Pentaquarks Containing A Heavy Quark-antiquark Pair In The Constituent Quark Model

Hardon spectrum reflects the structural information of hadrons,and is an important mean to understand the internal structure and dynamics of hadrons.From experimental and theoretical studies of hadron spectrum,it has been recognized that hadrons are strong interaction systems composed of quarks and gluons,while quantum chromodynamics(QCD)is recognized as the basic theory for describing strong interactions.The main features of QCD are asymptotic freedom,quark confinement,and spontaneous breaking of chiral symmetry.Due to the asymptotic freedom of QCD in the high energy region,the high energy processes can be treated with the perturbative QCD(PQCD).However,QCD is highly nonperturbative in the low energy region,and hence the non-perturbative QCD(NPQCD)methods needs to be developed to treat the low energy processes.Now,it is still difficult to try to obtain the structural properties of hadrons and strong interactions in the low energy region by strictly solving QCD,and exploring the effective degrees of freedom of QCD in the low energy region and explaining and predicting hadron structural properties are still scientific problems that need to be solved in hadron physics.The QCDinspired constituent quark potential models have achieved considerable success in describing the low excited states of conventional hadrons(mesons composed of quark and antiquark and baryons composed of three quarks),which opens the door for us to understand low energy QCD.However,QCD does not exclude the existence of exotic states such as tetraquark states,pentaquark states,hybrids,glueballs,etc.Since 2015,CERN’s LHCb collaboration has observed several hidden-charm pentaquark states,which has inspired the studies of its mass spectra from different theoretical models.In this thesis,under the framework of the constituent quark potential model,the mass spectra of the pentaquark states composed of the three light quark cluster and a heavy quarkanti quark pair are systematically calculated.Not only non-strange quark systems nnnQ(n=u,d,Q=c,b)are considered in this thesis,but also strange quark systems such as nnsQQ,nssQQ,sssQQ,and the constituent quark model used contains a one gluon exchange potential at short range and a linear potential with vector and scalar mixing at long range.Using the group theory methods,the orbital wavefunctions of negative parity L=0 and positive parity L=1 are constructed under the Jacobian coordinates,and coupled with the flavor,color,spin wavefunctions to form the total wavefunctions of the pentaquark states satisfying the Pauli principle.In the calculation of the mass spectra,the orbital wavefunction is expanded with the harmonic oscillator basis under the Jacobian coordinates,and the corresponding operator matrix elements are obtained with the help of the transformation coefficients of the harmonic oscillator basis at different Jacobian coordinates,and finally all Hamiltonian matrix elements are obtained.Comparing the results of the theoretical model calculation with the experimental data,the results show that Pc(4380)may be assigned as the ground state with quantum number I(JP)=1/2(1/2-),the angular parity of Pc(4440)correspond to 1/2±or 3/2±,Pc(4457)correspond to 3/2-or 5/2+,and Pcs(4459)correspond to 1/2-.Using the eigenvector to expand the wavefunctions,the proportion of the hidden color channel in the result can be obtained.And the wavefunctions under the q(1)q(2)q(3)(?)Q(4)Q(5)configuration can be transformed into the wavefunctions under the q(1)q(2)Q(4)(?)q(3)Q(5)configuration by orthogonal matrix,the proportion of the hidden color channel under the q(1)q(2)Q(4)(?)q(3)Q(5)configuration can also be obtained.Compared with the proportions of the hidden color channel under the two configurations,they are not small enough to be negligible.Therefore,the explanations of the pentaquark state candidates are given for Pc(4380),Pc(4440),Pc(4457),and Pcs(4459).Other theoretical calculations may also have corresponding pentaquark states,and it is hoped that future experiments will confirm their existence.The calculation results in this paper verify the correct rationality of the constituent quark potential model in the study of multiquark states mass spectra,provide some useful information for the study of the internal structure of the state,and provide direction for future experiments.