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Top Quark Physics

In the first years of operation, the LHC has proven to be a true top quark factory and top quark measurements are challenging the precision of theoretical calculations. The DESY CMS group is engaged in measurements of top quark pair cross sections (inclusive and differential), in the study of top quark pair events with additional particles in the final state, in top quark mass measurements and other top quark property measurements, like top quark polarization and spin correlation, as well as in the extraction of QCD parameters from the measurements.

The DESY CMS group has performed the first measurement of differential top quark pair cross sections in the dilepton channel, in which the top quarks decay into a final state with two isolated leptons (muons or electrons) of opposite charge and two jets. Moreover, the DESY CMS group was leading the first measurement of the inclusive top quark pair cross section at the current energy frontier of 13 TeV. A summary of these and other cross section measurements as a function of the center-of-mass energy as well as a dilepton candidate event are shown at the bottom of the page.

The top-quark mass (mt) is a crucial parameter of the SM. A precise measurement of this quantity is one of the most important inputs to global electroweak fits that provide constraints on the properties of the Higgs boson, as well as on models for physics beyond the SM. In higher-order QCD calculations, the mass of the top quark depends on the renormalization scheme and its value can differ considerably, e.g. for pole-mass or msbar-mass definitions. Therefore, it is important to understand how to interpret the experimental result in terms of the renormalization conventions. Direct measurements of mt rely on the reconstruction of kinematic observables sensitive to mt. These measurements depend highly on the detailed description of the corresponding signal and top-quark mass mtMC in Monte Carlo (MC) simulations. Alternatively, the top-quark mass can be derived indirectly from the cross section measurement. The group at DESY used the inclusive top-quark pair production cross section measured by CMS in the dilepton channel to extract the top-quark mass through its comparison to three different fully inclusive calculations at higher-order perturbative QCD, which provide an unambiguous definition of mt. The extraction provides an important test of the mass scheme as applied in MC simulations and gives complementary information, with different sensitivity to theoretical and experimental uncertainties than direct measurements of the top-quark mass mtMC, which rely on the kinematic details of the mass reconstruction. The results, both for the pole-mass and the mass definitions, are in very good agreement with similar measurements at the Tevatron and from ATLAS, and provide, for the first time, the determination of the top quark mass at the LHC.

[1] S. Chatrchyan et al. [CMS Collaboration], Measurement of differential top-quark pair production cross sections in pp colisions at sqrt(s)=7 TeV, Eur. Phys. J. C 73 (2013) 2339, arXiv:1211.2220
[2] S. Chatrchyan et al. [CMS Collaboration], Determination of the top-quark pole mass and strong coupling constant from the top quark pair production cross section in pp collisions at sqrt(s) = 7 TeV, PLB 728 (2014) 496, arXiv:1307.1907
[3] S. Chatrchyan et al. [CMS Collaboration], Measurement of the top quark pair production cross section in the dilepton channel in pp collisions at sqrt(s) = 8 TeV, JHEP 1402 (2014) 024, arXiv:1312.7582
[4] S. Chatrchyan et al. [CMS Collaboration], Measurement of jet multiplicity distributions in top quark pair production in pp collisions at sqrt(s) = 7 TeV, arXiv:1404.3171
[5] V. Khachatryan et al. [CMS Collaboration], Measurement of the top quark pair production cross section in proton-proton collisions at sqrt(s) = 13 TeV, arXiv:1510.05302

Top pair candidate event in the electron muon channel at 13 TeV.

Top quark pair cross section from 2 TeV to 13 TeV.

Last updated on 23-01-2018


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