As noted here and here, the pion peak is difficult to model using single-particle MC.  In single particle studies, the pion inv mass peak is reconstructed to narrow.  Instead of trying to manipulate the single particle MC to conform to the data (using smearing techniques) I decided instead to model the pion peak using identified pions from the filtered pythia MC sample, where identified in this context means the reconstructed pion is less than .01 (in eta-phi space) away from a thrown pythia pion.  As seen here, the mean and width of the peaks from the filtered pythia set match the data extremely well.

Goal:

Properly model the signal and background to the invariant mass plots using single particle MC background shapes and identified pion peak shapes from filtered pythia MC normalized to fit the data.  Further, to subtract out the background on a bin by bin basis to arrive at a raw yield for each pt bin.

Details:

Data Vs. Single Particle MC (see here) and Identified Pions from filtered pythia MC (see here)

Cuts:

• Data events pass L2gamma trigger and L2gamma software trigger
• filtered pythia MC events pass L2gamma software trigger
• Cand Pt > 5.5 GeV
• Charged Track Veto.
• At least one good strip in each SMD plane
• Z Vertex found and |vtx| < 60 cm.
• Zgg < 0.7
• |particle eta| < 0.7

Bins:

9 pt bins, with boundries {5.5, 6., 6.5, 7., 7.75., 9., 11.5, 13.5, 16., 21.}

Plots:

1)

Above is a plot of the invariant mass distributions for the 9 pt bins used in this analysis.  The black crosses represent the data (with errors.)  The four colored histograms show the invariant mass distributions of pion candidates found in identified pions from filtered pythia MC (purple,) Single photon MC (red,) single eta MC (blue,) and mixed-event MC (green.)  The four distributions are simultaneously fit to the data.

2)

The above plot shows a data/MC comparison, where the red MC curve is the sum of the four single particle curves shown in plot 1.  As you can see (especially in bins 3-7) the identified pion spectrum from filtered pythia MC much better recreates the mass peak than single particle (compare here.)

3)

The above plot shows the diphoton invariant mass spectrum for each of the 9 pt bins after the background shapes have been subtracted off.    To calculate the background subtracted raw yields, these peaks are integrated from mean - 3sigma to mean +3sigma of a gaussian fit to the peak.