The importance of pion and extended Brueckner–Hartree–Fock theory

The pion is important in nuclear physics. The fact that the pion is a pseudoscalar particle makes it very difficult to treat explicitly in the nuclear many-body problem. Matrix elements of the tensor component in the pion exchange interaction vanish in the spin-saturated Hartree–Fock (HF) state. Hence, we are forced to include 2 p − 2 h excitations from the HF state in the model space and use the variational principle to get an extended differential equation for single particle states in the HF state. The structure of this differential equation resembles to that of the Brueckner–Hartree–Fock (BHF) theory and we name the new many-body framework as extended BHF (EBHF) theory. We discuss the similarity and difference between the BHF and EBHF theories. We apply the EBHF theory to nuclear matter and study the equation of state (EOS) and the occupation probability of high momentum states.