9 Search Results

A measurement of the top quark pair-production cross section in the lepton + jets decay channel is presented. It is based on 4.6 fb^-1of $$\sqrt{s}$$ = 7 TeV pp collision data collected during 2011 by the ATLAS experiment at the CERN Large Hadron Collider. A three-class, multidimensional event classifier based on support vector machines is used to differentiate $$t\bar{t}$$ events from backgrounds. The $$t\bar{t}$$ production cross section is found to be σ_$$t\bar{t}$$ = 168.5 ± 0.7(stat)$$^{+6.2}_{-5.9}$$(syst)$$^{+ 3.4}_{-3.2}$$(lumi) pb. In conclusion, the result is consistent with the Standard Model prediction based on QCD calculations at next-to-next-to-leading order.

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 μb^-1 and 8.4 nb^-1, respectively. The BEC are measured for pairs of like-sign charged particles, each with |η|<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 investigatedmore » 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.« less

The production cross-section of a top quark in association with a W boson is measured using proton–proton collisions at √s = 8TeV. The dataset corresponds to an integrated luminosity of 20.2 fb^–1, and was collected in 2012 by the ATLAS detector at the Large Hadron Collider at CERN. The analysis is performed in the single-lepton channel. Events are selected by requiring one isolated lepton (electron or muon) and at least three jets. A neural network is trained to separate the tW signal from the dominant tt¯ background. The cross-section is extracted from a binned profile maximum-likelihood fit to a two-dimensionalmore » discriminant built from the neural-network output and the invariant mass of the hadronically decaying W boson. The measured cross-section is σ_tW = 26 ± 7pb, in good agreement with the Standard Model expectation.« less

A measurement of observables sensitive to spin correlations in $$t$$$\overline{t}$$ production is presented, using 36.1 fb^-1 of pp collision data at s√=13 TeV recorded with the ATLAS detector at the Large Hadron Collider. Differential cross-sections are measured in events with exactly one electron and one muon with opposite-sign electric charge as a function of the azimuthal opening angle and the absolute difference in pseudorapidity between the electron and muon candidates in the laboratory frame. The azimuthal opening angle is also measured as a function of the invariant mass of the $$t$$$\overline{t}$$ system. The measured differential cross-sections are compared to predictionsmore » by several NLO Monte Carlo generators and fixed-order calculations. The observed degree of spin correlation is somewhat higher than predicted by the generators used. The data are consistent with the prediction of one of the fixed-order calculations at NLO, but agree less well with higher-order predictions. Using these leptonic observables, a search is performed for pair production of supersymmetric top squarks decaying into Standard Model top quarks and light neutralinos. Top squark masses between 170 and 230 GeV are largely excluded at the 95% confidence level for kinematically allowed values of the neutralino mass.« less

The combination of measurements of the W boson polarization in top quark decays performed by the ATLAS and CMS collaborations is presented. The measurements are based on proton-proton collision data produced at the LHC at a centre-of-mass energy of 8 TeV, and corresponding to an integrated luminosity of about 20 fb$$^{−1}$$ for each experiment. The measurements used events containing one lepton and having different jet multiplicities in the final state. The results are quoted as fractions of W bosons with longitudinal (F$$_{0}$$), left-handed (F$$_{L}$$), or right-handed (F$$_{R}$$) polarizations. The resulting combined measurements of the polarization fractions are F$$_{0}$$ = 0.693more » ± 0.014 and F$$_{L}$$ = 0.315 ± 0.011. The fraction F$$_{R}$$ is calculated from the unitarity constraint to be F$$_{R}$$ = −0.008 ± 0.007. These results are in agreement with the standard model predictions at next-to-next-to-leading order in perturbative quantum chromodynamics and represent an improvement in precision of 25 (29)% for F$$_{0}$$ (F$$_{L}$$) with respect to the most precise single measurement. A limit on anomalous right-handed vector (V$$_{R}$$), and left- and right-handed tensor (g$$_{L}$$, g$$_{R}$$) tWb couplings is set while fixing all others to their standard model values. The allowed regions are [−0.11, 0.16] for V$$_{R}$$, [−0.08, 0.05] for g$$_{L}$$, and [−0.04, 0.02] for g$$_{R}$$, at 95% confidence level. Limits on the corresponding Wilson coefficients are also derived.[graphic not available: see fulltext]« less

