  The differential cross section for the process $Z/\gamma^*\rightarrow ll$
($l=e,\mu$) as a function of dilepton invariant mass is measured in pp
collisions at $\sqrt{s}=$ 7 TeV at the LHC using the ATLAS detector. The
measurement is performed in the $e$ and $\mu$ channels for invariant masses
between 26 GeV and 66 GeV using an integrated luminosity of 1.6 fb$^{-1}$
collected in 2011 and these measurements are combined. The analysis is extended
to invariant masses as low as 12 GeV in the muon channel using 35 pb$^{-1}$ of
data collected in 2010. The cross sections are determined within fiducial
acceptance regions and corrections to extrapolate the measurements to the full
kinematic range are provided. Next-to-next-to-leading-order QCD predictions
provide a significantly better description of the results than
next-to-leading-order QCD calculations, unless the latter are matched to a
parton shower calculation.
