Measurement of the 3-jet mass cross section in pp collisions at 7 TeV and determination of the strong coupling constant from 3-jet masses in the TeV range

This study presents a measurement of the double-differential cross section of 3-jet events at a center-of-mass energy of $\sqrt{s}=7\TeV$, using data corresponding to an integrated luminosity of $5\,\mathrm{fb}^{-1}$ collected with the CMS detector in 2011\@. The cross section is measured as a function of the invariant mass and maximum rapidity of the 3-jet system and is unfolded for detector effects. A comparison between the measurement and the prediction from perturbative QCD at next-to-leading order is performed. Within uncertainties, data and theory are in agreement. The sensitivity of the observable to parameters of the theory like the parton distribution functions of the proton and the strong coupling $\alpha_S$ is studied. A fit of all data points at central rapidity gives the value of the strong coupling at the scale of the $Z$ boson mass to be $\alpha_S(M_Z) = 0.1160 \pm_{0.0023}^{0.0025}\,(\mathrm{exp,PDF,NP})\pm_{0.0021}^{0.0068}\,(\mathrm{scale})$.