Measurement of the I = 1/2 Kπ S-Wave Amplitude from Dalitz Plot Analyses of η_c → KK̅π in Two-Photon Interactions

We study the processes γγ→K(S)⁰ K^± π^∓ and γγ→K⁺K⁻π⁰ using a data sample of 519 fb⁻¹ 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 η(c) 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 KπS-wave amplitude and phase. A comparison between the present measurement and those from previous experiments indicates similar behavior for the phase up to a mass of 1.5 GeV/c². 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/c² and Γ=271 ± 22 ± 29 MeV.