SU(6) symmetry and a unified mass formula for normal a Λ hypernuclei

We generalize the Weizsaecker mass formula to Λ hypernuclei by replacing the pairing term with one containing a Majorana operator. This construction is based on an SU(6) spin-flavour symmetry, and in the special case of non-strange nuclei it leads to the same results as a similar construction based on Wigner's SU(4) spin-isospin symmetry. We fit the model parameters to normal and hypernuclei separately and jointly in various mass ranges. We find that for non-strange nuclei the performance of our formula is significantly better than that of the conventional one, and that it is able to give a consistent description of non-strange and Λ hypernuclei. Besides the binding energy, we also calculate nucleon and Λ separation energies. A comparison with another generalization of the Weizsaecker mass formula is given, and a possible way of incorporating shell effects is also proposed. (Author)