurs independent of the transverse momentum of the pair.
Comparison of single-ratio two-particle correlation functions, C<sub>2</sub><sup>data</sup>(Q) and C<sub>2</sub><sup>MC</sup>(Q), with the two-particle double-ratio correlation function, R<sub>2</sub>(Q), for the high-multiplicity track (HMT) events using the opposite hemisphere (OHP) like-charge particles pairs reference sample for k<sub>T</sub> - interval 1000 < k<sub>T</sub> ≤ 1500 MeV.
Comparison of single-ratio two-particle correlation functions, C<sub>2</sub><sup>data</sup>(Q) and C<sub>2</sub><sup>MC</sup>(Q), with the two-particle double-ratio correlation function, R<sub>2</sub>(Q), for the high-multiplicity track (HMT) events using the unlike-charge particle (UCP) pairs reference sample for k<sub>T</sub> - interval 1000 < k<sub>T</sub> ≤ 1500 MeV.
The Bose-Einstein correlation (BEC) parameter R as a function of n<sub>ch</sub> for MB events using different MC generators in the calculation of R<sub>2</sub>(Q). The uncertainties shown are statistical. The lower panel of each plot shows the ratio of the BEC parameters obtained using EPOS LHC (red circles), Pythia 8 Monash (blue squares) and Herwig++ UE-EE-5 (green triangles) compared with the parameters obtained using Pythia 8 A2. The gray band in the lower panels is the MC systematic uncertainty, obtained as explained in the text.
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