  This paper presents a measurement of the inclusive 3-jet production
differential cross section at a proton-proton centre-of-mass energy of 7 TeV
using data corresponding to an integrated luminosity of 5 inverse femtobarns
collected with the CMS detector. The analysis is based on the three jets with
the highest transverse momenta. The cross section is measured as a function of
the invariant mass of the three jets in a range of 445-3270 GeV and in two bins
of the maximum rapidity of the jets up to a value of 2. 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 the strong coupling constant alpha[S] is
studied. A fit to all data points with 3-jet masses larger than 664 GeV gives a
value of the strong coupling constant of alpha[S](MZ) = 0.1171 +/- 0.0013 (exp)
+0.0073/-0.0047 (theo).

High Energy Physics - Experiment