Unexpected neutron/proton ratio and isospin effect in low-energy antiproton-induced reactions

Abstract

The inclusive spectra of preequilibrium nucleons produced in low-energy antiproton-nucleus collisions are thoroughly investigated within the framework of the Lanzhou quantum molecular dynamics model for the first time. All possible reaction channels such as elastic scattering, annihilation, charge exchange and inelastic scattering in antibaryon-baryon, baryon-baryon and meson-baryon collisions have been included in the transport model. The unexpected neutron/proton ratio in comparison to the pion and proton induced reactions is caused from the isospin effects of pion-nucleon collisions and the density dependence of symmetry energy. It is found that the π − -neutron collisions enhance the neutron emission in the antiproton annihilation in a nucleus. Different to heavy-ion collisions, the isospin effects are pronounced at the low kinetic energies. A soft symmetry energy with the stiffness of γs =0.5 at subsaturation densities is constrained from the available data of the neutron/proton spectra. On the other hand, the antiproton-nucleus collisions are motivated to many interesting issues, i.e., charmonium physics, strangeness physics, antiprotonic atom, symmetry, in-medium properties of hadrons, cold quark-gluon plasma, highly excited nucleus etc [7,8]. Recently, the antiproton-antiproton interaction was investigated by the STAR collaboration in relativistic heavy-ion collisions [9].In the past decades, the nuclear reactions induced by antiprotons were investigated with the facilities of the low-energy antiproton ring (LEAR) at CERN [10], the National Laboratory for High Energy Physics at KEK [11] and Brookhaven National Laboratory Alternating Gradient Synchrotron (AGS) accelerator [12]. A number of interesting phenomena were observed, e.g., the delayed fission from the decay of hypernuclei in antiproton annihilations on heavy nuclei [13], unexpected enhancement of the Λ/K 0 S ratio [14], decay mode of highly excited nucleus etc [15,16]. The low-energy antiprotons usually annihilate at the nucleus surface because of the large absorption cross section. The huge annihilation energy are available for producing the 2-6 pions [17,18]. The subsequent processes are complicated and also associated with the multiple pion-nucleon interaction, which result in the fragmentation of target nucleus and the preequilibrium emissions of complex particles [19,20].The dynamics of the antiproton-nucleus collisions is more complicated in comparison to hadron (proton, π, K etc) induced reactions and to heavy-ion collisions, in which the particles produced in the annihilation of the antiproton in a nucleus are coupled to the subsequent collisions with surrounding nucleons. The dynamics of antiproton-nucleus collisions is complicated, which is associated with the mean-field potentials of hadrons in nuclear medium, and also coupled to a number of reaction channels, i.e., the annihilation channels, charge-exchange reaction, elastic and inelastic collisions. There has been several approaches for describing the nuclear dynamics induced by anti...

Neutron skin thickness of 90Zr and symmetry energy constrained by charge exchange spin-dipole excitations*

“…Compared to the proton distribution, it is difficult to precisely measure the neutron distribution in nuclei experimentally since they are neutral particles [17][18][19][20]. A variety of experimental approaches have been attempted to extract the neutron skin thickness, since some experimental observables are sensitive to the density dependence of symmetry energy or the neutron skin thickness, such as parity violating electron scattering [21,22], proton elastic scattering [23], and antiprotonic atoms [24]. In Ref.…”