Search For E⁺e^-??x(3872)at BES? Experiment

In particle physics,the Quantum Chromodynamics(QCD)describing strong interaction allows for more complex combinations of quarks,such as multiquarks,glueballs,hybrid mesons and hadronic molecules.With the discovery of a large number of hadrons,some new hadrons seeming like charmonium states could not find their suitable positions in the charmonium spectrum predicted by the potential model.They share some properties with charmonium states,but can not be classified as conventional charmonium states.We call it charmonium-like states or the XYZ states.In recent years,the research of XYZ states has been a popular topic in hadron field.Belle observed the first charmonium-like state X(3872)in the decay B?K?+?-J/?in 2003.As one of the most important charmonium-like states,X(3872)has been extensively studied in experiments and theories.Searching for its production processes and decay modes in experiments provides important information for further understanding of its particle property.Up to now,X(3872)has been observed in B meson decays,?b baryon decays,pp collisions,PbPb collisions,Y(4260)radiative decays and two-photon processes.In addition,BES? also observed the process e+e-???cJ(1P).If X(3872)contains a component of the excited spin triplet state ?c1(2P),then the process e+e-??X(3872)may exist.In this thesis,we use the data samples collected by BES? at(?)=4.664-4.950 GeV to search for the process e+e-??X(3872),where ? is reconstructed by ???+?-?0,and X(3872)is reconstructed by X(3872)??0J/?,?0??+?-,J/??e+e-/?+?-.In order to improve the efficiency of signal reconstruction,we separately reconstruct the cases of five and six charged tracks,and then combine them together.This reconstruction strategy improves the signal significance greatly.We observe the process e+e-??X(3872)for the first time.The number of observed signal events is 24.6±5.3 with a statistical significance of 8.0?.We also measured the Born cross section ?(e+e-??X(3872))at each energy point.As the first charmonium-like state observed in experiments,X(3872)has always been at the core position of this field.The discovery of e+e-??X(3872)provides a new production mechanism for X(3872).This provides essential information for researchers to understand the internal structure of X(3872).