Progress on isospin-symmetry-breaking in nuclear forces and its effects
The charge-symmetry and charge-independence of strong nuclear forces are regarded as fundamental criteria in nuclear physics.However,the nucleon-nucleon scattering data suggest the charge-violating components of the nuclear force,that is,neutron-neutron interaction vnn is slightly more attractive than proton-proton strong interaction vpp,and neutron-proton interaction vnp is stronger than(vnn+vpp)/2.This review introduces the research progress on the isospin-symmetry-breaking component of nuclear forces and its effects,the challenges faced and the methods used to address them.In particular our recent work is reviewed.By employing the microscopic theory combined with bare nuclear forces,the effects of charge-violating components of the nuclear force on the average single-nucleon energy in nuclear matter are extracted,which are characterized by the correspondingly symmetry energy coefficients.With these results as calibrations,we constructed charge-symmetry-breaking(CSB)and charge-independence-breaking(CIB)effective two-body interactions in the Skyrme energy density functional theory.A generalized isobaric multiplet mass equation(GIMME)is established to address the Nolen-Schiffer anomaly(theoretically calculated Coulomb displacement energy turns out to be several percents smaller than the experimental ones persistently over a wide range of nuclei)and the breakdown of the Wigner's quadratic isobaric multiplet mass equation(IMME).For the first time,the high-order terms dTz3 and eTz4 as well as the specific expressions for d and e coefficients for the isobaric quartets and quintets are derived.The CSB interaction in the nuclear medium,combined with the core polarization effect induced by the Coulomb force,is responsible for the breakdown of the quadratic IMME.The magnitude of the calculated d coefficient exhibits an oscillation-like behavior with mass number,modulated by the shell effect,and the experimental d values extracted from the measured masses agree with the oscillation pattern qualitatively.If all excess neutrons in the Tz=T member of a multiplet fill a level above or below a large shell gap,the breakdown of the quadratic IMME tends to be strong,which is essential for guiding future experimental efforts.Finally,in the framework of the Skyrme-Hartree-Fock method with the inclusion of the effective CSB interactions,we constrain the effective Coulomb interaction in turn with the help of experimental Coulomb displacement energy.Accordingly,in order to phenomenologically embody many complicated corrections,an effective Coulomb coupling constant e02=e2(1+aexcZ-2/3)is introduced to replace the fine structure constant e2,where Z is the proton number.Then we established two Skyrme interactions IMP1 and IMP2 complemented by the effective Coulomb force and the calibrated CSB component of effective nuclear force,which help one to explore the isospin-symmetry-breaking effects in nuclear structure problems such as charge-exchange reactions,Gamow-Teller transitions,and β decays in the future.Meanwhile,the nuclear structure issues related to isospin-symmetry-breaking effects are also the important scientific problems studied at large scientific facilities in Lanzhou and Huizhou in China.
isospin symmetryisospin-symmetry-breakingnuclear forceisobaric multiplet mass equationNolen-Schiffer anomaly
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