Abstract: A measurement is presented of the t?t production cross section ?t?t in proton-proton collisions at a centre-of-mass energy of 7 TeV, in the all-jet final state that contains at least six jets, two of which are tagged as originating from b quarks. The data correspond to an integrated luminosity of 3.54 fb-1, collected with the CMS detector at the LHC. The cross section is determined through an unbinned maximum likelihood fit of background and t?t signal to the reconstructed mass spectrum of t?t candidates in the data, in which events are subjected to a kinematic fit assuming a t?t ? W+bW-?b ? 6 jets hypothesis. The measurement yields ?t?t = 139±10(stat.)±26(syst.)±3(lum.)pb, a result consistent with those obtained in other t?t decay channels, as well as with predictions of the standard model.

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