The BABAR collaboration has nearly completed a program of precise measurements of the cross sections for the dominant channels of e+e- --> hadrons from threshold to an energy of 3-5 GeV using the initial-state radiation (ISR) method, i.e. the measurement of the cross sections e+e- --> gamma hadrons with the energetic photon detected at large angle to the beams. These data are used as input to vacuum polarization dispersion integrals, in particular the hadronic contribution to the muon g-2 anomaly. In addition to the recently measured pi+pi- cross section, giving the dominant contibution, many multihadronic channels have been investigated, with some recent examples presented here. We give preliminary results for the process e+e- --> K+K-(gamma) using 232 fb-1 of data collected with the BABAR detector at e+e- center-of-mass energies near 10.6 GeV. The lowest-order contribution to the hadronic vacuum polarization term in the muon magnetic anomaly is obtained for this channel: amu-KK-LO=(22.95 +-0.14(stat) +-0.22(syst)) 10^-10, which is about a factor of three more precise than the previous world average value.
Download