  The physical reason why one can calculate with similar accuracy, as compared
to the experimental data, the absolute cross section associated with
two-nucleon transfer processes between members of pairing rotational bands,
making use of simple BCS (constant matrix elements) or of many-body
(Nambu-Gorkov (NG), nuclear field theory (NFT)) spectroscopic amplitudes, is
not immediately obvious. Restoration of spontaneous symmetry breaking and
associated emergent generalised rigidity in gauge space provides the answer,
and points to a new emergence: a physical sum rule resulting from the
intertwining of structure and reaction processes and closely connected with the
central role induced pairing interaction plays in structure together with the
fact that successive transfer dominates Cooper pair tunnelling.
