Quark Model Study Of Molecular States Of Five-quark Systems

In 2003,Bell collaboration discovered the exotic state of X（3872）.Since then,with the continuous improvement of high-energy physics experimental technology7 more and more hidden-charm and hidden-bottom hadron states were found,such as:Y（4260）,Z（3930）,2b（10610）,2b（10650）,etc.They are called "XYZ" particles.Theoretically,one of the most favorable explanation of these "XYZ”are four-quark statee.In fact,in 1997 D.V.Petrov and M.Polyakov proposed that there might be a five-quark state with a quark contents of ududs:（?）+,and then in 2003,LEPS found a singular S=+1 pentaquark state near 1540 MeV through the decay process of γn→ K⁺K^-n,which has a signal significance of 4.6σ.The experimental results have aroused the interest of many theorists and experimentalists.Many experimental groups have obtained positive results for the state.However,after 2004,due to the improvement of experimental ac-curacy,the state（?）+was denied by experiments.But the searching of the pentaquark state did not stop there.Finally,in 2015,the LHCb group found two hidden-charm pen-taquark states:P_c（4380）+and P_c（4450）from the decay process of ∧_b⁰→J/ΨK-p.In 2019,this result was updated by the LHCb experimental collaboration,there were three new P_c states:P_c（4312）,P_c（4440）,P_c（4457）.In addition,the research on the six-quark system is also underway.In 1997,the MIT bag model was used to predict H-dibaryon（uuddss）IJ^P=00⁺),and in 1989 Goldman et al.also proposed that there might be an"inevitable" dibaryon d^*（uuuddd,IJ^P=03⁺）.The research on the dibaryon d^*continues,a breakthrough appears in 2009 when CELSIUS/WASA and WASA@COSY experiment collaborations found a new resonance state d^*through the study of the Abashian-Booth-Crowe effect,the quantum number of the resonance state is I Jp=03⁺,the mass is about 2370～2380 MeV,and the decay width is about 70～80 MeV.Recently,the dibaryon d^*has been confirmed by the A2@MAMI experimental collaboration in the decay process of γ+d→p+n,and it is believed that the mass of dibaryon d^*is 80 MeV lower than the theoretical threshold of △△,80 MeV higher than the theoretical threshold of △πN The compact nature of the dibaryon d^*is very different from most of the "XYZ" particles which close to the threshold.In addition,the "XYZ" particles contain heavy quarks,and the d^*（2380）is made of light quarks.Therefore,it can be seen that as the accuracy of high-energy experiments increases,more and more exotic particles are found,so the study of the multiquark system is helpful to our understanding of the hadron world.The basic theory for studying the multi-quark system is Quantum Chromodynam-ics,which is the fundamental theory describing the strong interaction between quarks.According to the characteristics of quantum chromodynamics,the interaction between quarks and gluons can be treated perturbatively in βs within the high energy region ac-cording to the asymptotic freedom of QCD,but in the low and middle energy regions,the interaction between gluons-quarks becomes very strong,so non-perturbative methods can only be used to study the low energy phenomena,hadron properties,hadron-hadron inter-actions and multiquark systems.We introduced several commonly used non-perturbative methods,such as:lattice QCD,Dyson-Schwinger equation,QCD summation rules,and quark model etc.,among which the quark model is the most successful method to describe the experimental data.The quark model can not only give a good description of hadron properties and hadron-hadron interactions,but also it has distinct prediction power for the new particles.In addition to explaining the characteristics of five-quark states,this article also introduces the characteristics and methods of quark model,and the dynamic calculations of the possible five-quark states in the quark model.This thesis mainly studies some hadronic states from the perspective of pentaquark states,such as:Nφ,NK and △cc.First,looking for the bound states of Nφ in the chiral quark models.To study the model parameter dependence of the results,three sets of parameters are used.Through the dynamic calculations of five-quark systems with a quark component of uudss,it was found that all the states are unbound in the single channel calculations,and a bound state of Nφ appear in the chiral quark model with the first set of parameters and channel coupling.The results indicate that the channel coupling effects plays an important role on the forming of a bound state.Then,in the framework of quark delocalization color screening model,the resonating group method is used to study the five-quark state whose quark component is uudds.The results show that the states with quantum numbers I=0.J^P=1/2-of NK^*.I=1,J^P=1/2-of△K^*.and I=2,J^P=3/2-of △K^*are all bound states in the single-channel calculations.However,after coupling with the available open channels,all these bound states turn to be scattered states.In the system with Quantum numbers I=1.Jp=5/3-,there is only one state and it is bound state:△K^*,and the mass is 2110.5 MeV.At present,we only consider the S wave in all calculations,but the D wave of △K can be coupled with the S wave of △K^*by the tensor force,it is believed that the will show up as a resonance state through the S-D coupling.Recently,the LHCb experimental collaboration discovered several P_c states,so we believe that there is a high possibility that pentaquark states of P_c-like exist.For the five-quark states with strangeness of PC-like:AcC,we mainly study their properties in the framework of quark delocalization color screening model.The single channel calculations show that all channels with iJ^P=O1/2-is no bound state,but considering to the coupling channels,then the result shows that a bound state exist and the energy is 4094.3MeV.For the IJ^P=03/2-system,the results are similar to the IJ^P=01/2-,a bound state ΛjΨ is obtained by considering to the coupling channel,by contrast,the mass of ∧JΨ is the same as that of ∧_c（?）（4213）in Ref.[137].In addition,those results shows that the coupling channel effect has a great influence on the existence of bound states.For the IJ=11/2-,13/2-,system.because the effective potential of all channels is repulsive,so there are no bound state.At present,all the calculations are limited to the case with all the angular momentum is zero,all the states are in the ground state.In future work,we will consider all possible partial wave effects in the calculation.In addition,in the future,we will consider using unquenched quark model to study the exotic states.According to the theory of quantum chromodynamics and the experimental data of hadron spectrum,the quark-pair can be excited from the vacuum,and the energy needed to excite the quark-pair is about,even less than the energy to excite one unit of angular momentum.So mesons and baryons are not composed only of quark-antiquark and three quarks,there are four-quark and five-quark components in mesons and baryons.