Abstract
The inclusive cross section for top-quark pair production measured by the CMS experiment in proton-proton collisions at a center-of-mass energy of 7 TeV is compared to the QCD prediction at next-to-next-to-leading order with various parton distribution functions to determine the top-quark pole mass, mtpole, or the strong coupling constant, alphaS. With the parton distribution function set NNPDF2.3, a pole mass of 176.7 +3.0 -2.8 GeV is obtained when constraining alphaS at the scale of the Z boson mass, mZ, to the current world average. Alternatively, by constraining mtpole to the latest average from direct mass measurements, a value of alphaS(mZ) = 0.1151 +0.0028 -0.0027 is extracted. This is the first determination of alphaS using events from top-quark production.
Highlights
The Large Hadron Collider (LHC) has provided a wealth of proton–proton collisions, which has enabled the Compact Muon Solenoid (CMS) experiment [1] to measure cross sections for the production of top-quark pairs with high precision employing a variety of approaches [2,3,4,5,6,7,8,9,10]
These values are extracted under the assumption that the mt parameter in the Monte Carlo generator that was employed to obtain the mass-dependent acceptance correction of the measured cross section, shown in Fig. 1, is equal to the pole mass
A difference of 1.0 GeV between the two mass definitions [20] would result in changes of 0.3–0.6 GeV in the extracted pole masses, which is included as a systematic uncertainty
Summary
Since ttproduction at LHC energies is expected to occur predominantly via gluon–gluon fusion. The sensitivity of σttto mt might not be strong enough to make this approach competitive in precision, it yields results affected by different sources of systematic uncertainties compared to the direct mt measurements and allows for extractions of mt in theoretically well-defined mass schemes. The current world average for αS (mZ) is 0.1184 ± 0.0007 [26] In spite of this relatively precise result, the uncertainty on αS still contributes significantly to many QCD predictions, including expected cross sections for top-quark pairs or Higgs bosons. To account for the possible difference between the pole mass and the Monte Carlo generator mass [20], an additional uncertainty, assumed to be 1.00 GeV, is added in quadrature to the experimental uncertainty, resulting in a total uncertainty on the top-quark mass constraint, δmpt ole, of 1.4 GeV. The potential αS dependence of the direct mt measurements has not been explicitly evaluated, it is assumed to be covered by the quoted mass uncertainty
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