First observation of the M1 transition ψ(3686) →γη c(2S)

Abstract

Using a sample of 106×106 ψ(3686) events collected with the BESIII detector at the BEPCII storage ring, we have made the first measurement of the M1 transition between the radially excited charmonium S-wave spin-triplet and the radially excited S-wave spin-singlet states: ψ(3686) →γη c(2S). Analyses of the processes ψ(3686)→γη c(2S) with η c(2S) →KS0K ±π and K +K -π0 give an η c(2S) signal with a statistical significance of greater than 10 standard deviations under a wide range of assumptions about the signal and background properties. The data are used to obtain measurements of the η c(2S) mass (M(η c(2S))=3637.6±2.9 stat±1.6 systMeV/c2), width (Γ(η c(2S))=16.9±6.4 stat±4.8 systMeV), and the product branching-fraction (B(ψ(3686) →γη c(2S))×B(η c(2S) →KK̄π)=(1.30±0.20 stat±0.30 syst)×10 -5). Combining our result with a BABAR measurement of B(η c(2S)→KK̄π), we find the branching fraction of the M1 transition to be B(ψ(3686) →γη c(2S))=(6.8±1.1 stat±4. 5 syst)×10 -4. © 2012 American Physical Society.