Measurement of the 3-jet to 2-jet Cross Section Ratio in pp Collisions at sqrt s = 7 TeV

The ratio of the inclusive 3-jet to 2-jet cross sections as a function of the total jet transverse momentum in the range $0.2 {\rm ~TeV} < H_{\rm T}< 1 {\rm ~TeV}$ has been measured with the CMS detector at the LHC for proton-proton collisions at a center-of-mass energy of $\sqrt{s}=7\,\mathrm{TeV}$ using an integrated luminosity of $76\ensuremath{\,\text{nb}^{-1}}$. The jets are selected to have transverse momentum greater than 50 GeV and to be in the rapidity range $\vert y \vert < 2.5$. The measured ratio is in agreement with predictions of QCD-based Monte Carlo calculations.