Effects of nuclear symmetry energy on η meson production and its rare decay to the dark U-boson in heavy-ion reactions

Using a relativistic transport model ART1.0, we explore effects of nuclear symmetry energy on η meson production and its rare decay to the dark U-boson in heavy-ion reactions from 0.2 to 10 GeV/nucleon available at several current and future facilities. The yield of η mesons at sub-threshold energies is found to be very sensitive to the density dependence of nuclear symmetry energy. Above a beam energy of about 5 GeV/nucleon in Au + Au reactions, the sensitivity to symmetry energy disappears. Using the branching ratio of the rare η decay (η→γU) available in the literature, we estimate the maximum cross section for the U-boson production in the energy range considered, providing a useful reference for future U-boson search using heavy-ion reactions.