1. Measurement of the 3-jet to 2-jet Cross Sections Ratio in pp Collisions at 7TeV Update : Pythia8 tune 2C P.Kokkas, I.Papadopoulos, C.Fountas University of Ioannina, Greece High p T meeting 25 Feb 2011

2. Final Result P.Kokkas, Univ. of Ioannina2 The measured ratio rises with H T as the phase space opens up for a third jet to be emitted, as a result of the 50 GeV p T selection cut. The ratio R 32 reaches a plateau of ≈0.8 which is most sensitive to α s, although its exact value depends upon p T and y cuts as well as the jet finding algorithm. The error bars represent the statistical uncertainty. The yellow band indicates the size of the combined systematic error. Shown also in the same plot, are the predictions of the six MC generators considered in this analysis.

3. Final Result P.Kokkas, Univ. of Ioannina3 The figure on the left shows the ratio of MC R 32 over Data R 32. The yellow band represents the combined statistical and systematic uncertainty of the data. The predictions of Madgraph agree with the data throughout the H T range of the measurement. The prediction of Alpgen is consistently lower than those of Madgraph and underestimate the measurement for H T <1.5 TeV. The predictions of Pythia6, Pythia8 and Herwig++ are in agreement with the measurement for H T >0.5 TeV, but they overestimate the data for lower values of H T. In the kinematic range of this measurement Madgraph describes best the R 32 data.