  A likelihood-based discriminant for the identification of quark- and
gluon-initiated jets is built and validated using 4.7 fb$^{-1}$ of
proton-proton collision data at $\sqrt{s}$ = 7 TeV collected with the ATLAS
detector at the LHC. Data samples with enriched quark or gluon content are used
in the construction and validation of templates of jet properties that are the
input to the likelihood-based discriminant. The discriminating power of the jet
tagger is established in both data and Monte Carlo samples within a systematic
uncertainty of 10-20%. In data, light-quark jets can be tagged with an
efficiency of 50% while achieving a gluon-jet mis-tag rate of 25% in a $p_T$
range between 40 GeV and 360 GeV for jets in the acceptance of the tracker. The
rejection of gluon-jets found in the data is significantly below what is
attainable using a Pythia 6 Monte Carlo simulation, where gluon-jet mis-tag
rates of 10% can be reached for a 50% selection efficiency of light-quark jets
using the same jet properties.
