Search For The Radiative Transition Ψ(2S)→γη_c(2S)

Charmonium is a bound state of a charm quark and an anti-charm quark by strong interaction.The first member of this family,J/Ψ,was first observed in 1974. Subsequently,a series of charmonium states were discovered.Ψ(2S)is the heaviest charmonium state below the DD mass threshold:it can decay radiatively into charge-parity even charmnonia,such asηc(2S),χcJ(J=0,1,2),andηc(1S). So far,the radiative transitionΨ(2s)→γηc(2s)is still not observed.The lastest study on this transition is form the CLEO-c experiment,with no significant signal observed,the bounds of the production rates are determined.We search forΨ(2S)→γηc(2S)using the largestΨ(2S)data sample(106MΨ(2S)decays)collected with the BESⅢdetector.this sample is 4 times of the CLEO-c data sample.ηc(2S)is reconstrcuted withΚS0Κ+π-+c.c. final state,withΚS0 decays intoπ+π-. A 4.5σsignal is observed with the mass of 3636.0±2.0±0.7 McV/c2,width of 7±7±3 McV,and a product branching fraction ofΒ(Ψ(2S)→γηc(2S))×Β(ηc(2S)→ΚS0Κ+π-+c.c.)=(2.68±0.89±0.24)×10-6,with the first errors statistical and the second ones systematic.Taking the BaBar measurement ofΒ(ηc(2s)→ΚΚπ)=(1.9±0.4±1.1)%,we extractΒ(Ψ(2S)→γηc(2S)) (4.23±1.66±2.48)×10-4.This is the first strong evidence for the M1 transitionΨ(2S)→γηc(2S),the measurements of mass and width agree with previous experiments,and the transition rate is in agreement with theoretical model calculations.