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PHYSICS NEWS UPDATE

The American Institute of Physics Bulletin of Physics News

SUPERSYMMETRY IN ATOMIC NUCLEI. A new experiment provides solid evidence that fermions (objects with half-integral spin) and bosons (objects with integral-valued spin) are both governed by the same nuclear physics laws. (The operative term for this egalitarianism, supersymmetry, should not be confused with a similar word used in particle physics to denote the equivalence of fundamental bosons and fermions such as photons and quarks, and of all the physical forces, at energies approaching 10^19 GeV.) The nuclear shell model, dating from 1948, attempts to describe the nucleons (protons and neutrons) in an atomic nucleus as sorting themselves into shells much as electrons do in the atom as a whole. A further innovation in nuclear theory, the interacting boson model (c1974), holds that nucleons can even pair up within their shells, protons with protons and neutrons with neutrons. Individual nucleons are fermions but nucleon pairs are effectively bosons and as such are immune from Pauli's exclusion principle. This allows the pairs to fall into a sort of ground state, leaving only the outermost nucleons to determine the nature of the nucleus's energy level diagram (again analogous to an element's chemistry being determined mostly by its outermost "valence" electrons). In atomic energy diagrams the levels are separated by, at most, electron volts; in nuclear diagrams the levels are typically separated by100 keV or so. Studying these diagrams involves shooting beams (often of protons or deuterons) into a sample, in which nuclei can be promoted into a variety of excited states, and then detecting the telltale particles and high energy photons (gammas) that come out. Nuclei that have an even number of protons and an even number of neutrons possess perhaps a dozen excited energy levels below an energy of 2 MeV, and are relatively easy to probe experimentally. Pt-194 is an example. When the target nucleus has an odd number of either protons (e.g., Au-195) or neutrons (e.g., Pt-195), the number of low-energy excited states might be 20, making it harder to predict an energy diagram. Extending the interacting boson model further to nuclei with an odd number of both protons and neutrons (a nucleus which would consist, in effect, of many bosons and at least two unpaired fermions) entails another level of difficulty.

Harder still is experimentally mapping the energy level diagram for such a nucleus since it would have one hundred or more low-lying excited states. Nevertheless, an intrepid Swiss-German collaboration has now done exactly this for Au-196, a nucleus with 79 protons and 117 neutrons. Using high-resolution detectors they were able to sort through the complex energy-level terrain of Au-196, as well as those for the other three nuclei mentioned above, with results very close to theoretical predictions, demonstrating thereby that a single set of equations could indeed account for nuclei with all the different combinations of even or odd number of neutrons and protons. This is evidence for supersymmetry in nuclei: nuclear forces seem to treat fermions and bosons equivalently, at least for these four nuclei. According to Richard Casten of Yale, who is not a team member, this new research represents an important step forward in applying the interacting boson model.

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