The NΔ Molecular State Is Studied By The Quasi-potential Bethe-Salpeter Equation

Since the discovery of X（3872）by the Belle Collaboration in 2003,a lot of exotic resonance structures were observed in experiment,which can not be put into the frame of the conventional quark model.It is possible that these resonances are the exotic hadrons which have been searched for many years.However,up to now,the origins of those structures are still in the debate,and many interpretations exist in the literatures,such as compact multi-quark states,molecular states,and threshold effect.Among these interpretations,the molecular state is an important physical pictureAt present,many XYZ particles and the hidden-charm pentaquark Pc are interpreted to be molecular states.But the studies about the molecular dibaryons are relatively few.In fact,the study on the dibaryons has a long history.Since the H particle was proposed by Jaffe in 1977,the study of the dibaryons becomes an important research field of the intermediate and high energy nuclear physics.Many efforts have been made in both experimental and theoretical sides.Based on theoretical models,such as the one-boson-exchange model and the constituent quark model,many dibaryons were predicted theoretically.The observation of the d*（2380）at COSY in the process pn dπ0π0 excites the theorist to interpret its mass,width and internal structure,with different models,such as chiral quark model and quark delocalization color screening model.The theoretical studies suggest that the d*（2380）refers to be a compact hexaquarkRecently,the WASA-at-COSY Collaboration reported a hint of existence of a dibary-on N△(D₂₁)with quantum numbers I（JP）=2（1+）.Due to its closeness to the N△threshold,it refers to be a molecular state if it can be confirmed.In this thesis,we study all S-wave states of the N△ system by using the quasipotential Bethe-Salpeter equation combined with One-Boson-Exchange model to search for possible molecular state com-posed of a nucleon and a △ baryon.In the One-Boson-Exchange model,we consider the exchanges of π,ρ,and ω mesons.With the help of the effective Lagrangians,we derive the exchange potential following the Feynman rules.The coupling constants are deter-mined by the experiments and SU（3）symmetry.After inserting the exchange potential into the quasipotential Bethe-Salpeter equation,the scattering amplitude of nucleon and△ baryon can be obtained.The molecular state can be found by searching the poles of the scattering amplitude in the complex energy plane.We consider four S-wave states,that is,D11,D₁₂,D₂₁ and D₂₂.The results suggest that three of them,D₁₂,D₂₁ and D₂₂,can be bound by the interaction.With the value of parameter with which the experi-mental value of the binding energy for D₂₁ state is obtained,the D₂₁ and D₂₂ states are predicted to have binding energies of several and about ten MeV,respectively.Among three exchanges,the π exchange provides main attraction for all three bound states while the p exchange only provides marginal attraction for the D₁₂ state.Inclusion of the ωexchange even weakens the attraction of the N△ interaction.Besides,we find the direct diagram is important to produce the D₁₂ state while the D₂₁ and D₂₂ states are mainly from the cross diagram,which suggests that the contribution of the cross diagram can not be neglected...