Study of e+e− collisions with a hard initial state photon at BABAR

A study of several 3- and 4-body hadronic final states (π+π−π0, π+π−π+π−, K+K−π+π− and K+K−K+K−) accompanied by a hard photon is presented. These states are produced from e+e− collisions at the c.m. energy near the ϒ(4S) resonance using a 90 fb−1 data sample collected with the BABAR detector at the PEP-II collider. The invariant mass of the hadronic final state determines the virtual photon energy, so that the data can be compared with direct e+e− cross sections. Cross sections have been obtained from threshold up to 4.5 GeV with systematic errors at the 5% level. The accuracy of the results is comparable with the best direct e+e− results overall, but achieves amuch better precision in 1.4-2.5 GeV region where data are sparse. In addition to light meson spectroscopy these data can be used to improve the determination of R -the ratio of e+e−→ hadrons cross section to e+e−→μ+μ− - and thereby to impact the understanding of the recent (g−2)μ measurement through a better evaluation of the hadronic vacuum polarization contribution. The ISR technique also gives access to J/ψ production and allows one to measure branching ratios into 3- and 4-body final states above at a level of precision that is typically better than that obtained in the combined earlier measurements.