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).
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
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 $\sqrt{s}=13$ TeV collected by the ATLAS detector at the Large Hadron Collider. To cover a range of mass differences between the top squark $\tilde{t}$ and lighter supersymmetric particles, four possible decay modes of the top squark are targeted with dedicated selections: the decay $\tilde{t} \rightarrow b \tilde{\chi}_{1}^{\pm}$ into a $b$-quark and the lightest chargino with $\tilde{\chi}_{1}^{\pm} \rightarrow W \tilde{\chi}_{1}^{0}$, the decay $\tilde{t} \rightarrow t \tilde{\chi}_{1}^{0}$ into an on-shell top quark and the lightest neutralino, the three-body decay $\tilde{t} \rightarrow b W \tilde{\chi}_{1}^{0}$ and the four-body decay $\tilde{t} \rightarrow b \ell \nu \tilde{\chi}_{1}^{0}$. 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 $\tilde{t}$ and $\tilde{\chi}_{1}^{0}$ masses. The results exclude at 95% confidence level $\tilde{t}$ masses up to about 720 GeV, extending the exclusion region of supersymmetric parameter space covered by previous searches.
Two-body selection background fit results for the CRs of the SRC$^{2-body}_{110}$ background fit. The nominal expectations from MC simulation are given for comparison for those backgrounds ($t\bar t$, $t\bar t Z$) that are normalised to data in dedicated CRs. The Others category contains the contributions from $t\bar t W$, $t\bar t h$, $t\bar t WW$, $t\bar t t$, $t\bar t t\bar t$, $Wh$, $ggh$ and $Zh$ production. Combined statistical and systematic uncertainties are given. Entries marked $--$ indicate a negligible background contribution. Uncertainties on the predicted background event yields are quoted as symmetric except where the negative uncertainty extends to zero predicted events, in which case the negative uncertainty is truncated.
Two-body selection distribution of $n_{jets}$ in CR$^{2-body}_{top}$ after the background fits. The contributions from all SM backgrounds are shown as a histogram stack; the bands represent the total statistical and detector-related systematic uncertainty. The rightmost bin of each plot includes overflow events.
Two-body selection distribution of $R_{2\ell 2j}$ in $CR^{2-body}_{VV-SF}$ after the background fits. The contributions from all SM backgrounds are shown as a histogram stack; the bands represent the total statistical and detector-related systematic uncertainty. The rightmost bin of each plot includes overflow events.
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.
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.
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 paper presents a measurement of the triple-differential cross section for the Drell--Yan process $Z/\gamma^*\rightarrow \ell^+\ell^-$ where $\ell$ is an electron or a muon. The measurement is performed for invariant masses of the lepton pairs, $m_{\ell\ell}$, between $46$ and $200$ GeV using a sample of $20.2$ fb$^{-1}$ of $pp$ collisions data at a centre-of-mass energy of $\sqrt{s}=8$ TeV collected by the ATLAS detector at the LHC in 2012. The data are presented in bins of invariant mass, absolute dilepton rapidity, $|y_{\ell\ell}|$, and the angular variable $\cos\theta^{*}$ between the outgoing lepton and the incoming quark in the Collins--Soper frame. The measurements are performed in the range $|y_{\ell\ell}|<2.4$ in the muon channel, and extended to $|y_{\ell\ell}|<3.6$ in the electron channel. The cross sections are used to determine the $Z$ boson forward-backward asymmetry as a function of $|y_{\ell\ell}|$ and $m_{\ell\ell}$. The measurements achieve high-precision, below the percent level in the pole region, excluding the uncertainty in the integrated luminosity, and are in agreement with predictions. These precision data are sensitive to the parton distribution functions and the effective weak mixing angle.
Detailed breakdown of systematic uncertainties for the measurement in the central rapidity muon channel. Common systematic uncertainty on the luminosity measurment of 1.8% is not included. Correlated systematic uncertainties with the suffix :A should be treated as additive and with the suffix :M should be treated as multiplicative. The source 'sys,uncor' represents bin-to-bin uncorrelated systematic uncertainty. The cross sections are given at the Born QED level. 'C Dressed' represents the multiplicative correction factor to translate the cross sections to the dressed level with the cone radius of 0.1: SigmaDressed = C Dressed * SigmaBorn.
