2,430 research outputs found

    Measurements of the inclusive ttˉ\rm t\bar{t} production cross section at the ATLAS and CMS experiments

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    The results of the most recent measurements of the inclusive ttˉ\mathrm{t\bar{t}} cross section performed by the ATLAS and CMS experiments at the CERN LHC are summarized. These include results obtained in proton-proton collisions at different centre-of-mass energies, and the first observation of ttˉ\mathrm{t\bar{t}} production in proton-lead collisions. A new result at s=13 TeV\sqrt{s} = 13~\mathrm{TeV} by the CMS Collaboration is presented for the first time at this conference. In this analysis, the total cross section is determined both for a fixed top quark mass value in the simulation of mtMC=172.5 GeVm_{\mathrm{t}}^{\mathrm{MC}} = 172.5~\mathrm{GeV}, and simultaneously with mtMCm_{\mathrm{t}}^{\mathrm{MC}}.Comment: Proceedings for 11th International Conference on Top Quark Physic

    Measuring standard model parameters using top-quark cross sections in ATLAS and CMS

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    Theoretical predictions of top-quark cross sections depend on the values of the parameters of the quantum chromodynamics Lagrangian, such as the strong coupling constant and the mass of the top quark, but also on parameters of the electroweak sector of the standard model and parton distribution functions. Comparisons between state-of-the-art calculations and recent measurements at the ATLAS and CMS experiments at the CERN LHC allow for the precise determination of these parameters. In this proceeding, the most recent results by the two collaborations are summarized and discussed.Comment: Presented at the 13th International Workshop on Top Quark Physics, Durham UK (videoconference), 14-18 September 202

    Probing QCD using top quark pair production at s=13 TeV\sqrt{s} = 13~\mathrm{TeV} in CMS

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    Measurements of the top quark-antiquark pair production cross section, σttˉ\sigma_\mathrm{t\bar{t}}, can be used to constrain the strong coupling constant, αS\alpha_{S}, the top quark mass, mtm_\mathrm{t}, and the parton distribution functions (PDFs). In this poster, the two most recent relevant results published by the CMS Collaboration are presented. The analyses are performed using proton-proton collision data at a centre-of-mass energy of 13 TeV recorded by the CMS detector at the CERN LHC in 2016, corresponding to an integrated luminosity of 35.9 fb135.9~\mathrm{fb^{-1}}. In the first one, αS\alpha_{S} and mtm_\mathrm{t} are extracted independently from a measurement of the inclusive σttˉ\sigma_\mathrm{t\bar{t}}, using next-to-next-to-leading order theoretical predictions. In the second, a measurement of the normalized triple-differential ttˉ\mathrm{t\bar{t}} cross section is performed; the result is then used together with HERA deep-inelastic scattering data to perform a simultaneous determination of αS\alpha_{S}, mtm_\mathrm{t}, and the PDFs, at next-to-leading order. As a result, the uncertainty in the gluon PDF and its correlation with αS\alpha_{S} are significantly reduced at high parton momentum fraction, the kinematic range probed by ttˉ\mathrm{t\bar{t}} production. The result also yields the most precise determination of the top quark pole mass, to date.Comment: To appear in the Proceedings of the European Physical Society Conference on High Energy Physics (EPS-HEP 2019), Ghent, Belgium, 10-17 July 201

    First experimental investigation of the running of the top quark mass

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    The first experimental investigation of the running of the top quark mass is presented. The running, defined in the modified minimal subtraction (MS\mathrm{\overline{MS}}) renormalization scheme, is extracted from a measurement of the differential top quark-antiquark (ttˉ\mathrm{t\bar{t}}) production cross section as a function of the invariant mass of the ttˉ\mathrm{t\bar{t}} system using theoretical predictions at next-to-leading order. The measurement is performed using proton-proton collisions at a centre-of-mass energy of 13 TeV13~\mathrm{TeV} recorded by the CMS detector at the CERN LHC in 2016, corresponding to an integrated luminosity of 35.9 fb135.9~\mathrm{fb^{-1}}. Candidate ttˉ\mathrm{t\bar{t}} events are selected in the final state with an electron and a muon of opposite charge, and the differential cross section is determined at the parton level by means of a maximum-likelihood fit to multidifferential distributions. The extracted running is found to be compatible with the solution of the corresponding renormalization group equation, up to a scale on the order of 1 TeV1~\mathrm{TeV}

    Constraints on the χ_(c1) versus χ_(c2) polarizations in proton-proton collisions at √s = 8 TeV

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    The polarizations of promptly produced χ_(c1) and χ_(c2) mesons are studied using data collected by the CMS experiment at the LHC, in proton-proton collisions at √s=8  TeV. The χ_c states are reconstructed via their radiative decays χ_c → J/ψγ, with the photons being measured through conversions to e⁺e⁻, which allows the two states to be well resolved. The polarizations are measured in the helicity frame, through the analysis of the χ_(c2) to χ_(c1) yield ratio as a function of the polar or azimuthal angle of the positive muon emitted in the J/ψ → μ⁺μ⁻ decay, in three bins of J/ψ transverse momentum. While no differences are seen between the two states in terms of azimuthal decay angle distributions, they are observed to have significantly different polar anisotropies. The measurement favors a scenario where at least one of the two states is strongly polarized along the helicity quantization axis, in agreement with nonrelativistic quantum chromodynamics predictions. This is the first measurement of significantly polarized quarkonia produced at high transverse momentum