Testing the Standard Model of Nuclear Physics
S. Tabor 1*, P. Bender 1
1 Florida State University
Shell model calculations using optimized effective interactions are becoming the standard model of nuclear structure physics. Since most structure data in the past came from nuclei which could be studied using reactions of stable beams on stable targets, these interactions have generally been optimized for nuclear structure near the valley of stability. Modern radioactive beam facilities have allowed the study of nuclei much farther from stability. Evidence is accumulating that the filling of proton shells changes the neutron shell structure and vice-versa. The s-d shell provides an excellent testing ground for confronting theory with experiment. Full calculations are quite tractable and an excellent interaction (USD) exists. Recent refits of the effective interaction (USDA and USDB) with perhaps twice as much experimental data have not changed the results much. Therefore significant deviations between USD predictions and experiment highlight physics beyond the shell (intruder states) and perhaps changes in the single particle structure. New insights will come from experimental studies of nuclei farther from stability being made possible by the new facilities. This talk will focus on experimental results near and inside the "Island of Inversion" (nuclei with Z ~ 10 and N ~20 where intruder configurations dominate the ground states) and the comparison with shell model calculations. An example of new experimental results is shown in the figure below. Although 34P lies just outside the Island of Inversion, comparison with standard model calculations shows that only the ground state and first two excited states are essentially pure s-d states. All the higher states involve at least one particle in a higher shell. Current shell models work reasonably for the 1 particle-1 hole states, but overpredict the energy of what is likely the lowest 2p-2h fully aligned state (6235 keV) by 2 MeV. The highest state observed is a good candidate for a 3p-3h structure whose calculation awaits further work.
Experimental level scheme of 34P. Information in red is new. Only the three lowest levels are pure s-d states. All other states appear to involve one or more "intruder" particles in the f-p shell.