The First Observation Of Ψ（3770）→ηj/Ψ At BESⅢ

Experimental measurements of charmonium decays are essential for studying the structure and decay mechanisms of charmonium states.The BESⅢ is a large instrument for high brightness and high energy physics experiment in tau-charm physical energy regions.The BESⅢexperiment has the largest data sample generated by electron-positron collisions in the world,and the high statistical data provides an ideal place for us to study charmonium decay.The Ψ（3770）is a resonant state with the lowest mass higher than D（?） threshold.It has been popularly interpreted as a mixture of the cc system.Since its mass is higher than the D（?） threshold and its width is two orders larger than the width of Ψ（3686）magnitude,it is thought to decay almost entirely to pure D（?） final states before 2003.During the period from 2003 to 2008,the BES collaboration made extensive studies of Ψ（3770）production and decays.In 2003,they observed the first non-D（?） decay mode,Ψ（3770）→J/Ψπ+π-,and they made the first measurement of the non-D（?） decay branching fraction of Ψ（3770）.In 2005,the CLEO collaboration confirmed the observation of Ψ（3770）→J/Ψπ+π-by BES.The large inclusive non-D（?） branching fraction for Ψ（3770）decays may indicate that there are some effects we still don’t understand very well,which is about the traditional resonance（s）lying above the D（?） threshold.Measurement of the branching fractions or partial widths for Ψ（3770）decaying to non-D（?） final states can help us to understand the nature of the Ψ（3770）better.Using the e+e-collision data sample with a total integrated luminosity of 2.9 fb-1 at（?）=3.773 GeV collected with the BESⅢ detector operating at the BEPCⅡ storage ring during 2010 and 2011,the cross section for e+e-→ηJ/Ψ is measured to be（8.97±0.89（stat.）±0.47（sys.））pb.In addition,the decay Ψ（3770）→ηJ/Ψ is observed for the first time with statistic significance of 11.6 σ,and the corresponding branching fraction is measured to be（5.75 ±0.53（stat.）±0.40（sys.）±2.41（int.））× 10-4.The measured branching fraction is consistent with the previous measurement,but with much improved precision.The result is significantiy lower than the theoretical predicted branch ratio with large tetra-quark component in Ψ（3770）.It indicates that the Ψ（3770）might not have large tetra-quark component.