This Letter presents a measurement of the inelastic proton-proton cross section using 60 $\mu$b$^{-1}$ of $pp$ collisions at a center-of-mass energy $\sqrt{s}$ of $13$ TeV with the ATLAS detector at the LHC. Inelastic interactions are selected using rings of plastic scintillators in the forward region ($2.07<|\eta|<3.86$) of the detector. A cross section of $68.1\pm 1.4$ mb is measured in the fiducial region $\xi=M_X^2/s>10^{-6}$, where $M_X$ is the larger invariant mass of the two hadronic systems separated by the largest rapidity gap in the event. In this $\xi$ range the scintillators are highly efficient. For diffractive events this corresponds to cases where at least one proton dissociates to a system with $M_X>13$ GeV. The measured cross section is compared with a range of theoretical predictions. When extrapolated to the full phase space, a cross-section of $78.1 \pm 2.9$ mb is measured, consistent with the inelastic cross section increasing with center-of-mass energy.
The measured and extrapolated inelastic cross section. The statistical uncertainty is negligible and is therefore displayed as zero. The first systematic uncertainty is the experimental systematic uncertainty apart from the luminosity, the second is the luminosity uncertainty, and the third is the extrapolation uncertainty.
This article presents searches for the $Z\gamma$ decay of the Higgs boson and for narrow high-mass resonances decaying to $Z\gamma$, exploiting $Z$ boson decays to pairs of electrons or muons. The data analysis uses 36.1 fb$^{-1}$ of $pp$ collisions at $\sqrt{s} = 13$ TeV recorded by the ATLAS detector at the CERN Large Hadron Collider. The data are found to be consistent with the expected Standard Model background. The observed (expected - assuming Standard Model $pp\to H\to Z\gamma$ production and decay) upper limit on the production cross section times the branching ratio for $pp\to H\to Z\gamma$ is 6.6 (5.2) times the Standard Model prediction at the 95% confidence level for a Higgs boson mass of 125.09 GeV. In addition, upper limits are set on the production cross section times the branching ratio as a function of the mass of a narrow resonance between 250 GeV and 2.4 TeV, assuming spin-0 resonances produced via gluon-gluon fusion, and spin-2 resonances produced via gluon-gluon or quark-antiquark initial states. For high-mass spin-0 resonances, the observed (expected) limits vary between 88 fb (61 fb) and 2.8 fb (2.7 fb) for the mass range from 250 GeV to 2.4 TeV at the 95% confidence level.
The measured sigma(pp-->X)xB(X->Z gamma) limit with the hypothesis of spin-0 resonance.
The measured sigma(pp-->X)xB(X->Z gamma) limit with the hypothesis of spin-2 resonance via gluon-gluon initial states.
The measured sigma(pp-->X)xB(X->Z gamma) limit with the hypothesis of spin-0 resonance via qqbar initial states.
The analysis of the momentum difference between charged hadrons in high-energy proton-proton collisions is performed in order to study coherent particle production. The observed correlation pattern agrees with a model of a helical QCD string fragmenting into a chain of ground-state hadrons. A threshold momentum difference in the production of adjacent pairs of charged hadrons is observed, in agreement with model predictions. The presence of low-mass hadron chains also explains the emergence of charge-combination-dependent two-particle correlations commonly attributed to Bose-Einstein interference. The data sample consists of 190 inverse microbarns of minimum bias events collected with proton-proton collisions at a center-of-mass energy of 7 TeV in the early low-luminosity data taking with the ATLAS detector at the LHC.
The inclusive Delta as a function of $Q$, in the interval 0.02 GeV < Q < 2 GeV.
The Delta(3h), for 3-hadron chains with mass below 0.59 GeV, as a function of $Q$, in the interval 0.02 GeV < Q < 0.36 GeV.
The Dalitz plot, for 3-hadron chains with mass below 0.59 GeV. Coordinates X = sqrt(3)(T0-T2)/sum(T) , Y = 3T1/sum(T) - 1. T0/T1/T2 stand for kinetic energy of hadrons in the rest frame of the triplet ( hadrons 0 and 2 form like-sign pair).
