The Quark Model And The Dynamic Study Of The Pentaquark System

Since 2003,the BaBar,Belle,BESII,CDF,LHCb etc.Collaborations reported a large amount of exotic states called "XYZ states" that contain the heavy quarkonium(bb and cc),These discoveries arose the interesting in the study of multiquark states.In fact,the existence of multiquark states was proposed since the birth of quark model.In 1964,Dyson put forward the sin-quark systems(or dibaryons),Strottman studied the five quark systems that contain light:quarks in 1972.The famous dibaryon candidate,H-particle(uuddss)was proposed by Jaffe in 1977 within the approach of MIT bag mod-el.In 1987 Lipkin naimed the five-quark system as "pentaquark" for the first time.In 2003,a pentaquark state(?)+ with the quark contents of uudds was reported by LEPS Collaboration in the decay of γn → K+K-n,although the results were confirmed by other experiments in 2003 and 2004,it had been denied by more precise experiments later in 2004 and 2005.LEPS still insisted on his original point of view,and they found the pentaquark(?)+ signal with mass 1530 MeV in new decay process of γC→ K-X and γd→Λ*X.At that time,several other pentaquarks,[1]5(1862)and(?)c0(3100).were also reported.However they had been denied by more precise experimental data.In 2015,the LHCb collaboration reported the observation of two hidden-charm pentaquark states,denoted as Pc+(4380)and Pc+(4450),in the decay of Λb0 →J/ψK-p.Recently LHCb collaborations observed five new stats of Ωc0,which parity and spin has not been determined,Ωc0(3000),Ωc0(3050),Ωc0(3066),Ωc0(3090),Ωc0(3119).A lot of explanations of these states have been proposed,ordinary mesons or baryons,molecular states,compact multiquark states,kinematics effects and so on.So far,a concensus about these exotic states has not been arrived.One believes that,the studying of mullti-quark systems will deepen our understanding of the underlying theory.Quantum Chromodynamics(QCD)is the fundamental theory to describe the strong interaction between hadrons using gluons and quarks as the basic degree of freedom.Because of the non-perturbative properties of QCD in the low energy region,it is difficult to apply QCD to study the properties of hadrons and multi-quark states.The QCD sumrule is a very important non-perturbative method and has been an important tool for the study of hadron physics and nuclear physics.The processing of the ground states and the low excited states of the hadron states in this approach can obtain results which agree well with the experimental data.,but there are some problems when dealing with the high excited states of hadrons.Lattice QCD is a non-perturbation method for calculating some physical properties of hadrons from the first principle of the fundamental theory.However,this calculation is very time-consuming,lack of visual physics images,and there is always a problem of systematic error.Dyson-Schwinger equation is another non-perturbative approach for hadron physics.it achieved great success in meson and baryon properties,however it is very difficult to extend to multi-quark systems.Quark model is a successful method for describing the properties of hadrons and hadron-ha.dron interactions,and is easy to extend to the multiquark systems.Although there are problems in quark model,for example,missing states in baryon sector,the nature of Roper resonance,it is still an effective method.The present work is to study the pentaquark states in the framework of quark model.The often used quark models are,MIT bag model,constituent quark model,soliton model etc..The models used in the present work arc chiral quark model and quark delocalization color screening model.They all can give a good description of the properties of hadrons and hadron-hadron interactions.It is expected that they can obtain some meaningful results on pentaquark states.Another feature of the present work is that the high precision few-body method,gaussian expansion method(GEM)are employed.Since the report of pentaquark PC states by LHCb Collaboration,the existence of Pc-like states,with the replacement of cc by ss,was proposed.The study on the hadron level showed that.the nucleon resonances N*(1875)and N*(2100)observed experimentally may be the Pc-like states,the molecular states of ∑*K and ∑K*.The first part of our work is to study these states on the quark level in quark delocalization color screening model by using resonating group method.All the states with I=1/2 and ss are calculated.Our results show that no attractive potential between Λ and K,no bound state can be formed in ΛK channel,whereas the effective potentials between E(*)and K(*)are attractive,and resonances are formed in E(*)K(*)channels.The quantum numbers and the masses of IJP = 1/23/2 ∑*K and IJP=1/23/2 of ∑K*are consistent with the experimental data of N*(1875)and N*(2100).Our work supports the explanation of N*(1875)and N*(2100)as baryon-meson molecular states.The second part of our work is to do the dynamical calculation of non-strange pen-taqaurk states by using GEM.For the parity negative states,one resonance,△π with IJP =1/23/2 is found with the binding energy 3 MeV.No resonance state is found for IJP ＝1/21/2 states.So our calculation does not support the assignment of N*(1535)as the pentaquark state.For the positive parity states,no bound state is found,too.In the present calculation,because the huge amount of computation,the hidden-color channels are not taken into account,which is our future work.Other part of the interaction,tensor and spin-orbital interaction,will also be taken into account in the future work.