  The dijet double-differential cross section is measured as a function of the
dijet invariant mass, using data taken during 2010 and during 2011 with the
ATLAS experiment at the LHC, with a center-of-mass energy of 7 TeV. The
measurements are sensitive to invariant masses between 70 GeV and 4.27 TeV with
center-of-mass jet rapidities up to 3.5. A novel technique to correct jets for
pile-up (additional proton-proton collisions) in the 2011 data is developed and
subsequently used in the measurement. The data are found to be consistent with
fixed-order NLO pQCD predictions provided by NLOJET++. The results constitute a
stringent test of pQCD, in an energy regime previously unexplored. The dijet
analysis is a confidence building step for the extraction of the signal of hard
double parton scattering (DPS) in four-jet events, and subsequent extraction of
the effective overlap area between the interacting protons, expressed in terms
of the variable, \sigma(eff). The measurement of DPS is performed using the
2010 ATLAS data. The rate of DPS events is estimated using a neural network. A
clear signal is observed, under the assumption that the DPS signal can be
represented by a random combination of exclusive dijet production. The fraction
of DPS candidate events is determined to be f(DPS) = 0.081 +- 0.004 (stat.)
+0.025-0.014 (syst.) in the analyzed phase-space of four-jet topologies.
Combined with the measurement of the dijet and four-jet cross sections in the
appropriate phase-space regions, the effective cross section is found to be
\sigma(eff) = 16.0 +0.5-0.8 (stat.) +1.9-3.5 (syst.) mb. This result is
consistent within the quoted uncertainties with previous measurements of
\sigma(eff) at center-of-mass energies between 63 GeV and 7 TeV, using several
final states.
