  An azimuthal angle correlation between the two hardest jets is studied in the
ttbar production process at the 14 TeV LHC. The event samples are generated by
merging the tree level matrix elements for the ttbar plus up to 2 or 3 partons
with parton showers. The generated event samples show a strong correlation in
the azimuthal angle difference between the two hardest jets, as predicted in
the analysis based on the tree level matrix elements for the ttbar+2 partons.
The effects of merging the matrix elements for the ttbar+3 partons on the
correlation are studied in detail. It is found that they play important roles
in improving the prediction of the correlation.
