Purity Calculation Method for Events Samples with Two Identical   Particles
  This paper studies a method of a two dimensional background calculation for an analysis of events with two particles of the same type registered in experiments in high-energy physics. The standard two-dimensional integration is replaced by an approximation of a specially constructed one-dimensional function. The number of the signal events is found by the subtraction of the background events. It allows calculating the purity of the selection. The procedure does not require a hypothesis about background and signal shapes. Monte Carlo examples of double J/psi samples are used to demonstrate high performance of the purity calculation method. A comparison of the method with standard two-dimensional fit of the signal revealed a systematic shift of the fit results to lower values. 
Valentin Kuzmin
hep-ex