This paper presents a study of $WW\gamma$ and $WZ\gamma$ triboson production using events from proton--proton collisions at a centre-of-mass energy of $\sqrt{s} = 8$ TeV recorded with the ATLAS detector at the LHC and corresponding to an integrated luminosity of 20.2 fb$^{-1}$. The $WW\gamma$ production cross-section is determined using a final state containing an electron, a muon, a photon, and neutrinos ($e\nu\mu\nu\gamma$). Upper limits on the production cross-section of the $e\nu\mu\nu\gamma$ final state and the $WW\gamma$ and $WZ\gamma$ final states containing an electron or a muon, two jets, a photon, and a neutrino ($e\nu jj\gamma$ or $\mu\nu jj\gamma$) are also derived. The results are compared to the cross-sections predicted by the Standard Model at next-to-leading order in the strong-coupling constant. In addition, upper limits on the production cross-sections are derived in a fiducial region optimised for a search for new physics beyond the Standard Model. The results are interpreted in the context of anomalous quartic gauge couplings using an effective field theory. Confidence intervals at 95% confidence level are derived for the 14 coupling coefficients to which $WW\gamma$ and $WZ\gamma$ production are sensitive.
Computed fiducial cross section in the $e\nu\mu\nu\gamma$ channel. The first uncertainty shown is the statistical uncertainty and the second one is the total systematic uncertainty including the uncertainty due to the luminosity. The theoretical prediction is determined with the VBFNLO generator and its uncertainty does not account for an uncertainty related to the scale introduced by restricting the jet multiplicity in the fully leptonic channel.
Observed and expected cross-section upper limits at 95\% CL for the different final states using the CL$_{\text{s}}$ method. The expected cross-section limits are computed assuming no signal is present. The last column shows the theory prediction for the signal cross-section ($\sigma_{\text{theo}}$) computed with the VBFNLO generator and corrected to particle level. The $\ell \nu jj \gamma$ cross-section corresponds to the average cross-section per lepton flavour in the semileptonic analysis and all events of the $e \nu jj \gamma$ and $\mu\nu jj \gamma$ final states are employed for the determination of this limit.
Observed and expected cross-section upper limits at 95\% CL using the CL$_{\text{s}}$ method for the different final states with the photon \et threshold optimised for maximal aQGC sensitivity. The expected cross-section limits are computed assuming the absence of $WV\gamma$ production. The last column shows the theory prediction for the SM signal cross-section computed with the VBFNLO generator and corrected to particle level. The $\ell \nu jj \gamma$ cross-section corresponds to the average cross-section per lepton flavour in the semileptonic analysis and all events of the $e \nu jj \gamma$ and $\mu\nu jj \gamma$ final states are employed for the determination of this limit.
The cross-section for the production of two jets in association with a leptonically decaying Z boson ($Zjj$) is measured in proton-proton collisions at a centre-of-mass energy of 13 TeV, using data recorded with the ATLAS detector at the Large Hadron Collider, corresponding to an integrated luminosity of 3.2 fb$^{-1}$. The electroweak $Zjj$ cross-section is extracted in a fiducial region chosen to enhance the electroweak contribution relative to the dominant Drell-Yan $Zjj$ process, which is constrained using a data-driven approach. The measured fiducial electroweak cross-section is $\sigma^{Zjj}_{EW}= 119\pm 16 (\mathrm{stat.}) \pm 20 (\mathrm{syst.})\pm 2 (\mathrm{lumi.})$ for dijet invariant mass greater than 250 GeV, and $34.2\pm 5.8 (\mathrm{stat.})\pm 5.5 (\mathrm{syst.})\pm 0.7 (\mathrm{lumi.})$ for dijet invariant mass greater than 1 TeV. Standard Model predictions are in agreement with the measurements. The inclusive $Zjj$ cross-section is also measured in six different fiducial regions with varying contributions from electroweak and Drell-Yan $Zjj$ production.
