The physics of hypernuclei

Abstract

Aldo Zenoni, Universitii di Brescia and INFN Sezione di Pavia, Brescia, Italy Paola Gianotti, INFN Laboratori Nazionali di Frascati, Frascati, Italy The physics of hypernuclei nuclei (with new quantum numbers), in direction of other exotic nuclear systems (charmed nuclei and so on). A hypernucleus is generally indicated with the symbol of the parent nucleus with the suffix A, indicating that a A particle has replaced a neutron. ~ C means a nuclear system composed of 6 protons, 5 neutrons and one A particle. Three events of double hypernuclei were observed, in which 2 nucleons are substitutedby 2 A particles. One example is ,J. He, a system composed of2 protons, 2 neutrons and 2 A particles. The concept of hypernuclei can be extended to nuclei where a nucleon is replaced by hyperons other than the A one. Experimental evidence was claimed ofthe existence ofE-hypernuclei, in which a nucleon is replaced by a L hyperon. This is rather surprising since, in nuclear matter, the L can decay through a strong interaction process (L + N~A + N) giving very largewidths for the hypernuclear states, unless a strong suppression mechanism is at work. However, the experimental evidence of the existence of narrow L-hypernuclei is rather controversial and only the case of tHe is considered unambiguous. After the very first evidence of hyperfragments in cosmic ray interactions with nuclei, starting from the end of the Sixties, the installation ofbeams ofKmesons at particle accelerators made it possible to study the formation of hypernuclei in the laboratory. Hypernuclei were produced copiouslywith little background, the required negative for A hyperon production being present in the beam. The typical utilized was the strangeness where a neutron hit by a Kis changed into a A hyperon emitting a 1t(K+ n~ A + rr-). The experimental techniques used in these experiments were photographic emulsions and bubble chambers (filled with He or heavy liquids) exposed to Kbeams. These experiments mainly measured the hyperon binding energies, by means of the kinematical analysis of the disintegration star, and observed the principal decay modes of the hypernuclei. Starting from the Seventies, the study ofhypernuclei was continued with Kbeams by means of counter techniques with magnetic spectrometers and the introduction of new particle detectors (multiwire proportional chambers and drift chambers). The identification ofwell defined excited hypernuclear levels, by means of the kinematical analysis of the production reaction, was one of the most important results of a first series of experiments started at CERN and continued at Brookhaven (USA). The physical interpretation ofthese spectra was possible in terms of microscopic descriptions of the A-nucleus and A-nucleon potentials. In the Eighties, a new technique for hypernucleus production was introduced at the Brookhaven laboratory,bymeans ofintense beams ofhigh energy 1t+. With this technique the A hyperon is produced inside the nucleus by an associated production reaction (1t+ + n ~ A + K+). This has a reduced cross section, compared to the strangeness exchange reaction, however this drawback is over compensated by the greater intensities of the 1t+ beams. The technique was fully exploited at the KEK laboratory in Japan where, for more than ten years, a great wealth of excellent hypernuclear data were produced, concerning both spectroscopyand decay ofhypernuclei. The interest in these new data has triggered the attention ofthe nuclear physics community and, in the last few years, different laboratories around the world have started a hypernuclear physics program: COSY at Jiilich in Germany, TJNAF at Newport News in USA, Nuclotron in Dubna (Russia). Inparticular, it isworth mentioning the Italian hypernudear project, the FINUDA experiment (acronim of Flsica NUcleare a DAne) at the Laboratori Nazion'. ,.