Study of B ^±→K ^±(K _SKπ) ⁰ decay and determination of η _c and η _c(2S) parameters

We report the results of a study of B ^±→K ^±η _c and B ^±→K ^±η _c(2S) decays followed by η _c and η _c(2S) decays to (K _SKπ) ⁰. The results are obtained from a data sample containing 535 million BB _--meson pairs collected by the Belle experiment at the KEKB e ⁺e ^- collider. We measure the products of the branching fractions B(B±→K±ηc)B(ηc→KSK±π∓)=(26.7±1.4(stat)-2.6+2.9(syst)±4.9(model))×10-6 and B(B±→K±ηc(2S))B(ηc(2S)→KSK±π∓)=(3.4-1.5+2.2(stat+model)-0.4+0.5(syst))×10-6. Interference with the non-resonant component leads to significant model uncertainty in the measurement of these product branching fractions. Our analysis accounts for this interference and allows the model uncertainty to be reduced. We also obtain the following charmonia masses and widths: M(ηc)=(2985.4±1.5(stat)-2.0+0.5(syst)) MeV/c2, Γ(ηc)=(35.1±3.1(stat)-1.6+1.0(syst)) MeV/c2, M(ηc(2S))=(3636.1-4.2+3.9(stat+model)-2.0+0.7(syst)) MeV/c2, Γ(ηc(2S))=(6.6-5.1+8.4(stat+model)-0.9+2.6(syst)) MeV/c2.