The Research For Multiquark States

Quantum Chromodynamics(QCD)is a theory that describes strong interaction.At present the research indicates strong interaction has three important properties including asymptotic freedom,quark confinement and approximate chiral symmetry and spontaneous broken,but we can not systematically get the hadron spectrum according to QCD theory.with three properties many phenomenological models were brought in to calculate hadron spectrum except Lattice QCD.quark model is the most successful one among these models,which classifies hadron into meson and baryon,and meson is made up of two quarks,baryon is made up of three quarks.Gellmann explain octet of vector meson,pseuoscalar meson and baryon and decuplet of baryon by SU(3)flavor group successfully.Godfrey-Isgur got basic and excitation state of meson and baryon spectrum by relative potential model.With experiment progress famous collaborations all over the world reported many charmonium-like and bottomnium-like states in the decades,we do not know the internal struc-ture of these states,so we called them XYZ particles,which consist two quarks and two an-tiquarks,then they also called hidden tetraquark.in addition two hidden-charm pentaquark,Pc(4380)and Pc(4450)were reported in the J/?p invairant mass spectrum by LHCb collabo-ration in 2015,all of them support a good chance to research multiquarks,we brought various theoretical model to study these new multiquark states,such as one boson exchange model,one? exchange,chiral untairy,chiral quark and QCD sum rule.X(3872)is the most famous among these states,although thousands of articles try to clear its nature,no of them can do.Some multiquarks have a special property,whose mass is near threshold of two charm mesons or two bottom mesons,which means these states are molecules,we will use one boson exchange model to research these special states,and consider into all possible bosons acting as propagators,these bosons have different functions,for example ? and ? participate in short distance strong interaction,? and ? for middle distance and ? for long distance.because ?and ?' have a mixing angle,we also think of ?' when we calculate.the D wave has important contribution about formation bound states due to deuteron,which is also our consideration.after we calculate and compare with some multiquark states experiment found,we conclude X(3872)is a bound state near DD*+ h.c threshold,Zc(3900)is not a bound state near DD*threshold,Zc(4020)is not D*D*+ h.c,Zb(10610)is not B*B + h.c and Zb(10650)is not B*B*+ h.c.The main content of each chapter as follow:The first chapter;at first we introduce the fundamental structure of matter which people are exploring,cognition about fundamental structure of matter at present and the interaction force between these fundamental structures.next we present the several symmetries of these interactions,which would simplify our question.then we introduce Yukawa model of QED,which have deep relation with one boson exchange model.at last we introduce some famous collaborations about multiquark state experiment.The second chapter:because the width of these multiquark states discovered by collabora-tion at present are big,we need a way to calculate both mass and width.we will introduce two outstanding measures,chiral unitary method and complex scaling method.chiral unitary theory favors for coupling channel,but it just considers basic state.we use complex scaling method to research,which is good at calculating both S-D mixing and coupling channel systems.The third chapter:first we introduce various method about research multiquark states and mention one boson exchange model.then introduce heavy quark effective theory when we induce effective potential between heavy light quark mesons.next we introduce how to infer these effective potentials and list the potential of each system.last we list the number results of each system,and we also compare with multiquark states discovered.The forth chapter:we summarize the results of P-P system,P-P*system and P*-P*system,comparing with experiment.we are hopeful for researching multi quark states.