Detailed breakdown of systematic uncertainties for the measurement in the central rapidity electron channel. Common systematic uncertainty on the luminosity measurment of 1.8% is not included. Correlated systematic uncertainties with the suffix :A should be treated as additive and with the suffix :M should be treated as multiplicative. The source 'sys,uncor' represents bin-to-bin uncorrelated systematic uncertainty. The cross sections are given at the Born QED level. 'C Dressed' represents the multiplicative correction factor to translate the cross sections to the dressed level with the cone radius of 0.1: SigmaDressed = C Dressed * SigmaBorn.
Detailed breakdown of systematic uncertainties for the measurement in the forward rapidity electron channel. Common systematic uncertainty on the luminosity measurment of 1.8% is not included. Correlated systematic uncertainties with the suffix :A should be treated as additive and with the suffix :M should be treated as multiplicative. The source 'sys,uncor' represents bin-to-bin uncorrelated systematic uncertainty. The cross sections are given at the Born QED level. 'C Dressed' represents the multiplicative correction factor to translate the cross sections to the dressed level with the cone radius of 0.1: SigmaDressed = C Dressed * SigmaBorn.
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 measurement of $b$-hadron pair production is presented, based on a data set corresponding to an integrated luminosity of 11.4 fb$^{-1}$ of proton--proton collisions recorded at $\sqrt{s}=8$ TeV with the ATLAS detector at the LHC. Events are selected in which a $b$-hadron is reconstructed in a decay channel containing $J/\psi \rightarrow \mu\mu$, and a second $b$-hadron is reconstructed in a decay channel containing a muon. Results are presented in a fiducial volume defined by kinematic requirements on three muons based on those used in the analysis. The fiducial cross section is measured to be $17.7 \pm 0.1 ($stat.$) \pm 2.0 ($syst.$)$ nb. A number of normalised differential cross sections are also measured, and compared to predictions from the Pythia8, Herwig++, MadGraph5\_aMC@NLO+Pythia8 and Sherpa event generators, providing new constraints on heavy flavour production.
Normalised differential cross sections and corresponding uncertainties in bins of $\Delta R(J/\psi,\mu)$.
Transfer functions for $\Delta R(b-hadron,b-hadron)$.
$B_{c}$ and $B + D$ corrections removed from the fitted data for $\Delta R(J/\psi,\mu)$.
A search is performed for resonant and non-resonant Higgs boson pair production in the $\gamma\gamma b\bar{b}$ final state. The data set used corresponds to an integrated luminosity of 36.1 fb$^{-1}$ of proton-proton collisions at a centre-of-mass energy of 13 TeV recorded by the ATLAS detector at the CERN Large Hadron Collider. No significant excess relative to the Standard Model expectation is observed. The observed limit on the non-resonant Higgs boson pair cross-section is 0.73 pb at 95% confidence level. This observed limit is equivalent to 22 times the predicted Standard Model cross-section. The Higgs boson self-coupling ($\kappa_\lambda = \lambda_{HHH} / \lambda_{HHH}^{\rm SM}$) is constrained at 95% confidence level to $-8.2 < \kappa_\lambda < 13.2$. For resonant Higgs boson pair production through X $\rightarrow$ HH $\rightarrow$ $\gamma\gamma b\bar{b}$, the limit is presented, using the narrow-width approximation, as a function of $m_X$ in the range 260 GeV $< m_X <$ 1000 GeV. The observed limits range from 1.1 pb to 0.12 pb over this mass range.
Number of data events for the 1-tag category with the loose selection in bins of diphoton mass.
Number of data events for the 2-tag category with the loose selection in bins of diphoton mass.
Number of data events for the 1-tag category with the tight selection in bins of diphoton mass.
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