Study Of Double- And Hidden-charm Multiquark States

Since the establishment of quark model and Quantum Chromodynamics(QCD)theory and subsequent complet,ion of standard model,peoples achieve great suc-cess on understanding the inner structures and properties of particles.Because of the nonperturbative properties of QCD in low-energy region,however,it is still not possible to directly study the problems related with hadron structures and spectra from first principles.For exotic hadron states in particular,the problems become more complicated.Therefore,effective theories and phenomenological models based on the spirit of QCD are widely used in various studies.In recent years,a series of extioc mesons and baryons have been found on experiments,whose inner structures are difficult or unable to be understood with the conven-tional quark model.For example,LHCb Collaboration observed the pent,aquark states Pc(4380)and Pc(4450)in the invariant mass of J/?p channel in 2015.Later in 2019,a new structure Pc(4312)+was detected in the same channel and the Pc(4450)was found to be split into two states,Pc(4440)+and Pc(4457)+.One cannot interpret these structures as orbitally excited states of the nucle-on.They should contain at least five quarks.Actually,before the observation of such hidden-charm pentaquark states,theorists in our country had predicted such exotic structures and performed lots of related discussions.To explain the struct,ures and properties of exot,ic states and to predict new exotica is important for us to get a clear understanding on the nature of strong interactions.Up to now,triply heavy baxyons predicted in the quark model have not been observed yet.The conventional baryons containing two heavy quarks were not confirmed until 2017.In that year,LHCb observed the doubly charmed baryon?cc++(ccu)which was first observed by SELEX in 2002.The confirmation of this baryon facilitates researches on double-charm exotic states.In this thesis,we focus on frontiers and new experimental observations in hadron physics.By using improved or proposed approaches in the color-magnetic interaction(CMI)model or the one-boson-exchange potential model,we study on hidden-charm pentaquark states urudcc and udscc,tetraquark states with four d-ifferent flavors q1q2q344,double-heavy tetraquark states QQqq,and double-charm pentaquark-like molecular stat,es(qqc)(cq).The findings are given below.·In the study of hidden-charm pentaquarks,we discuss the possible quantum number and structure of Pc(4312),Pc(4440)and Pc(4457)pentaquark,and then predict the properties of their partner states.In compact pent,aquark picture,we systematically estimate the spectra and rearrangement decay width of uudcc and udscc with CMI model and simple rearrangement decay model.In the hypothesis that rearrangement decay width is the total width of pentaquark above,there is an assignment for pentaquark(uud)8c(cc)8c,that is,Pc(4457),Pc(4440),Pc(4312)? Jp=3/2-,1/2-,3/2-.We could also find by a comparison,that the three Pc states could also be S-wave molecular states(cnn)1c(cn)1c(n=u,d)with assign-ment JP=3/2-(1/2-),1/2-(3/2-),1/2-.The P,(4312)'s quantum number is evidently different in these two pictures,which provide an available information for experiments to distinguish their inner structures.The prediction on spec-tra and width of the other hidden-charm pentaquarks,especially for the stable or narrow state(uds)8c(cc)8c with low mass,could be helpful to test the two pictures and comprehend the rule of strong interaction further.·In the study of tetraquark with four different flavors,we adopt CMI model with new mass estimation method and simple rearrangement decay model to study the spectra and decay width of 9 tetraquark system F1-9=bcsn,bscn,bncs,bucd,bcud,busd,bsud,cusd,csud.(0.1)When estimating the hidden-charm pentaquarks,we take the threshold of relat-ed charmed meson-baryon as input,however,this handling is not suitable for tertraquark states.When we try to get spectra of tetraquark(F1-9),it's mo re reasonable to take X(4140)as input under the assumption that X(4140)is a cscs tetraquark.After analysis on spectra and decay of tetraquark,one could find:(1)Most of tetraquarks have rearrangement decay channels,but the lowest bsud and csud states with I(JP)=0(0+),0(1+)could be st,able;(2)The width is mainly in the region of tens MeV(doubly heavy state bcud with 1(JP)=0(0+),0(1+)could be even smaller than 10 MeV),but the unstable busd state could reach 100 MeV.(3)The X(5568)observed by DO Collaboration couldn't be consistently explained in this work.·In the study of double-charm tetraquarks,we use the heavy diquark-antiquark symmetry(HDAS)to determine the mass of the highest spin tetraquark QQqq with that of doubly heavy baryon QQq,then systematically study the spectra of different system with CMI model.This work provides a different viewpoint from the method with meson-meson thresholds.One could find:(1)There is no stable state existing in the double-charm system ccqq,which is different from the result estimated with threshold of D(*)D(*).(2)The double-bottom system bbud has a stable state with mass around 10490 MeV in the case I(JP)=0(1+).This value is very close to the result estimated with threshold of B(*)B(*).(3)The lowest state in bottoi-charmed(bcnn)system have quantum numbers I(JP)=0(0+),whose mass is 7167 MeV.It is slightly higher than threshold of BD and close to the mass 7151 MeV in the previous study(tetraquark with four different fla-vor).This fact that different estimation methods give a similar conclusion on(bcnn)case,indicates that this state should be close to the BD threshold.In addition,considering the strong attractive effect from color-Coulomb potential between heavy quarks,we discuss the mass constraints on doubly and triply heavy baryons which have not been observed yet.·In the study of double-charm pentaquark within the molecular picture,we det,ailedly analyze the multichannel molecular system AcD-?cD-?cD*-?cD*-?c*D*based on one-boson-exchange model.A molecule is a bound state mainly determined by long-range and medium-range meson-exchange potentials.In the adopted model,we consider the long range ? exchange potential,medium range ?exchange potential,and short range heavy quark exchange potential.To consider the size effects of hadrons on molecule bound energy,we introduce the form factor containing a cutoff parameter.Another concerned parameter is ? dependent coupling constant.Using the gaussian expansion method,we could solve the Schrodinger equation of single channel and multichannel coupling system,and get binding energy and size information of molecular state.In this study,we consider two cutoff parameterized schemes-common cutoff and scaled cutoff.For ? dependent coupling parameter,we also discuss two selection schemes:GT relation scheme and quark model scheme.The results show,(1)Two cutoff parameterized schemes give a result without evident difference;(2)Two coupling constant selection schemes give a result with evident difference.Results in the quark model scheme is evidently deeper in no matter the single channel and multichannel cases,and one can get a bound state with smaller cutoff in the quark model scheme;(3)If the cutoff around 1 GeV is a reasonable choice,the formation of a double-charm molecule(qqc)(cq)is likely;(4)Just like the S-D wave mixing effect in deuteron,the coupled channel effect plays an import,ant role in this work.The cutoff dependence in the coupled channel case is evidently more sensitive than the single channel case;(5)In the condition that RMS radius is larger than 0.7 fm,the binding energies are all around tens of MeV.Along with the experimental improvements,more exotic hadrons would be found,and people will get a deeper understanding on the way how quarks and gluons formhadrons.We hope this and following works could be helpful to further understanding of this problem.