This is a study using the measured pion, kaon (K0short) spectra to estimate the D0 background for the Run 10 62 GeV collisions.

The charged pion spectra were taken from the STAR publication Phys. Lett. B 655 (2007) 104. The K0short spectra were taken from the STAR recent measurement (paper in GPC now) AuAu 62 GeV V0 particle paper. I use the similar procedure as I did for the HT trigger rate estimation for the fast MC calculation. Then I take all random combinations for the pion-kaon pair in the event and estimate the background level. I also take assumptions about the mis-PID, basically, assuming dE/dx PID up to 0.6 GeV/c (p) and TOF PID up to 1.6 GeV/c (p). For all the pions beyond the momentum region, I also allow it can be mis-identified as kaons. The following plot shows the estimated invmass distribution per event for K-pi+ pairs (one charge sign combination) for all kaons and pions within |eta|<1 region.

In the D0 mass region, the background level per event for the dE/dx PID case is ~3/event/MeV (Here the reconstruction efficiency is not included). Assuming the efficiency is ~20-30% (same as the D0's), D0 mass peak width is ~ 15 MeV and then I take +/- 3 sigma to count the background, then multiply by 2 to take into account two charge sign combinations (D0+D0bar), then I get the total background per event for AuAu 62 GeV is about 3*90*2*0.3 = 160.

In this calculation, I didn't include the mis-PID case that the kaon is mis-identified as pions (20% increase?), and also the contribution from protons. Nevertheless, the above number is in the ballpark as what Sarah has got from looking at the Run 4 62 GeV data. The background/event she got is about 250/event.