Fiducial regions definitions
Measured and predicted inclusive Zjj production cross-sections in the six fiducial regions
Measured and predicted EW-Zjj production cross-sections in the EW-enriched fiducial regions with and without an additional kinematic requirement of $m_{jj} > $ 1 TeV
A combined measurement of differential and inclusive total cross sections of Higgs boson production is performed using 36.1 fb$^{-1}$ of 13 TeV proton-proton collision data produced by the LHC and recorded by the ATLAS detector in 2015 and 2016. Cross sections are obtained from measured $H \rightarrow \gamma \gamma$ and $H \rightarrow ZZ^* \rightarrow 4\ell$ event yields, which are combined taking into account detector efficiencies, resolution, acceptances and branching fractions. The total Higgs boson production cross section is measured to be 57.0$^{+6.0}_{-5.9}$ (stat.) $^{+4.0}_{-3.3}$ (syst.) pb, in agreement with the Standard Model prediction. Differential cross-section measurements are presented for the Higgs boson transverse momentum distribution, Higgs boson rapidity, number of jets produced together with the Higgs boson, and the transverse momentum of the leading jet. The results from the two decay channels are found to be compatible, and their combination agrees with the Standard Model predictions.
Differential cross sections in the full phase space obtained from the H->gammagamma and H->4l combined measurement for Higgs boson transverse momentum ptH. The NNLOPS ggF prediction scaled to the N3LO cross section is also provided.
Differential cross sections in the full phase space obtained from the H->gammagamma and H->4l combined measurement for the Higgs boson rapidity |yH|. The NNLOPS ggF prediction scaled to the N3LO cross section is also provided.
Differential cross sections in the full phase space obtained from the H->gammagamma and H->4l combined measurement for the number of jets Njets with pT > 30 GeV. The NNLOPS ggF prediction scaled to the N3LO cross section is also provided.
This paper presents single lepton and dilepton kinematic distributions measured in dileptonic $t\bar{t}$ events produced in 20.2 fb$^{-1}$ of $\sqrt{s}=8$ TeV $pp$ collisions recorded by the ATLAS experiment at the LHC. Both absolute and normalised differential cross-sections are measured, using events with an opposite-charge $e\mu$ pair and one or two $b$-tagged jets. The cross-sections are measured in a fiducial region corresponding to the detector acceptance for leptons, and are compared to the predictions from a variety of Monte Carlo event generators, as well as fixed-order QCD calculations, exploring the sensitivity of the cross-sections to the gluon parton distribution function. Some of the distributions are also sensitive to the top quark pole mass; a combined fit of NLO fixed-order predictions to all the measured distributions yields a top quark mass value of $m_t^{\rm pole}=173.2\pm 0.9\pm0.8\pm1.2$ GeV, where the three uncertainties arise from data statistics, experimental systematics, and theoretical sources.
Absolute differential cross-section in the fiducial region as a function of lepton pT. The first column gives the cross-section including contributions from leptonic tau decays, the second without. Systematic uncertainties are given for ttbar modelling (ttmod), lepton calibration (lept), jet and b-tagging calibration (jet), backgrounds (bkg) and integrated luminosity and beam energy (leb).
Normalised differential cross-section in the fiducial region as a function of lepton pT. The first column gives the cross-section including contributions from leptonic tau decays, the second without. Systematic uncertainties are given for ttbar modelling (ttmod), lepton calibration (lept), jet and b-tagging calibration (jet), backgrounds (bkg) and integrated luminosity and beam energy (leb).
Absolute differential cross-section in the fiducial region as a function of lepton eta. The first column gives the cross-section including contributions from leptonic tau decays, the second without. Systematic uncertainties are given for ttbar modelling (ttmod), lepton calibration (lept), jet and b-tagging calibration (jet), backgrounds (bkg) and integrated luminosity and beam energy (leb).
A search for an invisibly decaying Higgs boson or dark matter candidates produced in association with a leptonically decaying $Z$ boson in proton--proton collisions at $\sqrt{s} =$ 13 TeV is presented. This search uses 36.1 fb$^{-1}$ of data collected by the ATLAS experiment at the Large Hadron Collider. No significant deviation from the expectation of the Standard Model backgrounds is observed. Assuming the Standard Model $ZH$ production cross-section, an observed (expected) upper limit of 67% (39%) at the 95% confidence level is set on the branching ratio of invisible decays of the Higgs boson with mass $m_H = $ 125 GeV. The corresponding limits on the production cross-section of the $ZH$ process with the invisible Higgs boson decays are also presented. Furthermore, exclusion limits on the dark matter candidate and mediator masses are reported in the framework of simplified dark matter models.
