Physics - Hypernuclei

Replacing an up or a down quark with a strange quark in a nucleon, which is bound in a nucleus, leads to the formation of a hypernucleus. A new quantum number, strangeness, is introduced into the nucleus, adding a third axis to the nuclear chart. Due to experimental limitations the third dimension has only scarcely been explored in the past. Single and double Λ-hypernuclei were discovered 50 and 40 years ago, respectively.

However, only 6 double Λ-hypernuclei are presently known, in spite of a considerable experimental effort during the last 10 years. Thanks to the use of p beams and the skilful combination of experimental techniques, copious production at PANDA is expected, with even higher numbers than at (planned) dedicated facilities. A new chapter of strange nuclear physics will be opened whose first result will be the determination of the ΛΛ strong interaction strength, not feasible with direct scattering experiments.

The hyperon - usually a Λ particle - is not restricted by the Pauli principle in populating all possible nuclear states, in contrast to neutrons and protons. The description of hyperons occupying the allowed single-particle states is without the complications encountered in ordinary nuclei, like pairing interactions. The strength of the Λ-N strong interaction may be extracted with a description of the pure single-particle states by well known wave functions.

Furthermore, the decomposition into the different spin-dependent contributions may be analyzed. For these contributions, significantly different predictions exist from meson exchange current and quark models. At the same time, the Λ-N weak interaction can be studied where the Pauli principle acts in the opposite way: the decay of the Λ into N-π is suppressed, since all nucleon states in the nucleus are occupied. In contrast, the process ΛN → NN is allowed, opening a unique window for four-baryon, strangeness non-conserving interaction.