304 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

The total cross section and differential cross sections for the production of $$B_c^± $$ mesons, times their branching fraction to J/Ψπ^±, re measured relative to those for the production of $B^±$ mesons, times their branching fraction to J/Ψπ^±. The data used for this study correspond to an integrated luminosity of 20.3 fb^-1 of pp collisions recorded by the ATLAS detector at the Large Hadron Collider in 2012 at a center-of-mass energy of $$\sqrt{s}$$ = 8 TeV. The measurement is performed differentially in bins of transverse momentum p_T for 13 GeV $$\langle$$ p_T ($$B_c^± $$) $$\langle$$ 22 GeV and p_T ($$B_c^±more » $$) $$\rangle$$ 22 GeV and in bins of rapidity y for |y| $$\langle$$ 0.75 and 0.75 $$\langle$$ | y | $$\langle$$ 2.3. The relative cross section times branching fraction for the full range p_T $$\langle$$ 13 GeV and |y| $$\langle$$ 2.3 is (0.34 ± 0.0 4_stat $$^{0.06}_{0.02sys}$$ ± 0.01_lifetime)%. The differential measurements suggest that the production cross section of the $$B_c^± $$ decreases faster with p_T than the production cross section of the $B^±$, while no significant dependence on rapidity is observed.« less

One correction is noted for the paper. A wrong cross-section was used for the theory prediction in figure 6 due to not taking into account the VHH contamination properly in the rescaling formula for the signal samples. The change in the theory prediction leads to stricter exclusion limits on the di-vector-boson–di-Higgs-boson coupling modifier κ_2V. The observed excluded region corresponds to κ_2V < –0.43 and κ_2V > 2.56, while the expected exclusion is κ_2V < –0.55 and κ_2V > 2.72. Erratum JHEP 01 (2021) 145 fixed the description in the text but contains the uncorrected figure. The correct figure is shownmore » in the new erratum.« less

A measurement of the $$B_{s}^{0} \rightarrow J/\psi \phi $$ decay parameters using 80.5 fb^-1 of integrated luminosity collected with the ATLAS detector from 13 TeV proton–proton collisions at the LHC is presented. The measured parameters include the CP-violating phase ϕ_s, the width difference ΔΓ_s between the $$B_{s}^{0}$$ meson mass eigenstates and the average decay width Γ_s. The values measured for the physical parameters are combined with those from 19.2 fb^-1 of 7 TeV and 8 TeV data, leading to the following: ϕ_s = -0.087 ± 0.036 (stat.) ± 0.021 (syst.) rad ΔΓ_s = 0.0657 ± 0.0043 (stat.) ± 0.0037 (syst.)more » ps^-11 Γ_s = 0.6703 ± 0.0014 (stat.) ± 0.0018 (syst.) ps^-1 Results for ϕ_s and ΔΓ_s are also presented as 68% confidence level contours in the ϕ_s–ΔΓ_s plane. Furthermore the transversity amplitudes and corresponding strong phases are measured. ϕ_s and ΔΓ_s measurements are in agreement with the Standard Model predictions.« less

One correction is noted for the paper. A wrong cross-section was used for the theory prediction in figure 6 due to not taking into account the VHH contamination properly in the rescaling formula for the signal samples.

The integrated fiducial cross-section and unfolded differential jet mass spectrum of high transverse momentum $$Z\rightarrow b\overline{b}$$ decays are measured in $$Z\gamma$$ events in proton-proton collisions at $$\sqrt{s} = 13$$ TeV. The data analysed were collected between 2015 and 2016 with the ATLAS detector at the Large Hadron Collider and correspond to an integrated luminosity of 36.1 fb$$^{-1}$$. Photons are required to have a transverse momentum $$p_{\mathrm{T}}>175$$ GeV. The $$Z\rightarrow b\overline{b}$$ decay is reconstructed using a jet with $$p_{\mathrm{T}} > 200$$ GeV, found with the anti-$$k_{t}$$ $$R$$ = 1.0 jet algorithm, and groomed to remove soft and wide-angle radiation and tomore » mitigate contributions from the underlying event and additional proton-proton collisions. Two different but related measurements are performed using two jet grooming definitions for reconstructing the $$Z\rightarrow b\overline{b}$$ decay: trimming and soft drop. These algorithms differ in their experimental and phenomenological implications regarding jet mass reconstruction and theoretical precision. To identify $$Z$$ bosons, $$b$$-tagged $$R$$ = 0.2 track-jets matched to the groomed large-$$R$$ calorimeter jet are used as a proxy for the $$b$$-quarks. The signal yield is determined from fits of background templates extracted from the data to the different jet mass distributions for the two grooming methods. Integrated fiducial cross-sections and unfolded jet mass spectra for each grooming method are compared with leading-order theoretical predictions. The results are found to be in good agreement with Standard Model expectations within the current statistical and systematic uncertainties.« less

The jet energy scale, jet energy resolution, and their systematic uncertainties are measured for jets reconstructed with the ATLAS detector in 2012 using proton–proton data produced at a centre-of-mass energy of 8 TeV with an integrated luminosity of 20fb^-1. Jets are reconstructed from clusters of energy depositions in the ATLAS calorimeters using the anti-k_t algorithm. A jet calibration scheme is applied in multiple steps, each addressing specific effects including mitigation of contributions from additional proton–proton collisions, loss of energy in dead material, calorimeter non-compensation, angular biases and other global jet effects. The final calibration step uses several in situ techniquesmore » and corrects for residual effects not captured by the initial calibration. Furthermore, these analyses measure both the jet energy scale and resolution by exploiting the transverse momentum balance in γ + jet, Z + jet, dijet, and multijet events. A statistical combination of these measurements is performed. In the central detector region, the derived calibration has a precision better than 1% for jets with transverse momentum 150 GeV < p_T < 1500 GeV, and the relative energy resolution is (8.4 ± 0.6)% for p_T=100 GeV and (23 ± 2)% for p_T=20 GeV. The calibration scheme for jets with radius parameter R = 1.0, for which jets receive a dedicated calibration of the jet mass, is also discussed.« less

Corrections for five figures and six tables are noted for the paper, which do not affect the conclusions reported.