The low mass background is the result of single photons being artifically split by the detector (specifically the SMD.)  The SMD fails in it's clustering algorithm and one photon is reconstructed as two, which, by definition, comprises a pion candidate.  These will show up with smaller invariant masses than true pions.  Below is a slide from a recent presentation that explaines this in more detail and with pictures.

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We can reproduce this background by looking at singly thrown photons and trying to find those that are artificially split by the clustering algorithm.  Indeed when we do this, we find that a small fraction (sub 1%) do indeed get split.  We can then plot the invariant mass of these pion candidats.  The results can be seen below.  (x-axis is mass in GeV/c^2.)

These mass distributions will later be combined with other pieces and normalized to the data.  For those interested in the errors on these histograms please see below.