Dispersive calculation of B7(3/2) and B8(3/2) in the chiral limit

We show how the isospin vector and axialvector current spectral functions ρV,3 and ρA,3 can be used to determine in leading chiral order the low energy constants B7(3/2) and B8(3/2). This is accomplished by matching the Operator Product Expansion to the dispersive analysis of vacuum polarization functions. The data for the evaluation of these dispersive integrals has been recently enhanced by the ALEPH measurement of spectral functions in tau decay, and we update our previous phenomenological determination. Our calculation yields in the NDR renormalization scheme and at renormalization scale μ=2 GeV the values B7(3/2)=0.55±0.07±0.10 and B8(3/2)=1.11±0.16±0.23 for the quark mass values ms+m̂=0.1 GeV.