Observed E<sub>T</sub><sup>miss</sup> distribution in the ee channel compared to the signal and background predictions. The error band shows the total statistical and systematic uncertainty on the background prediction. The background predictions are presented as they are before being fit to the data. The ratio plot gives the observed data yield over the background prediction (black points) as well as the signal-plus-background contribution divided by the background prediction (blue or purple line) in each E<sub>T</sub><sup>miss</sup> bin. The rightmost bin contains the overflow contributions. The ZH → ℓℓ + inv signal distribution is shown with BR<sub>H → inv</sub> =0.3, which is the value most compatible with data. The simulated DM distribution with m<sub>med</sub> = 500 GeV and m<sub>χ</sub> = 100 GeV is also scaled (with a factor of 0.27) to the best-fit contribution.
Observed E<sub>T</sub><sup>miss</sup> distribution in the μμ channel compared to the signal and background predictions. The error band shows the total statistical and systematic uncertainty on the background prediction. The background predictions are presented as they are before being fit to the data. The ratio plot gives the observed data yield over the background prediction (black points) as well as the signal-plus-background contribution divided by the background prediction (blue or purple line) in each E<sub>T</sub><sup>miss</sup> bin. The rightmost bin contains the overflow contributions. The ZH → ℓℓ + inv signal distribution is shown with BR<sub>H → inv</sub> =0.3, which is the value most compatible with data. The simulated DM distribution with m<sub>med</sub> = 500 GeV and m<sub>χ</sub> = 100 GeV is also scaled (with a factor of 0.27) to the best-fit contribution.
DM exclusion limit in the two-dimensional phase space of WIMP mass m<sub>χ</sub> vs mediator mass m<sub>med</sub> determined using the combined ee+μμ channel. Both the observed and expected limits are presented, and the 1σ uncertainty band for the expected limits is also provided. Regions bounded by the limit curves are excluded at the 95% CL. The grey line labelled with "m<sub>med</sub> = 2m<sub>χ</sub>'' indicates the kinematic threshold where the mediator can decay on-shell into WIMPs, and the other grey line gives the perturbative limit (arXiv 1603.04156). The relic density line (arXiv 1603.04156) illustrates the combination of m<sub>χ</sub> and m<sub>med</sub> that would explain the observed DM relic density.
A search for a narrow scalar resonance decaying into an opposite-sign muon pair produced in events with and without $b$-tagged jets is presented in this paper. The search uses 36.1 fb$^{-1}$ of $\sqrt{s}= 13$ TeV proton-proton collision data recorded by the ATLAS experiment at the LHC. No significant excess of events above the expected Standard Model background is observed in the investigated mass range of 0.2 to 1.0 TeV. The observed upper limits at 95$\%$ confidence level on the cross section times branching ratio for $b$-quark associated production and gluon-gluon fusion are between 1.9 and 41 fb and 1.6 and 44 fb respectively, which is consistent with expectations.
The distribution of di-muon invariant mass in the b-tag signal region (SRbTag).
The distribution of di-muon invariant mass in the b-veto signal region (SRbVeto).
Inclusive jet and dijet cross-sections are measured in proton-proton collisions at a centre-of-mass energy of 13 TeV. The measurement uses a dataset with an integrated luminosity of 3.2 fb$^{-1}$ recorded in 2015 with the ATLAS detector at the Large Hadron Collider. Jets are identified using the anti-${k_t}$ algorithm with a radius parameter value of $R=0.4$. The inclusive jet cross-sections are measured double-differentially as a function of the jet transverse momentum, covering the range from 100 GeV to 3.5 TeV, and the absolute jet rapidity up to $|y|=3$. The double-differential dijet production cross-sections are presented as a function of the dijet mass, covering the range from 300 GeV to 9 TeV, and the half absolute rapidity separation between the two leading jets within $|y|<3$, $y*$, up to $y*=3$. Next-to-leading-order, and next-to-next-to-leading-order for the inclusive jet measurement, perturbative QCD calculations corrected for non-perturbative and electroweak effects are compared to the measured cross-sections.
rapidity bin 0 < |Y| < 0.5 anti-kt R=0.4
rapidity bin 0.5 < |Y| < 1.0 anti-kt R=0.4
rapidity bin 1.0 < |Y| < 1.5 anti-kt R=0.4
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