Distorting Nuclear Mirror Physics 14, s20 Two “mirror” nuclei, in which the numbers of neutrons and protons are interchanged, have markedly different shapes—a finding that defies current nuclear theories. × The development of nuclear theories was aided by a postulated symmetry of the strong interaction holding nucleons together. This interaction, it was initially thought, should be independent of particle charge, allowing neutrons and protons to be regarded as two states of the same particle. However, this “isospin” symmetry isn’t exact—as demonstrated, for instance, by the slight mass difference between protons and neutrons. Researchers have previously accounted for such manifestations of isospin symmetry breaking by tweaking nuclear theories. But now, Kathrin Wimmer, of the Institute for the Structure of Matter in Spain and of the University of Tokyo, and colleagues have shown that two “mirror” nuclei exhibit a degree of symmetry violation that current models can’t explain [1].