Study Of E~+e~-→K~+K~-Ψ(2S) At BESⅢ

According to the quark model,the meson and baryon are composed of two and three quarks,respectively.However,according to the Quantum Chromodynamics(QCD),hadrons can also have more complex structures,e.g.glueballs,hybrids,multi-quark states,and molecules.In the past two decades,many new hadrons have been discovered in experiments with some properties different from the regular hadrons,which are considered as the possible candidates of exotics.They are also known as XYZ state.Here,Y represents the vector quarkonium-like states.The first Y state Y(4260)was discovered in 2005 by the BABAR experiment(now is called Y(4230)).Recently,the BESⅢ experiment(BESⅢ)measured the cross section of e~+e~-→K~+K~-J/ψ in the energy region of 4.127-4.600 GeV and found a new state Y(4500).The decay Y(4230)→K~+K~-J/ψ is also observed.As hot topics in particle physics,study on the exotic hadrons is important for us to understand the perturbative and non-perturbative QCD.BESⅢ collected a large number of data samples above 4 GeV and produced a number of important research results.The threshold of e~+e~-→K~+K~-ψ(2S)is 4.686 GeV.We first studied the process of e~+e~-→K~+K~-ψ(2S)by using data samples collected by BESⅢ with an energy higher than 4.699 GeV.Since these data points are close to the threshold of the process,the generated K+ and K-have low momentum which lead to low detection efficiency.In order to improve the efficiency of signal reconstruction,we adopt a variety of reconstruction methods to reconstruct the signals.Because the ψ(2S)has a large branching fraction(61.4%)to decay to the final states contain a J/ψ,we firstly reconstruct ψ(2S)→J/ψ+anything by tagging a J/ψ.This can effectively improve the signal efficiency.Secondly,due to the low momentum of the produced K mesons in the e~+e~-→K~+K~-ψ(2S)process,we reconstruct e~+e~-→K~+K~-ψ(2S),ψ(2S)→J/ψπ+π-process to improve the signal reconstruction efficiency by missing a Kaon in the final states.In addition,ψ(2S)→(?)~+(?)-((?)=e,μ)also has large branching fraction and is used to reconstruct the signals.With these reconstruction methods,we observe the process e~+e~-→K~+K~-ψ(2S)and measure its cross section at each energy point.Assuming the K~+K~-→(2S)signals come from a single resonance,we extract mass and width of the resonance to be M=4787.7±17.7±1.0 MeV/c~2 and Γ=110.3±33.9±0.7 MeV,respectively,with a statistical significance of 5.6σ.The first error is statistical error and the second is systematic error.This is the first measurement of the e~+e~-→K~+K~-ψ(2S)process and its cross section in experiment.