Ope Constraints and the Leading Order Hadronic Contribution to (g − 2) Μ

OPE constraints are studied as a means of distinguishing between the versions of the I = 1 vector spectral function extracted from (i) inclusive I = 1 hadronic electroproduction cross-sections and (ii) inclusive I = 1 hadronic τ decay data, with the goal of clarifying expectations for the leading order hadronic contribution to aµ = (g − 2)µ/2 in the Standard Model. The constraints are shown to, at present, favor the τ decay data, and hence a Standard Model prediction for aµ compatible with the BNL E821 experimental result. The relative role of the ππ and 4π contributions to the discrepancy between the integrated electroproduction results and the corresponding OPE constraints is also investigated, and the significance, in this context, of forthcoming re-measurements of the e + e − → π + π − π 0 π 0 cross-sections pointed out.