Measurement of the I = 1/2 Kπ S-wave amplitude from Dalitz plot analyses of η c →KKπ in two-photon interactions recorded with the BABAR detector operating at the SLAC PEP-II asymmetric-energy e + e − collider at center-of-mass energies at and near the Υ (nS) (n = 2, 3, 4) resonances. We observe ηc decays to both final states and perform Dalitz plot analyses using a model-independent partial wave analysis technique. This allows a model-independent measurement of the mass-dependence of the I = 1/2 Kπ S-wave amplitude and phase. A comparison between the present measurement and those from previous experiments indicates similar behaviour for the phase up to a mass of 1.5 GeV/c 2 . In contrast, the amplitudes show very marked differences. The data require the presence of a new a0(1950) resonance with parameters m = 1931 ± 14 ± 22 MeV/c 2 and Γ = 271 ± 22 ± 29 MeV.

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The e + e − → K + K − cross section and charged-kaon electromagnetic form factor are measured in the e + e − center-of-mass energy range (E) from 2.6 to 8.0 GeV using the initial-state radiation technique with an undetected photon. The study is performed using 469 fb −1 of data collected with the BABAR detector at the PEP-II e + e − collider at center-of-mass energies near 10.6 GeV. The form factor is found to decrease with energy faster than 1/E 2 , and approaches the asymptotic QCD prediction. Production of the K + K − final state through the J/ψ and ψ(2S) intermediate states is observed. The results for the kaon form factor are used together with data from other experiments to perform a model-independent determination of the relative phases between electromagnetic (singlephoton) and strong amplitudes in J/ψ and ψ(2S) → K + K − decays. The values of the branching fractions measured in the reaction e + e − → K + K − are shifted relative to their true values due to interference between resonant and nonresonant amplitudes. The values of these shifts are determined to be about ±5% for the J/ψ meson and ±15% for the ψ(2S) meson.