Vacuum polarisation phenomenology: the mu mesic atom X-ray discrepancy and the 2p/sub 3/2/-2s/sub 1/2/ separation in the (mu/sup 4/he)/sup +/ system

The possible existence of non-perturbative vacuum polarisation modifications and the possible existence of a weakly-coupled light scalar boson have been recently proposed to explain the apparent mu- mesic atom X-ray discrepancy. Their effects are shown to introduce in the separation of the 2p/sub 3/2/ and 2s/sub 1/2/ levels of the (mu /sup 4/he)/sup +/ system a discrepancy between recent experiment and theory-otherwise in agreement-of the same level as the original mu- mesic atom X-ray discrepancy. (9 refs).