First study of η _c(1S), η(1760) and X(1835) production via η ^′π ⁺π ^- final states in two-photon collisions

The invariant mass spectrum of the η ^′π ⁺π ^- final state produced in two-photon collisions is obtained using a 673fb ^-1 data sample collected in the vicinity of the Υ(4S) resonance with the Belle detector at the KEKB asymmetric-energy e ⁺e ^- collider. We observe a clear signal of the η _c(1S) and measure its mass and width to be M(η _c(1S))=(2982.7±1.8(stat)±2.2(syst)±0.3(model)) MeV/c2 and Γ(η _c(1S))=(37.8-5.3+5.8(stat)±2.8(syst) ±1.4(model))MeV/c2. The third error is an uncertainty due to possible interference between the η _c(1S) and a nonresonant component. We also report the first evidence for η(1760) decay to η ^′π ⁺π ^-; we find two solutions for its parameters, depending on the inclusion or not of the X(1835), whose existence is of marginal significance in our data. From a fit to the mass spectrum using coherent X(1835) and η(1760) resonant amplitudes, we set a 90% confidence level upper limit on the product Γ _γγB(η ^′π ⁺π ^-) for the X(1835).