Chinese Acad Sci, Inst High Energy Phys, Beijing 100039, Peoples R China
; Chinese Acad Sci, Inst Theoret Phys, Beijing 100080, Peoples R China
; Sichuan Univ, Dept Phys, Chengdu 610064, Peoples R China
; Natl Lab Heavy Ion Accelerator, Ctr Theoret Nucl Phys, Lanzhou 730000, Peoples R China
; Tsing Hua Univ, Dept Phys, Beijing 100084, Peoples R China
; China Ctr Adv Sci & Technol, World Lab, Beijing 100080, Peoples R China
The electromagnetic field in nuclear Fermi gas and nuclei has been studied. Because in the nuclear Fermi gas the expectation value of the electric charge density operator is not zero, different from that in vacuum, the U (1) local gauge symmetry of electric charge is spontaneously broken in the nuclear Fermi gas, and consequently, the photon gains an effective mass. Perturbation calculation of the photon self-energy shows that the effective mass of photon is about 5.42MeV in the symmetric nuclear Fermi gas at the saturation density p(0) = 0.16fm(-3) and about 2.0MeV at the surface of U-238. Furthermore, it is concluded that it is just the two-body decay of a massive photon that causes the discovery of sharp lines of electron-positron pairs in low energy U-238 + Th-232 heavy ion collisions experiments.