Study On Excited ?_b States In The Extended Local Hidden Gauge Theory

The properties and internal structure of hadron and the interaction between hadrons are important topics in hadron physics.Hadrons are made up of quarks(q)and anti-quarks(q).qq mesons and qqq baryons are considered to be the hadrons with the simplest internal components,the so-called ordinary hadrons.The quark model of hadrons and the fundamental theory describing strong interactions,quantum chromodynamics(QCD),allow for the existence of exotic hadrons with more complex structures.A few baryonic states containing heavy flavor quarks(including excited ?c states and excited ?b states)have been experimentally observed in recent years.And they are considered as candidates of exotic hadrons.The study on exotic hadron states not only enriches the existing hadron spectrum,but also deepens our understanding of theory on strong interaction.In this thesis,the extended local hidden gauge theory,which is successful in re-producing dynamically the excited ?c states,is applied to the study of the excited?b states.The S-wave interacting meson-baryon systems,with quantum numbers of the excited ?b state,are investigated,including pseudoscalar meson-baryon(PB1/2)system with spin-parity JP=1/2-(i.e.?B and the coupled channels),pseudoscalar meson-baryon(PB3/2)system with spin-parity JP=3/2-(i.e.?*B and the coupled channels),vector meson-baryon(VB1/2)system with spin-parity JP=1/2-,3/2-(i·e.?B*and the coupled channels).The lowest scattering amplitude(or transition poten-tial)of meson-baryon scattering is derived from the Lagrangian of the meson-baryon interaction with a vector meson exchange mechanism.By solving Bethe-Salpeter e-quation in coupled channels,several excited Qb states,with molecular structure,are generated dynamically.The mass MR,the width FR,the spin-parity JP and the main components of the excited Qb states are predicted.The only free parameter in our theoretical calculation is the three-momentum cutoff qmax,used to renormalize the loop function of the meson-baryon propagator.In the present work,the cutoff parameter is taken as qmax=650 MeV,which is used to reproduce well the exited ?c states.However,with this value of qmax,the obtained masses of excited Qb states in our calculation are not consistent with those observed by LHCb experiment in early 2020.To investigate whether the difference is caused by the improper value of qmax,we adjust the value of qmax by fitting the new experimental data of the excited Qb states from LHCb,and redo the calculations.The restudy of excited Qb states shows that,in our theoretical framework,the four excited Qb states,newly observed in LHCb experiment,cannot be obtained by adjusting the value of the three-momentum cutoff qmax.Based on the above research results and combined with that the prediction for ordinary excited states from quark model is in good agreement with the observed results from LHCb,we suggest that the four excited Qb states newly observed in LHCb experiment may be ordinary baryons and should not be interpreted as meson-baryon molecular states.The molecular ?b states predicted in the present work are with higher energies than those observed in LHCb experiment.We look forward to more data for excited ?_b~- states in future experiments,which can be used to test our theoretical predictions about molecular states.