Estimating Condensates from the Difference of Tau Branching Fractions
  The difference of tau partial decay widths into even and odd number of pions may be considered as another important constraint on the hadronic spectral function, in addition to four classic sum rules of current algebra. Within certain reasonable assumptions its value may be used to test the factorization hypothesis and estimate the contribution of dimension 6 condensates into the tau hadronic decay width. 
Vakhtang Kartvelishvili and Murman Margvelashvili (High Energy Physics   Institute, Tbilisi State University, GE-380086, Tbi...