Measurements of ${\alpha }_s$ in $pp$ Collisions at the LHC

Abstract

The coupling of the strong force, ${\alpha }_s$, is deemed to be a fundamental parameter of Nature, and, beyond the quark masses, constitutes the only free parameter in the QCD Lagrangian. Provided is an overview of CERN Large Hadron Collider (LHC) measurements of ${\alpha }_s(M_Z)$ evaluated at the $Z$-boson mass and of the running of ${\alpha }_s(Q)$ as a function of energy-momentum transfer $Q$. The measurements were performed by the ATLAS and CMS Collaborations using proton-proton ($pp$) collisions with centre-of-mass energies of 7 TeV and data samples with time-integrated luminosities up to 5 fb${}^{-1}$. Four different categories of observable were used in the described extractions of ${\alpha }_s$: inclusive jet cross sections, 3-jet to 2-jet inclusive cross-section ratios, 3-jet mass cross sections, and top-quark pair production cross sections. These results, which include the first NNLO measurement of ${\alpha }_s$ at a hadron collider and the first determinations of ${\alpha }_s$ at energy scales above 1 TeV, are consistent with each other, with the world-average value, and with QCD predictions of their running with $Q$.

arXiv commentInvited talk presented at CIPANP2015 (Twelfth Conference on the Intersections of Particle and Nuclear Physics, Vail, Colorado, USA, 19 - 24 May 2015); 13 pages, LaTeX, 13 PDF figures

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