Topological variables distribution from data:
(1) total bkg was estimated with same event like sign plus residual bkg.
(2) residual bkg = arithmetic average of side band from (same event unlike sign - same event like sign)

Advantage: reasonable even at S/B is very small
Dis-advantage: need run mix-event to reduce error

DCAxy vs DCAz 2D input for fast simulation: change the range back to (-1, 1) in order to reduce secondary track contribution