The results of a search for electroweakino pair production $$pp \rightarrow \tilde\chi^\pm_1 \tilde\chi^0_2$$ in which the chargino ($$\tilde\chi^\pm_1$$) decays into a $$W$$ boson and the lightest neutralino ($$\tilde\chi^0_1$$), while the heavier neutralino ($$\tilde\chi^0_2$$) decays into the Standard Model 125 GeV Higgs boson and a second $$\tilde\chi^0_1$$ are presented. The signal selection requires a pair of $$b$$-tagged jets consistent with those from a Higgs boson decay, and either an electron or a muon from the $$W$$ boson decay, together with missing transverse momentum from the corresponding neutrino and the stable neutralinos. The analysis is based on data corresponding to 139 $$\mathrm{fb}^{-1}$$more » of $$\sqrt{s}=13$$ TeV $pp$ collisions provided by the Large Hadron Collider and recorded by the ATLAS detector. No statistically significant evidence of an excess of events above the Standard Model expectation is found. Limits are set on the direct production of the electroweakinos in simplified models, assuming pure wino cross-sections. Masses of $$\tilde{\chi}^{\pm}_{1}/\tilde{\chi}^{0}_{2}$$ up to 740 GeV are excluded at 95% confidence level for a massless $$\tilde{\chi}^{0}_{1}$$.« less

This paper presents the combinations of single-top-quark production cross-section measurements by the ATLAS and CMS Collaborations, using data from LHC proton-proton collisions at $$ \sqrt{s} $$ = 7 and 8 TeV corresponding to integrated luminosities of 1.17 to 5.1 fb$$^{−1}$$ at $$ \sqrt{s} $$ = 7 TeV and 12.2 to 20.3 fb$$^{−1}$$ at $$ \sqrt{s} $$ = 8 TeV. These combinations are performed per centre-of-mass energy and for each production mode: t-channel, tW, and s-channel. The combined t-channel cross-sections are 67.5 ± 5.7 pb and 87.7 ± 5.8 pb at $$ \sqrt{s} $$ = 7 and 8 TeV respectively. Themore » combined tW cross-sections are 16.3 ± 4.1 pb and 23.1 ± 3.6 pb at $$ \sqrt{s} $$ = 7 and 8 TeV respectively. For the s-channel cross-section, the combination yields 4.9 ± 1.4 pb at $$ \sqrt{s} $$ = 8 TeV. The square of the magnitude of the CKM matrix element V$$_{tb}$$ multiplied by a form factor f$$_{LV}$$ is determined for each production mode and centre-of-mass energy, using the ratio of the measured cross-section to its theoretical prediction. It is assumed that the top-quark-related CKM matrix elements obey the relation |V$$_{td}$$|, |V$$_{ts}$$| ≪ |V$$_{tb}$$|. All the |f$$_{LV}$$V$$_{tb}$$|$$^{2}$$ determinations, extracted from individual ratios at $$ \sqrt{s} $$ = 7 and 8 TeV, are combined, resulting in |f$$_{LV}$$V$$_{tb}$$| = 1.02 ± 0.04 (meas.) ± 0.02 (theo.). All combined measurements are consistent with their corresponding Standard Model predictions.« less

This paper presents combinations of inclusive and differential measurements of the charge asymmetry (A$$_{C}$$) in top quark pair $$ \left(\mathrm{t}\overline{\mathrm{t}}\right) $$ events with a lepton+jets signature by the ATLAS and CMS Collaborations, using data from LHC proton-proton collisions at centre-of-mass energies of 7 and 8 TeV. The data correspond to integrated luminosities of about 5 and 20 fb$$^{−1}$$ for each experiment, respectively. The resulting combined LHC measurements of the inclusive charge asymmetry are A$$_{C}^{CHC7}$$  = 0.005 ± 0.007 (stat) ± 0.006(syst) at 7 TeV and A$$_{C}^{CHC8}$$  = 0.0055 ± 0.0023 (stat) ± 0.0025 (syst) at 8 TeV. These values, as well as the combination of A$$_{C}$$ measurements as a function of the invariantmore » mass of the $$ \mathrm{t}\overline{\mathrm{t}} $$ system at 8 TeV, are consistent with the respective standard model predictions.« less

Combined ATLAS and CMS measurements of the Higgs boson production and decay rates, as well as constraints on its couplings to vector bosons and fermions, are presented. The combination is based on the analysis of five production processes, namely gluon fusion, vector boson fusion, and associated production with a W or a Z boson or a pair of top quarks, and of the six decay modes H → ZZ, W W , γγ, ττ, bb, and μμ. All results are reported assuming a value of 125.09 GeV for the Higgs boson mass, the result of the combined measurement by themore » ATLAS and CMS experiments. The analysis uses the CERN LHC proton-proton collision data recorded by the ATLAS and CMS experiments in 2011 and 2012, corresponding to integrated luminosities per experiment of approximately 5 fb$$^{−1}$$ at $$ \sqrt{s}=7 $$ TeV and 20 fb$$^{−1}$$ at $$ \sqrt{s}=8 $$ TeV. The Higgs boson production and decay rates measured by the two experiments are combined within the context of three generic parameterisations: two based on cross sections and branching fractions, and one on ratios of coupling modifiers. Several interpretations of the measurements with more model-dependent parameterisations are also given. The combined signal yield relative to the Standard Model prediction is measured to be 1.09 ± 0.11. The combined measurements lead to observed significances for the vector boson fusion production process and for the H → ττ decay of 5.4 and 5.5 standard deviations, respectively. The data are consistent with the Standard Model predictions for all parameterisations considered.« less