  Additional jet activity in dijet events is measured using $pp$ collisions at
ATLAS at a centre-of-mass energy of 7 TeV, for jets reconstructed using the
anti-kt algorithm with radius parameter R=0.6. This is done using variables
such as the fraction of dijet events without an additional jet in the rapidity
interval bounded by the dijet subsystem and correlations between the azimuthal
angles of the dijets. They are presented, both with and without a veto on
additional jet activity in the rapidity interval, as a function of the mean
transverse momentum of the dijets and of the rapidity interval size. The double
differential dijet cross section is also measured as a function of the interval
size and the azimuthal angle between the dijets. These variables probe
differences in the approach to resummation of large logarithms when performing
QCD calculations. The data are compared to POWHEG, interfaced to the PYTHIA 8
and HERWIG parton shower generators, as well as to HEJ with and without
interfacing it to the ARIADNE parton shower generator. None of the theoretical
predictions agree with the data across the full phase-space considered;
however, POWHEG+PYTHIA 8 and HEJ+ARIADNE are found to provide the best
agreement with the data.These measurements use the full data sample collected
with the ATLAS detector in 7 TeV $pp$ collisions at the LHC and correspond to
integrated luminosities of 36.1 pb$^-1$ and 4.5 fb$^-1$ for data collected
during 2010 and 2011 respectively.
